UNITED NATIONS ENVIRONMENT PROGRAMME
INTERNATIONAL LABOUR ORGANISATION
WORLD HEALTH ORGANIZATION
INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY
ENVIRONMENTAL HEALTH CRITERIA 184
Diflubenzuron
This report contains the collective views of an international group of
experts and does not necessarily represent the decisions or the stated
policy of the United Nations Environment Programme, the International
Labour Organisation, or the World Health Organization.
First draft prepared by Dr M. Tasheva, Sofia, Bulgaria
Published under the joint sponsorship of the United Nations
Environment Programme, the International Labour Organisation, and the
World Health Organization, and produced within the framework of the
Inter-Organization Programme for the Sound Management of Chemicals.
World Health Organization
Geneva, 1996
The International Programme on Chemical Safety (IPCS) is a joint
venture of the United Nations Environment Programme, the International
Labour Organisation, and the World Health Organization. The main
objective of the IPCS is to carry out and disseminate evaluations of
the effects of chemicals on human health and the quality of the
environment. Supporting activities include the development of
epidemiological, experimental laboratory, and risk-assessment methods
that could produce internationally comparable results, and the
development of manpower in the field of toxicology. Other activities
carried out by the IPCS include the development of know-how for coping
with chemical accidents, coordination of laboratory testing and
epidemiological studies, and promotion of research on the mechanisms
of the biological action of chemicals.
The Inter-Organization Programme for the Sound Management of
Chemicals (IOMC) was established in 1995 by UNEP, ILO, the Food and
Agriculture Organization of the United Nations, WHO, the United
Nations Industrial Development Organization and the Organisation for
Economic Co-operation and Development (Participating Organizations),
following recommendations made by the 1992 UN Conference on
Environment and Development to strengthen cooperation and increase
coordination in the field of chemical safety. The purpose of the IOMC
is to promote coordination of the policies and activities pursued by
the Participating Organizations, jointly or separately, to achieve the
sound management of chemicals in relation to human health and the
environment.
WHO Library Cataloguing in Publication Data
Diflubenzuron.
(Environmental health criteria ; 184)
1. Diflubenzuron - adverse effects 2. Diflubenzuron - toxicity
3. Insecticides - adverse effects 4. Insecticides - toxicity
5. Environmental exposure I. Series
ISBN 92 4 157184 1 (NLM Classification: WA 240)
ISSN 0250-863X
The World Health Organization welcomes requests for permission to
reproduce or translate its publications, in part or in full.
Applications and enquiries should be addressed to the Office of
Publications, World Health Organization, Geneva, Switzerland, which
will be glad to provide the latest information on any changes made to
the text, plans for new editions, and reprints and translations
already available.
(c) World Health Organization 1996
Publications of the World Health Organization enjoy copyright
protection in accordance with the provisions of Protocol 2 of the
Universal Copyright Convention. All rights reserved. The designations
employed and the presentation of the material in this publication do
not imply the expression of any opinion whatsoever on the part of the
Secretariat of the World Health Organization concerning the legal
status of any country, territory, city or area or of its authorities,
or concerning the delimitation of its frontiers or boundaries. The
mention of specific companies or of certain manufacturers' products
does not imply that they are endorsed or recommended by the World
Health Organization in preference to others of a similar nature that
are not mentioned. Errors and omissions excepted, the names of
proprietary products are distinguished by initial capital letters.
CONTENTS
ENVIRONMENTAL HEALTH CRITERIA FOR DIFLUBENZURON
Preamble
1. SUMMARY AND EVALUATION; CONCLUSIONS AND RECOMMENDATIONS
1.1. Summary
1.1.1. Identity, physical and chemical properties, and
analytical methods
1.1.2. Sources of human and environmental exposure
1.1.3. Environmental transport, distribution and
transformation
1.1.4. Environmental levels and human exposure
1.1.5. Kinetics and metabolism in laboratory animals
1.1.6. Effects on laboratory mammals and in vitro test
systems
1.1.7. Effects on humans
1.1.8. Effects on other organisms in the laboratory and
field
1.2. Evaluation
1.2.1. Evaluation of human health risks
1.2.2. Evaluation of effects on the environment
1.2.3. Toxicological criteria for setting guidance values
1.3. Conclusions and recommendations
2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, ANALYTICAL METHODS
2.1. Identity
2.2. Physical and chemical properties
2.3. Conversion factor
2.4. Analytical methods
3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE
3.1. Natural occurrence
3.2. Anthropogenic sources
3.2.1. Production levels and processes
3.2.2. Formulations
3.2.3. Uses
4. ENVIRONMENTAL TRANSPORT, DISTRIBUTION, TRANSFORMATION AND FATE
4.1. Appraisal
4.2. Transport and distribution between media
4.2.1. Soil mobility
4.2.2. Dissipation
4.2.3. Evaporation
4.2.4. Crop residue data
4.3. Transformation
4.3.1. Abiotic degradation
4.3.1.1 Photolysis
4.3.1.2 Hydrolysis
4.3.2. Biodegradation
4.3.2.1 Water
4.3.2.2 Soil
4.4. Bioaccumulation and biomagnification
4.5. Interaction with other physical, chemical or
biological factors
4.6. Ultimate fate following use
5. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE
5.1. Environmental levels
5.1.1. Air
5.1.2. Water
5.1.3. Food and feed
5.1.4. Forest plants and litter
5.1.5. Aquatic organisms
5.2. General population exposure
5.3. Occupational exposure during manufacture, formulation or use
6. KINETICS AND METABOLISM IN LABORATORY ANIMALS AND HUMANS
6.1. Absorption
6.2. Distribution
6.3. Metabolic transformation
6.3.1. Metabolites - distribution, excretion, retention
and turnover
6.4. Elimination and excretion
6.5. Retention and turnover
6.5.1. Biological half-life
7. EFFECTS ON LABORATORY MAMMALS AND IN VITRO TEST SYSTEMS
7.1. Single exposure
7.2. Short-term exposure
7.3. Long-term exposure
7.4. Skin and eye irritation; sensitization
7.5. Reproductive toxicity, embryotoxicity and teratogenicity
7.6. Mutagenicity and related end-points
7.7. Carcinogenicity
7.8. Other special studies
7.8.1. Special studies on met- and sulfhaemoglobin
formation
7.9. Toxicity of metabolites
7.9.1. Carcinogenicity studies with 4-chloroaniline
8. EFFECTS ON HUMANS
9. EFFECTS ON OTHER ORGANISMS IN THE LABORATORY AND FIELD
9.1. Laboratory experiments
9.1.1. Microorganisms
9.1.1.1 Water
9.1.1.2 Soil
9.1.2. Aquatic organisms
9.1.2.1 Microorganisms
9.1.2.2 Plants
9.1.2.3 Invertebrates
9.1.2.4 Vertebrates
9.1.3. Terrestrial organisms
9.1.3.1 Plants
9.1.3.2 Invertebrates
9.1.3.3 Vertebrates
9.2. Field observations
9.2.1. Microorganisms
9.2.1.1 Water
9.2.1.2 Soil
9.2.2. Aquatic organisms
9.2.2.1 Plant
9.2.2.2 Invertebrates
9.2.2.3 Vertebrates
9.2.3. Terrestrial organisms
9.2.3.1 Invertebrates
9.2.3.2 Vertebrates
10. PREVIOUS EVALUATIONS BY INTERNATIONAL BODIES
REFERENCES
RESUME
RESUMEN
NOTE TO READERS OF THE CRITERIA MONOGRAPHS
Every effort has been made to present information in the criteria
monographs as accurately as possible without unduly delaying their
publication. In the interest of all users of the Environmental Health
Criteria monographs, readers are requested to communicate any errors
that may have occurred to the Director of the International Programme
on Chemical Safety, World Health Organization, Geneva, Switzerland, in
order that they may be included in corrigenda.
* * *
A detailed data profile and a legal file can be obtained from the
International Register of Potentially Toxic Chemicals, Case postale
356, 1219 Châtelaine, Geneva, Switzerland (Telephone No. 9799111).
* * *
This publication was made possible by grant number 5 U01
ES02617-15 from the National Institute of Environmental Health
Sciences, National Institutes of Health, USA, and by financial support
from the European Commission.
The proprietary information contained in this document cannot
replace documentation for registration purposes, because the latter
has to be closely linked to the source, the manufacturing route, and
the purity/impurities of the substance to be registered. The data
should be used in accordance with paragraphs 82-84 and recommendations
paragraph 90 of the Second FAO Government Consultation (1982).
Environmental Health Criteria
PREAMBLE
Objectives
In 1973 the WHO Environmental Health Criteria Programme was
initiated with the following objectives:
(i) to assess information on the relationship between exposure to
environmental pollutants and human health, and to provide
guidelines for setting exposure limits;
(ii) to identify new or potential pollutants;
(iii) to identify gaps in knowledge concerning the health effects of
pollutants;
(iv) to promote the harmonization of toxicological and
epidemiological methods in order to have internationally
comparable results.
The first Environmental Health Criteria (EHC) monograph, on
mercury, was published in 1976 and since that time an ever-increasing
number of assessments of chemicals and of physical effects have been
produced. In addition, many EHC monographs have been devoted to
evaluating toxicological methodology, e.g., for genetic, neurotoxic,
teratogenic and nephrotoxic effects. Other publications have been
concerned with epidemiological guidelines, evaluation of short-term
tests for carcinogens, biomarkers, effects on the elderly and so
forth.
Since its inauguration the EHC Programme has widened its scope,
and the importance of environmental effects, in addition to health
effects, has been increasingly emphasized in the total evaluation of
chemicals.
The original impetus for the Programme came from World Health
Assembly resolutions and the recommendations of the 1972 UN Conference
on the Human Environment. Subsequently the work became an integral
part of the International Programme on Chemical Safety (IPCS), a
cooperative programme of UNEP, ILO and WHO. In this manner, with the
strong support of the new partners, the importance of occupational
health and environmental effects was fully recognized. The EHC
monographs have become widely established, used and recognized
throughout the world.
The recommendations of the 1992 UN Conference on Environment and
Development and the subsequent establishment of the Intergovernmental
Forum on Chemical Safety with the priorities for action in the six
programme areas of Chapter 19, Agenda 21, all lend further weight to
the need for EHC assessments of the risks of chemicals.
Scope
The criteria monographs are intended to provide critical reviews
on the effect on human health and the environment of chemicals and of
combinations of chemicals and physical and biological agents. As
such, they include and review studies that are of direct relevance for
the evaluation. However, they do not describe every study carried
out. Worldwide data are used and are quoted from original studies,
not from abstracts or reviews. Both published and unpublished reports
are considered and it is incumbent on the authors to assess all the
articles cited in the references. Preference is always given to
published data. Unpublished data are only used when relevant
published data are absent or when they are pivotal to the risk
assessment. A detailed policy statement is available that describes
the procedures used for unpublished proprietary data so that this
information can be used in the evaluation without compromising its
confidential nature (WHO (1990) Revised Guidelines for the Preparation
of Environmental Health Criteria Monographs. PCS/90.69, Geneva, World
Health Organization).
In the evaluation of human health risks, sound human data,
whenever available, are preferred to animal data. Animal and
in vitro studies provide support and are used mainly to supply
evidence missing from human studies. It is mandatory that research on
human subjects is conducted in full accord with ethical principles,
including the provisions of the Helsinki Declaration.
The EHC monographs are intended to assist national and
international authorities in making risk assessments and subsequent
risk management decisions. They represent a thorough evaluation of
risks and are not, in any sense, recommendations for regulation or
standard setting. These latter are the exclusive purview of national
and regional governments.
Content
The layout of EHC monographs for chemicals is outlined below.
* Summary - a review of the salient facts and the risk evaluation
of the chemical
* Identity - physical and chemical properties, analytical methods
* Sources of exposure
* Environmental transport, distribution and transformation
* Environmental levels and human exposure
* Kinetics and metabolism in laboratory animals and humans
* Effects on laboratory mammals and in vitro test systems
* Effects on humans
* Effects on other organisms in the laboratory and field
* Evaluation of human health risks and effects on the environment
* Conclusions and recommendations for protection of human health
and the environment
* Further research
* Previous evaluations by international bodies, e.g., IARC, JECFA,
JMPR
Selection of chemicals
Since the inception of the EHC Programme, the IPCS has organized
meetings of scientists to establish lists of priority chemicals for
subsequent evaluation. Such meetings have been held in: Ispra, Italy,
1980; Oxford, United Kingdom, 1984; Berlin, Germany, 1987; and North
Carolina, USA, 1995. The selection of chemicals has been based on the
following criteria: the existence of scientific evidence that the
substance presents a hazard to human health and/or the environment;
the possible use, persistence, accumulation or degradation of the
substance shows that there may be significant human or environmental
exposure; the size and nature of populations at risk (both human and
other species) and risks for environment; international concern, i.e.
the substance is of major interest to several countries; adequate data
on the hazards are available.
If an EHC monograph is proposed for a chemical not on the
priority list, the IPCS Secretariat consults with the Cooperating
Organizations and all the Participating Institutions before embarking
on the preparation of the monograph.
Procedures
The order of procedures that result in the publication of an EHC
monograph is shown in the flow chart. A designated staff member of
IPCS, responsible for the scientific quality of the document, serves
as Responsible Officer (RO). The IPCS Editor is responsible for
layout and language. The first draft, prepared by consultants or,
more usually, staff from an IPCS Participating Institution, is based
initially on data provided from the International Register of
Potentially Toxic Chemicals, and reference data bases such as Medline
and Toxline.
The draft document, when received by the RO, may require an
initial review by a small panel of experts to determine its scientific
quality and objectivity. Once the RO finds the document acceptable as
a first draft, it is distributed, in its unedited form, to well over
150 EHC contact points throughout the world who are asked to comment
on its completeness and accuracy and, where necessary, provide
additional material. The contact points, usually designated by
governments, may be Participating Institutions, IPCS Focal Points, or
individual scientists known for their particular expertise. Generally
some four months are allowed before the comments are considered by the
RO and author(s). A second draft incorporating comments received and
approved by the Director, IPCS, is then distributed to Task Group
members, who carry out the peer review, at least six weeks before
their meeting.
The Task Group members serve as individual scientists, not as
representatives of any organization, government or industry. Their
function is to evaluate the accuracy, significance and relevance of
the information in the document and to assess the health and
environmental risks from exposure to the chemical. A summary and
recommendations for further research and improved safety aspects are
also required. The composition of the Task Group is dictated by the
range of expertise required for the subject of the meeting and by the
need for a balanced geographical distribution.
The three cooperating organizations of the IPCS recognize
the important role played by nongovernmental organizations.
Representatives from relevant national and international associations
may be invited to join the Task Group as observers. While observers
may provide a valuable contribution to the process, they can only
speak at the invitation of the Chairperson.
Observers do not participate in the final evaluation of the chemical;
this is the sole responsibility of the Task Group members. When the
Task Group considers it to be appropriate, it may meet in camera.
All individuals who as authors, consultants or advisers
participate in the preparation of the EHC monograph must, in addition
to serving in their personal capacity as scientists, inform the RO if
at any time a conflict of interest, whether actual or potential, could
be perceived in their work. They are required to sign a conflict of
interest statement. Such a procedure ensures the transparency and
probity of the process.
When the Task Group has completed its review and the RO is
satisfied as to the scientific correctness and completeness of the
document, it then goes for language editing, reference checking, and
preparation of camera-ready copy. After approval by the Director,
IPCS, the monograph is submitted to the WHO Office of Publications for
printing. At this time a copy of the final draft is sent to the
Chairperson and Rapporteur of the Task Group to check for any errors.
It is accepted that the following criteria should initiate the
updating of an EHC monograph: new data are available that would
substantially change the evaluation; there is public concern for
health or environmental effects of the agent because of greater
exposure; an appreciable time period has elapsed since the last
evaluation.
All Participating Institutions are informed, through the EHC
progress report, of the authors and institutions proposed for the
drafting of the documents. A comprehensive file of all comments
received on drafts of each EHC monograph is maintained and is
available on request. The Chairpersons of Task Groups are briefed
before each meeting on their role and responsibility in ensuring that
these rules are followed.
WHO TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR DIFLUBENZURON
Members
Dr T. Bailey, US Environmental Protection Agency, Washington DC, USA
Dr A.L. Black, Department of Human Services and Health, Canberra,
Australia
Mr D.J. Clegg, Carp, Ontario, Canada
Dr S. Dobson, Institute of Terrestrial Ecology, Monks Wood, Abbots
Ripton, Huntingdon, Cambridgeshire, United Kingdom
(Vice-Chairman)
Dr P.E.T. Douben, Her Majesty's Inspectorate of Pollution, London,
United Kingdom (EHC Joint Rapporteur)
Dr P. Fenner-Crisp, US Environmental Protection Agency, Washington DC,
USA
Dr R. Hailey, National Institute of Environmental Health Sciences,
National Institutes of Health, Research Triangle Park, USA
Ms K. Hughes, Environmental Health Directorate, Health Canada,
Ottawa, Ontario, Canada (EHC Joint Rapporteur)
Dr D. Kanungo, Central Insecticides Laboratory, Government of India,
Ministry of Agriculture & Cooperation, Directorate of Plant
Protection, Quarantine & Storage, Faridabad, Haryana, India
Dr L. Landner, MFG, European Environmental Research Group Ltd,
Stockholm, Sweden
Dr M.H. Litchfield, Melrose Consultancy, Denmans Lane, Fontwell,
Arundel, West Sussex, United Kingdom (CAG Joint Rapporteur)
Professor M. Lotti, Institute of Occupational Medicine, University of
Padua, Padua, Italy (Chairman)
Professor D.R. Mattison, University of Pittsburgh, Graduate School of
Public Health, Pittsburgh, Pennsylvania, USA
Dr J. Sekizawa, National Institute of Health Sciences, Tokyo, Japan
Dr P. Sinhaseni, Chulalongkorn University, Bangkok, Thailand
Dr S.A. Soliman, King Saud University, Bureidah, Saudi Arabia
Dr M. Tasheva, National Centre of Hygiene, Medical Ecology and
Nutrition, Sofia, Bulgaria (CAG Joint Rapporteur)
Mr J.R. Taylor, Pesticides Safety Directorate, Ministry of
Agriculture, Fisheries and Food, York, United Kingdom
Dr H.M. Temmink, Wageningen Agricultural University, Wageningen, The
Netherlands
Dr M.I. Willems, TNO Nutrition and Food Research Institute, Zeist,
The Netherlands
Representatives of GIFAPa (Groupement International des
Associations Nationales de Fabricants de Produits Agrochimiques)
Dr M. Bliss, Jr., ISK Biosciences Corporation, Mentor, Ohio, USA
Dr A.C. Dykstra, Registration Department BPID, Solvay-Duphar BV, CP
Weesp, The Netherlands
Dr H. Frazier, ISK Biosciences Corporation, Mentor, Ohio, USA
Dr R. Gardiner, GIFAP, Brussels, Belgium
Dr B. Julin, Regulatory Affairs, Du Pont de Nemours (Belgium),
Agricultural Products Department, Mercure Centre, Brussels,
Belgium
Dr S.M. Kennedy (Environmental Science), Du Pont de Nemours (Belgium),
Agricultural Products Department, Mercure Centre, Brussels,
Belgium
Dr J. Killeen, ISK Biosciences Corporation, Mentor, Ohio, USA
Dr Th. S.M. Koopman, Toxicology Department, Solvay-Duphar BV, CP
Weesp, The Netherlands
Dr R.L. Mull, Du Pont Agricultural Products, Wilmington, Delaware, USA
Dr J.L.G. Thus, Environmental Research Department, Solvay-Duphar BV,
CP Weesp, The Netherlands
Secretariat
Ms A. Sundén Byléhn, International Register of Potentially
Toxic Chemicals, United Nations Environment Programme,
Châtelaine, Switzerland
Dr P. Chamberlain, International Programme on Chemical Safety, World
Health Organization, Geneva, Switzerland
a Participated as required for exchange of information.
Dr J. Herrman, International Programme on Chemical Safety, World
Health Organization, Geneva, Switzerland
Dr K. Jager, International Programme on Chemical Safety, World Health
Organization, Geneva, Switzerland
Dr P. Jenkins, International Programme on Chemical Safety, World
Health Organization, Geneva, Switzerland
Dr W. Kreisel, World Health Organization, Geneva, Switzerland
Dr M. Mercier, International Programme on Chemical Safety, World
Health Organization, Geneva, Switzerland
Dr M.I. Mikheev, Occupational Health, World Health Organization,
Geneva, Switzerland
Dr G. Moy, Food Safety, World Health Organization, Geneva, Switzerland
Mr I. Obadia, International Labour Organisation, Geneva, Switzerland
Dr R. Pleœtina, International Programme on Chemical Safety, World
Health Organization, Geneva, Switzerland
Dr E. Smith, International Programme on Chemical Safety, World Health
Organization, Geneva, Switzerland (EHC Secretary)
Mr J. Wilbourn, International Agency for Research on Cancer, Lyon,
France
ENVIRONMENTAL HEALTH CRITERIA FOR DIFLUBENZURON
The Core Assessment Group (CAG) of the Joint Meeting on Pesticide
Residues met in Geneva from 25 October to 3 November 1994.
Dr W. Kreisel of the WHO welcomed the participants on behalf of WHO,
and Dr M. Mercier, Director, IPCS, on behalf of the IPCS and its
cooperating organizations (UNEP/ILO/WHO). The Group reviewed and
revised the draft monograph and made an evaluation of the risks for
human health and the environment from exposure to diflubenzuron.
The first draft of the monograph was prepared by Dr M. Tasheva,
Sofia, Bulgaria. The second draft, incorporating comments received
following circulation of the first draft to the IPCS contact points
for Environmental Health Criteria monographs, was prepared by the IPCS
Secretariat.
Dr K.W. Jager and Dr P.G. Jenkins, both members of the IPCS
Central Unit, were responsible for the overall scientific content and
technical editing, respectively.
The fact that Solvay-Duphar, BV, made available to the IPCS its
proprietary toxicological information on diflubenzuron is gratefully
acknowledged. This allowed the CAG to make its evaluation on a more
complete database.
The efforts of all who helped in the preparation and finalization
of the monograph are gratefully acknowledged.
ABBREVIATIONS
ADI acceptable daily intake
a.i. active ingredient
AP alkaline phosphatase
bw body weight
4-CPU 4-chlorophenylurea
DFB diflubenzuron
2,6-DFBA 2,6-difluorobenzoic acid
ECD electron capture detection
G granular formulation
GC gas chromatography
GLC gas-liquid chromatography
Hb haemoglobin
HPLC high performance liquid chromatography
MATC maximum acceptable toxicant concentration
MCH mean cell haemoglobin
MCHC mean cell haemoglobin concentration
MCV mean cell volume
NOAEC no-observed-adverse-effect concentration
NOEL no-observed-effect level
NPD nitrogen-phosphorus detector
PCA para-chloroaniline (4-chloroaniline)
PCV packed cell volume
SAP serum alkaline phosphatase
SGOT serum glutamic-oxaloacetic transaminase (aspartate
aminotransferase)
SGPT serum glutamic-pyruvic transaminase (alanine
aminotransferase)
TLC thin-layer chromatography
WP wettable powder
1. SUMMARY AND EVALUATION; CONCLUSIONS AND RECOMMENDATIONS
1.1 Summary
1.1.1 Identity, physical and chemical properties, and analytical
methods
Diflubenzuron is a member of the benzoylphenylurea group of
insecticides. Its insecticidal action is due to interaction with
chitin synthesis and/or deposition. It forms odourless white crystals
with a melting point of 230-232°C. It is sparingly soluble in water
(0.2 mg/litre at 20°C) and is virtually non-volatile. It is
relatively stable in acidic and neutral media but it hydrolyses in
alkaline conditions.
Diflubenzuron is produced by the reaction of 2,6-difluoro-
benzamide with 4-chlorophenylisocyanate.
Diflubenzuron residues may be measured in water, biological
samples and soils by HPLC with UV detection or by GC with ECD for
analysis of the intact molecule or following derivatization of the
liberated 4-chloroaniline with trifluoroacetic anhydride.
1.1.2 Sources of human and environmental exposure
Diflubenzuron is a synthetic compound used in agriculture,
forestry and public health programmes to control insect pests and
vectors. Different formulations of diflubenzuron are available for
these uses. There is no relevant information on human exposure to
diflubenzuron.
1.1.3 Environmental transport, distribution and transformation
Diflubenzuron is usually applied directly to plants and water.
Uptake of diflubenzuron through plant leaves does not occur.
The adsorption of diflubenzuron to soil is rapid. It is
immobilized in the top 10 cm layer of soil to which it is applied. It
is unlikely to leach. Diflubenzuron is degraded in soils of various
types and origin under aerobic or anaerobic conditions with a half-
life of a few days. The rate of degradation depends greatly on the
diflubenzuron particle size. The main metabolic pathway (over 90%) is
hydrolysis leading to 2,6-difluorobenzoic acid and 4-chlorophenylurea;
these are degraded with half-lives of about 4 and 6 weeks,
respectively. Free 4-chloroaniline has not been detected in soils.
Diflubenzuron degrades rapidly in neutral or alkaline waters.
Studies of application of diflubenzuron to water show rapid partition
to sediment; the parent compound and 4-chlorophenylurea may persist on
sediment for more than 30 days.
Diflubenzuron does not bioaccumulate in fish.
1.1.4 Environmental levels and human exposure
Exposure of the general population to diflubenzuron via water or
food as a result of its use in agriculture, against forest insects or
in mosquito control is negligible.
1.1.5 Kinetics and metabolism in laboratory animals
In experimental animals, diflubenzuron is absorbed from the
digestive tract and to a lesser extent through the skin. There is a
saturable absorption mechanism in the rat gastrointestinal tract.
Consequently a large proportion of orally administered diflubenzuron
is found in the faeces. Diflubenzuron has widespread distribution in
the tissues, but it does not accumulate.
The metabolic fate of diflubenzuron has been studied in various
species. The major route of metabolism in mammals is via
hydroxylation. Hydrolysis of diflubenzuron may occur at any of the
three carbon-nitrogen bonds. In pigs and chickens the major route of
hydrolysis is at the ureido bridge. In rats and cows the major
metabolic pathway is hydroxylation. The major metabolites in sheep,
swine and chickens are 2,6-difluorobenzoic acid and 4-chloro-
phenylurea; minor metabolites are 2,6-difluorobenzamide and
4-chloroaniline. In rats and cattle 80% of the metabolites are
2,6-difluoro-3-hydroxydiflubenzuron, 4-chloro-2-hydroxy-diflubenzuron
and 4-chloro-3-hydroxydiflubenzuron. The metabolic studies indicate
that little or no 4-chloroaniline is formed in rats or cattle.
The major route of elimination is via the faeces, ranging from 70
to 85% in cats, pigs and cattle. In sheep elimination is roughly
equally distributed between the urine and faeces. Urinary excretion
in rats and mice decreases proportionally with increasing dosage
level. Less than 1% of an oral dose is recovered in exhaled air.
Only trace residues are found in milk.
No human studies on the kinetics and metabolism of diflubenzuron,
including the extent of biotransformation to 4-chloroaniline, are
available.
1.1.6 Effects on laboratory mammals and in vitro test systems
Diflubenzuron has low acute toxicity by any route of exposure.
It has been classified by WHO as a "product unlikely to present an
acute hazard in normal use", based on an acute oral LD50 of more than
4640 mg/kg body weight in rats. The acute dermal LD50 in rats is
greater than 10 000 mg/kg body weight while the acute inhalational
LC50 for rats is greater than 2.49 mg/litre. No signs of
intoxication have been observed during the 14-day period following
single administration of diflubenzuron by various routes to a variety
of animal species.
Diflubenzuron is not a skin irritant (in rabbits) and not a skin
sensitizer (in guinea-pigs). It is marginally irritating to the eyes
of rabbits.
Diflubenzuron causes methaemoglobinaemia and sulfhaemo-
globinaemia. Dose-related methaemoglobinaemia has been demonstrated
after oral, dermal or inhalatory exposure to diflubenzuron in various
species. This effect is the most sensitive toxicological end-point in
experimental animals. The NOEL based on methaemoglobin formation is
2 mg/kg body weight per day in rats and dogs and 2.4 mg/kg body weight
per day in mice. In long-term toxicity studies with mice and rats,
treatment-related changes were principally associated with oxidation
of haemoglobin or with hepatocyte changes.
In carcinogenicity studies in mice and rats at dietary levels up
to 10 000 mg/kg in the diet, there were no treatment-related effects
on tumour incidence. Specifically, there were no mesenchymal
neoplasms of the spleen or liver as observed in carcinogenicity
studies with 4-chloroaniline.
In several reproductive toxicity studies on rats, mice, rabbits
and three avian species, no effects were seen on reproduction and
there was no embryotoxicity. Teratogenicity studies in rats and
rabbits demonstrated no teratogenic effects.
Diflubenzuron and its main metabolites have been examined in a
variety of in vitro and in vivo mutagenicity tests. Neither
diflubenzuron nor its major metabolites have a mutagenic effect.
The minor metabolite, 4-chloroaniline, was shown to be positive
in several in vitro mutagenicity assays using various end-points.
It is carcinogenic in rats and mice. The neoplastic lesions related
to administration of 4-chloroaniline were benign and malignant
mesenchymal tumours in the spleens of male rats and haemangiomas and
haemangiosarcomas, primarily in the spleen and liver of male mice.
1.1.7 Effects on humans
The diflubenzuron metabolite, 4-chloroaniline, has been reported
to cause methaemoglobinaemia in exposed workers and in neonates
inadvertently exposed. Some individuals who are deficient in
NADH-methaemoglobin reductase may be particularly sensitive to
4-chloroaniline and hence to diflubenzuron exposure.
1.1.8 Effects on other organisms in the laboratory and field
All chitin-synthesizing organisms show susceptibility to
diflubenzuron.
Bacteria were not affected by diflubenzuron at concentrations of
500 mg/kg soil; some stimulation of nitrogen fixation was seen.
Diflubenzuron acetone solutions were degraded; the acetone was used
as carbon source. Algal biomass increased at a diflubenzuron
concentration of 1 µg/litre. There were no adverse effects at
concentration above the limit of diflubenzuron solubility. Fungi were
temporarily affected at 0.1 µg/litre in laboratory streams.
Aquatic invertebrates show variable responses to diflubenzuron.
Molluscs are insensitive, the LC50 being greater than 200 mg/litre.
LC50 values for other invertebrates ranged from 1 to > 1000
µg/litre, reflecting the effects of the compound on juvenile,
moulting stages. A MATC for Daphnia has been estimated at > 40 and
< 93 ng/litre; as expected, larval mayflies and other aquatic insect
juveniles are highly susceptible. Overspray of water bodies would be
expected to kill some aquatic invertebrates.
In ecosystems and field experiments where diflubenzuron was
applied directly to the water, the effects on most taxa were less
severe than predictions from acute laboratory tests. No effects on
aquatic organisms have been found after aerial applications to
forests.
The LC50 values for fish are > 150 mg/litre. In field
experiments, fish kills have never been recorded.
The oral and contact LD50 for honey-bees is greater than
30 µg/bee. Honey-bee colonies were not affected after aerial
application of 350 g diflubenzuron/ha.
A 5-day dietary study on the mallard duck and bobwhite quail with
levels of up to 4640 mg/kg feed revealed no observable signs of
toxicity. Small songbirds in the forest ecosystem were not affected
after aerial application of diflubenzuron at 350 g/ha.
Small mammal species showed no reductions in numbers after
application of diflubenzuron at 67 g/ha to a forest.
1.2 Evaluation
1.2.1 Evaluation of human health risks
The primary manifestation of diflubenzuron toxicity is
methaemoglobin induction. This toxicity occurs in a range of test
animal species. It is attributable to the metabolite, 4-chloroaniline,
which is known to induce methaemoglobin formation in several animal
species and in humans.
Diflubenzuron does not cause other toxicities on chronic dietary
administration. It is not mutagenic or carcinogenic in mice or rats.
However, its metabolite, 4-chloroaniline, is mutagenic in vitro and
is carcinogenic in mice and male rats. Although 4-chloroaniline is a
minor urinary metabolite of diflubenzuron in rats, the extent to which
it is formed in vivo in various animal species remains unknown.
Similarly, the comparative degree of absorption of its parent compound
in various species is unknown.
The sensitivity of human haemoglobin to methaemoglobin formation
by 4-chloroaniline in vivo is not known. However, since induction
of methaemoglobinaemia is consistently the most sensitive measure of
diflubenzuron toxicity in the various animal species tested, it may be
used as the basis to estimate the levels causing no toxicological
effect.
1.2.2 Evaluation of effects on the environment
Diflubenzuron adsorbs readily to soil with little subsequent
desorption. Its mobility in soil is very low, practically all
residues remaining within 15 cm of the top, even in sandy loam soils;
diflubenzuron does not leach. It is only partly removed from foliage
by heavy rainfall. Nevertheless, some diflubenzuron may be present in
surface water shortly after application, due to flooding of treatment
areas or agricultural run-off.
Dissipation of diflubenzuron from water is rapid. Adsorption to
sediment occurs within 4 days; both parent compound and 4-chloro-
phenylurea metabolite may persist on sediment for at least 30 days.
Uptake of diflubenzuron by plants through the leaves after aerial
application does not occur. Some uptake of soil residues does occur
in plants and this may be translocated. At the highest application
rate (1 kg a.i./ha), following 1 month ageing of residues, up to
1 mg/kg residue may be found in various crops.
Photolysis of diflubenzuron is slow with a calculated half-life
of 40 days. Under environmental conditions abiotic degradation in
water and soil represents a minimum route of break-down. Aerobic
degradation in water is a microbial process with a half-life of a few
days under both laboratory and field conditions. In the field,
degradation of diflubenzuron applied at practical rates is influenced
by pH, temperature, formulation, organic matter content and depth of
the water.
Degradation in soil through microbial hydrolysis is a rapid
process, with a half-life of a few days, depending on diflubenzuron
particle size. The major break-down products are 2,6-difluorobenzoic
acid and 4-chlorophenylurea; a minor metabolite is parachloroaniline.
All these are irreversibly bound to soil and/or further metabolized.
The half-life of diflubenzuron residues on citrus fruits is
significantly decreased by high temperature and humidity.
Anaerobic degradation in water and sediment is slower than
aerobic.
Fish bioconcentrate diflubenzuron and some bioaccumulation takes
place during extended exposure up to a plateau, depending on the water
concentration, owing to fast degradation of diflubenzuron and
excretion of metabolites; the depuration half-life is less than one
day. The 4-chloroaniline metabolite has not been detected in fish.
Fish are not sensitive to diflubenzuron, the LC50 values being
> 150 mg/litre. Metabolites of diflubenzuron are also of low
toxicity to fish. Chronic exposure has shown no effects on fish at
recommended application rates; the compound does not persist in water
and no chronic exposure is expected.
Diflubenzuron is not phytotoxic to duckweed at the diflubenzuron
solubility limit concentration.
Honey-bees were not affected by topical applications of
> 30 µg/bee or dietary concentrations of up to 1000 mg/kg diet.
Brood in hives was reduced when bees were fed syrup at 59 mg
diflubenzuron/kg. Brood was also reduced following exposure of
flying colonies.
Earthworms were not affected at a concentration of 780 mg/kg
soil, which is at least three orders of magnitude above reported soil
residues.
Diflubenzuron has low acute toxicity to birds, the oral and
dietary LD(LC)50 values being greater than 3000 mg/kg diet.
Following recommended application rates diflubenzuron is not expected
to pose a hazard to birds.
Extensive field studies have shown minimal or reversible effects
on most aquatic invertebrates; daphnids were most seriously affected,
with short-term reductions in populations of up to 75% following a
single application of diflubenzuron. Fish were not affected by water
overspraying. Neither bird nor mammal populations were adversely
affected following forest spraying with diflubenzuron.
A summary of risk quotients for birds, fish and aquatic
invertebrates is given in Table 1.
1.2.3 Toxicological criteria for setting guidance values
The toxicological studies on diflubenzuron of relevance for
setting guidance values are shown in Table 2.
Table 1. Toxicity/exposure ratios for birds, fish and aquatic invertebrates based on
application rates of 2.5 kg a.i./ha of diflubenzuron to soybeans (worst case)
Risk category LC50 (mg/litre Estimated exposure Toxicity/exposure
or mg/kg diet) (mg/litre or ratio (TER)c
mg/kg diet)a,b
Acute bird 3762 73.7-535.7 51.0-7.0
Acute fish (stream) 150 0.0007 214 300
Acute fish (pond) 150 0.01 15 000
Acute aquatic
invertebrate (stream) 0.005 0.0007 7.1
Acute aquatic
invertebrate (pond) 0.005 0.01 0.5
a Estimated environmental concentration in the terrestrial environment (for bird
exposure) is based on the stated application rate and the assumption of
deposition on short grass using the US EPA nomogram.
b Aquatic exposure concentrations were taken from the STREAM model based on a
single application and estimated runoff into water; no direct overspray is
included.
c TER is the toxicity (as LC50) divided by the exposure; values at or below
1.0 indicate likely exposure to toxic concentrations by organisms in the
different risk categories.
Table 2. Toxicological criteria for estimating guidance values for diflubenzuron
Exposure scenario Relevant route/effect/ Result/remarks
(technical species
diflubenzuron)
Short-term dermal, irritation, rabbit non-irritant
(1-7 days)
ocular, irritation, rabbit marginal, high dose
dermal, sensitization, non-sensitizing
guinea-pig
inhalational, toxicity, rat LC50 > 2.49 mg/litre
(single exposure)
Mid-term
(1-26 weeks)
3 weeks; 5 days dermal, irritation, rabbit NOEL = 70 mg/kg body
per week weight per day
3 weeks; 5 days inhalational, methaemoglobin NOAEL = < 0.12 mg/litre
per week formation, rat
Long-term dietary, methaemoglobin NOEL = 2 mg/kg body weight
formation, rat per day
dietary, methaemoglobin NOEL = 2.4 mg/kg body weight
formation, mouse per day
dietary, methaemoglobin NOEL = 2 mg/kg body weight
formation, dog per day
1.3 Conclusions and recommendations
Considering the toxicological characteristics of diflubenzuron,
both qualitatively and quantitatively, it was concluded, on the basis
of the NOEL of 2 mg/kg body weight per day derived in long-term
toxicity studies on mice, rats and dogs and applying a 100-fold
uncertainty factor, that 0.02 mg/kg body weight per day will probably
not cause adverse effects in humans whatever the route of exposure.
Biomonitoring of 4-chloroaniline during occupational exposures
needs to be carried out.
2. IDENTITY, PHYSICAL AND CHEMICAL PROPERTIES, ANALYTICAL METHODS
2.1 Identity
Molecular structure
Empirical formula C14H9ClF2N2O2
Common name Diflubenzuron
Common trade names Dimilin; Micromite; Vigilante
Common abbreviation DFB
IUPAC name 1-(4-chlorophenyl)-3-(2,6-difluorobenzoyl)-
urea
CAS chemical name N-[[(4-chlorophenyl) amino] carbonyl]-
2,6-difluorobenzamide
CAS registry number 35367-38-5
RTECS registry number YS6200000
Technical diflubenzuron contains > 95% pure compound.
2.2 Physical and chemical properties
Diflubenzuron is an odourless white crystalline solid. It is
almost insoluble in water and poorly soluble in apolar organic
solvents. In polar to very polar solvents, the solubility is moderate
to good, e.g., in acetone it is 6.5 g/litre at 20°C. Diflubenzuron is
highly soluble in N-methylpyrolidone (200 g/litre), dimethyl-
sulfoxide and dimethylformamide (both 120 g/litre).
Some physical and chemical properties of diflubenzuron are given
in Table 3.
Table 3. Physical and chemical properties of diflubenzuron
Relative molecular mass 310.7
Melting point technical > 95% 210-230°C
> 99% pure 230-232°C
Vapour pressure at 25°C 0.00012 mPa
Volatility
solid material < 4%
from water pH 5.6 < 2% (virtually non-volatile)
Specific gravity 1.56
n-Octanol/water partition coefficient
(log Kow) 5000
Solubility in water (at 25°C and pH 5.6) 8 × 10-5 g/litre
Stability in water (0.0001 g/litre 4% decomposition after 3 weeks at pH 5
in the dark) 8% decomposition after 3 weeks at pH 7
26% decomposition after 3 weeks at pH 91
2.3 Conversion factor
1 ppm = 12.7 mg/m3 at 25°C
1 mg/m3 = 0.079 ppm at 25°C
2.4 Analytical methods
Analytical methods for determining diflubenzuron in crops, soil,
water and biological samples are summarized in Table 4.
A review of the analytical methods has been presented by Rabenort
et al. (1978). Two general types of assay procedures for
diflubenzuron are available: high performance liquid chromatography
(HPLC) and gas chromatography (GC).
Table 4. Methods for the determination of diflubenzuron residues
Sample type Extraction/clean-up Analytical Limit of Comments Reference
method detection
Crops, soil, water dichloromethane; clean-up HPLC 0.03 mg/kg Rabenort et al. (1978)
on a Florisil column
Milk ethyl acetate HPLC 0.1 mg/kg Corley et al. (1974)
Crops acetone (n-hexane) HPLC 0.01 mg/kg Nakayama (1977a)
Apples acetonitrile HPLC 0.008 mg/kg Goto (1977a)
Tea acetone/dichloromethane HPLC 0.1 mg/kg Nakayama (1977b)
Tea acetone or water HPLC 0.2 mg/kg Goto (1977b)
Crops, soil, sediment; acetonitrile HPLC 0.05 mg/kg the procedures involve Celite Di Prima et al. (1978)
aquatic and forest liquid-liquid partition, and
foliage; fish and Florisil-aluminasilica gel
shellfish; animal column chromatography;
tissues 20 g sample
Crops acetone-hexane GLC-ECD 0.20 mg/kg Lawrence & Sundaram
(1+4) (1976); Di Prima (1976)
Soybean acetonitrile for process GC-ECD 0.05 mg/kg after hydrolysis and Lawrence & Sundaram
fractions, hulls and meal; derivatization (1976); Di Prima (1976)
hexane-acetonitrile for
oil
Water dichloromethane TLC 0.1 mg/kg Singh & Kaira (1989)
Table 4 (Con't)
Sample type Extraction/clean-up Analytical Limit of Comments Reference
method detection
Water & soil hexane/ethyl acetate; GC/ECD 0.05 ng 100 ml sample of water Smith et al. (1983)
evaporate to dryness; or 10 g sample of soil
dissolve residue in
benzene; derivatize with
trifluoroacetic anhydride
(with trimethylamine as
catalyst); LC on Florisil/
hexane: ethylether
(9:1 v/v)
Water ethyl acetate, KCl; GC/ECD 20 µg/litre % DEGS-LAC 728 on Cooke & Ober (1980)
derivatize with Chromosorb W-AW at 165°C
trifluoroacetic anhydride;
LC on Florisil
Exposure pads methylene chloride or HPLC/UV 3 ng 103.2 cm2 pads Bogus et al. (1985)
other solvents; clean-up (254 nm)
on SEPPAC C18; elute with
methanol
The HPLC method is recommended by CIPAC as a method of choice
(van Rossum et al., 1984). An alternative method for analysis of
residues in crops, soil, mud and water using Celite column
chromatography has been described by Di Prima et al. (1978). A gas
chromatographic method used on the acetylated derivative of
diflubenzuron was described by Worobey & Webster (1977) but has not
been applied to crop samples. The formation of 4-chloroaniline from
diflubenzuron under acidic conditions provides the basis for the GC
method.
Most of the recommended extraction procedures use acetonitrile or
acetone followed by n-hexane or dichloromethane.
Wie & Hammock (1982, 1984) developed three enzyme-linked
immunosorbent assays (ELISA) for diflubenzuron. All three assays were
based on antibodies raised against an N-carboxypropyl hapten of
diflubenzuron, while a diflubenzuron phenylacetic acid derivative
coupled to a carrier other than the immunizing antigen was used as the
coating antigen. None of these assays demonstrated significant cross-
reactivity with benzamide, urea, phenylurea or aniline components of
diflubenzuron. Each of the three assays was shown to be as sensitive
as the recommended HPLC methodology for the analysis of diflubenzuron
in water. Using ELISA, DFB was detected in milk at a level of
1-2 µg/litre without any sample extraction procedure.
Wimmer et al. (1991) developed a gas chromatography/mass
spectrometry (GC/MS) method using deuterated diflubenzuron as internal
standard and claimed high sensitivity.
The Joint FAO/WHO Codex Alimentarius Commission has given
recommendations for the methods of analysis to be used in determining
diflubenzuron residues (FAO/WHO, 1989).
3. SOURCES OF HUMAN AND ENVIRONMENTAL EXPOSURE
3.1 Natural occurrence
Diflubenzuron does not occur naturally in the environment.
3.2 Anthropogenic sources
3.2.1 Production levels and processes
Diflubenzuron was first commercialized by Philips-Duphar BV, The
Netherlands (now Solvay Duphar BV). Solvay Duphar BV produces
diflubenzuron under the trade name Dimilin, but production figures are
not available.
Diflubenzuron is synthesized by the reaction of 2,6-difluoro-
benzamide with p-chlorophenyl isocyanate.
3.2.2 Formulations
Technical diflubenzuron is made into diflubenzuron 90%
concentrate by air-milling with a grinding aid and sufficient kaolin
to attain 90% active material. This is the product from which all
other formulations are made; these are listed below.
Dry products
* Dimilin 25W: a 25% wettable powder (more or less the standard
product)
* Dimilin 5W: a local Italian formulation containing 5% active
ingredient
* Various granular formulations used locally in specific
situations; these products are expected to be removed from the
market within 2 or 3 years
Water-based products
* Dimilin SC-48: a suspension concentrate containing 48% active
ingredient
* Dimilin SC-15: a suspension concentrate containing 15% active
ingredient for the French market
* Dimilin 4L, a suspension concentrate (0.4 kg/litre) containing
48% active ingredient for the USA market
Oil-based products
* Dimilin ODC-45: an oil-based dispersible concentrate containing
45% active ingredient to be diluted with mineral or vegetable oil
for spraying operations; this formulation may not be mixed with
water
* Dimilin OF-6: a dispersion in oil ready for direct spraying,
containing 6% active ingredient; this product must not be diluted
or mixed with water
* Dimilin 2F: an oil-based suspension concentrate containing 24%
active ingredient; it must not be diluted with water for spraying
and is a local formulation development for the USA market
The all-round formulations are Dimilin 25W, Dimilin 5W, Dimilin
SC-48, Dimilin SC-15 and Dimilin 4L. Dimilin ODC-45 was developed
specially for aerial spraying operations on non-food crops and
forestry. Dimilin OF-6 was developed for broadcast aerial spraying
operations to control locusts and grasshoppers. Dimilin 2F was
developed for those purposes where oil must be added to improve spray
deposit tenacity on crops such as cotton.
3.2.3 Uses
Diflubenzuron was the first benzoylphenylurea to be discovered.
Its insecticidal properties were first described by van Daalen et al.
(1972).
Diflubenzuron is effective as a stomach and contact insecticide,
acting by inhibiting chitin synthesis and so interfering with the
formation of the cuticle. Thus, all stages of insects that form new
cuticles should be susceptible to diflubenzuron exposure. It has no
systemic activity and does not penetrate plant tissue. Consequently,
plant sucking insects are generally unaffected, and this forms the
basis of its selectivity.
The recommended application rates for diflubenzuron are given in
Table 5.
Diflubenzuron is effective at a concentration of 15-300 mg
a.i./litre of water against leaf-feeding larvae and leaf miners in
forestry (Lymantria dispar, Thaumethopoea pityocampa), top fruit
( Cydia pomonella, Psylla spp), citrus (Phyllocoptruta oleivora),
field crops including cotton and soybeans (Anthonomus grandis,
Anticarsia gemmatalis), and horticultural crops (Pieris
brassicae). It is also effective against the larvae of Sciaridae
and Phoridae in mushrooms (1 g/m2 casing at case mixing or
as a drench in 2.5 litre of water to the finished casing), against
mosquito larvae (20-45 g/ha water surface) and against fly
larvae (Stomoxys calcitrans, Musca domestica) as a surface
application in animal housings (0.5-1.0 g/m2 surface) (Worthing &
Walker, 1987).
Table 5. Recommended application rates for diflubenzuron on
different cropsa
Pest Crop Rate/concentration
Apple rust mite apples/pears 0.01-0.015% a.i.
Codling moth apples/pears 0.01-0.015% a.i.
Leaf miners apples/pears 0.01-0.015% a.i.
Leaf rollers 0.01-0.02% a.i.
Pear suckers 0.01 (+0.3% crop oil)% a.i.
0.02-0.03% (without oil) a.i.
Winter moth 0.02% a.i.
Plum fruit moth plum 0.02% a.i.
Olive moth plum 0.01-0.02% a.i.
Citrus rust mite citrus fruit 0.0075-0.0125% a.i.
Citrus weevil citrus fruit 0.015-0.03% a.i.
Cotton ball weevil cotton 70 g/ha a.i.
Army worms cotton 150-300 g/ha a.i.
Army worms maize and Sorghum 70-150 g/ha a.i.
Cotton leaf worms 75-150 g/ha a.i.
Beet army worms peanuts 150-300 g/ha a.i.
Rice water weevil rice 75-150 g/ha a.i.
Fall army worms rice 70-100 g/ha a.i.
Mosquitoes up to 100 g/ha a.i.
Rice leaf rollers rice 75-250 g/ha a.i.
Various pests peanuts up to 75 g/ha a.i.
Various pests oil palm 50-150 g/ha a.i.
Various pests soybean 20-150 g/ha
a Solvay Duphar (1994)
4. ENVIRONMENTAL TRANSPORT, DISTRIBUTION, TRANSFORMATION AND FATE
4.1 Appraisal
Diflubenzuron is hydrolysed and photolysed slowly (see section
2.2). Residues in the aquatic environment may decrease rapidly, due
to adsorption by organic and inorganic matter. This process greatly
reduces the availability of diflubenzuron to aquatic organisms.
4.2 Transport and distribution between media
Diflubenzuron is generally applied either directly on plants or
on water for mosquito control.
4.2.1 Soil mobility
Diflubenzuron and its two formulations, Dimilin WP-25 and Dimilin
SC-48, were applied separately at 17.23, 51.69 and 155.07 µg a.i.
(corresponding to 70, 210 and 630 g a.i./ha) to the top layers of
columns (30 × 5.6 cm internal diameter) packed with either sandy or
clay loam forest soils. Water (1.25 litre) equivalent to 50.8 cm of
precipitation (representing an average annual rainfall) was allowed to
leach through each column. After leaching, the columns were divided
into five segments from bottom to top as follows: two 10-cm
increments, one 5-cm increment and two 2.5 cm increments.
Diflubenzuron residues in soils were extracted and analysed by HPLC.
Diflubenzuron mobility was low and did not increase with dosage. At a
deposit rate equivalent to 70 g a.i./ha, nearly all the residues were
found within the top 2.5 cm of the column. Even at 630 g a.i./ha,
only about 9% of the technical diflubenzuron, 7% of Dimilin SC-48 and
4% of Dimilin WP-25 moved below the 2.5 cm level in sandy loam. The
mobility of diflubenzuron in clay loam was lower than in sandy loam.
No residues were found below the 10 cm level or in the leachates in
either soil type at any dosage levels. The mobility of diflubenzuron
was also influenced by the additives present in the formulation, the
mobilities being in the following order: technical diflubenzuron
> Dimilin SC-48 > Dimilin WP-25 (Sundaram & Nott, 1989).
Helling (1985) investigated the movement of 14C-labelled
diflubenzuron in five soils and classified it as immobile in all of
them. After six treatments of cotton fields with 14C-labelled
diflubenzuron, most radioactivity was detected in the top 10 cm layer
of soil (Bull & Ivie, 1978). Diflubenzuron was found to adsorb very
rapidly to eight soil types (greater than 87% of the initial amount),
and there was only limited desorption (Booth et al., 1987).
Fourteen days after a single foliar application of 14C-labelled
diflubenzuron to field-grown cotton, only just over 10% of the dose
was absorbed into the plants. After 21 days and following a heavy
rainfall, approximately 23% of the applied diflubenzuron remained on
the treated leaf surfaces (Bull & Ivie, 1978).
No leaching occurred when 14C-labelled diflubenzuron was applied
to soil at the rate of 134.52 g/ha in an area with a normal rainfall
of 32 cm (Danhaus et al., 1976).
4.2.2 Dissipation
Diflubenzuron might enter an estuary either as a result of
flooding of treated supra-tidal mosquito breeding lagoons during
spring tides or from agricultural run-off after significant rainfall
(Cunningham & Myers, 1986).
Following aerial application at 67.26 g/ha to a watershed,
diflubenzuron was found to reach the stream channel. It was also
washed from the foliage as a result of several subsequent rainfalls
(Jones & Konchenderfer, 1988). However, these discharges were very
short-lived.
No residues were found in sediments from a lake treated with
diflubenzuron, suggesting rapid dissipation before or upon reaching
the bottom sediment (Apperson et al., 1978).
Pritchard & Bourquin (1981) demonstrated some affinity of
diflubenzuron for sediments, i.e. a partition coefficient of 380 in
simulated estuarine conditions. According to Cunningham & Myers
(1986), sediment appeared to be a major site for diflubenzuron
adsorption in a supra-tidal salt marsh. Carringer et al. (1975) found
that the organic content of soil was the most important factor in
determining adsorption and dissipation of diflubenzuron, and that
adsorption was inversely related to the water solubility of
diflubenzuron.
4.2.3 Evaporation
When diflubenzuron was applied as Dimilin WP 80 at a
concentration of 75 g/ha a.i. to bare soil (less than 1.5% organic
matter) and red kidney bean leaves, no significant evaporation was
measured under the following simulated climatological conditions: wind
speed 1-2 m/s; temperature 20-21°C; relative humidity 25-45% (van der
Laan-Straathof & Thus, 1994).
4.2.4 Crop residue data
When soybean and maize (corn) seedlings and potato tubers were
planted into soil treated with 3H- or 14C-labelled diflubenzuron,
only small amounts of radioactivity were taken up (Nimmo & de Wilde,
1976a). When 3H- or 14C-labelled diflubenzuron was applied to soil
in which the seedlings of wheat and rice were already present, the
14C residues in rice and wheat leaves were between 0.1 and 0.5 µg/kg.
The residues consisted mainly of 4-chlorophenylurea and polar
conjugates. The 14C residues in the wheat seeds were 0.02-0.04 mg/kg
and 3H residues were lower (Nimmo & de Wilde, 1976b).
The fate of diflubenzuron was studied following application to
soybeans both in greenhouse and field conditions. It was found that
75 to 100% of the total residues in soybean plants consisted of
unaltered diflubenzuron. There was no significant absorption or
translocation of residues. Less than 0.05 mg/kg of the total residues
was found in harvested soybean seed (Gustafson & Wargo, 1976).
The diflubenzuron spray residue on aerial parts of plants is
essentially stable. Leaf permeation does not occur and the compound
is not translocated to other parts of the plant. It has been
demonstrated that there is virtually no absorption, translocation or
metabolism of foliar-applied diflubenzuron on greenhouse cotton plants
(Nimmo & de Wilde, 1974; Nimmo, 1976a,b; Mansager et al., 1979).
Plant metabolism studies in corn, soybean, cabbage and apples
have demonstrated that no degradation products are found in plant
tissues. The only residue component present was the parent compound
diflubenzuron. Similar results were reported for cotton. Studies on
citrus fruits, apples and soybeans have confirmed that the only
residue component is the parent compound diflubenzuron. It can be
concluded that plants do not metabolize diflubenzuron (Nimmo & de
Wilde, 1974; Nimmo et al., 1978; Bull & Ivie, 1978; Nigg, 1989;
Joustra et al., 1989; Serra & Joustra, 1990; van Kampen & Joustra,
1991; Thus & van der Laan, 1993).
4.3 Transformation
4.3.1 Abiotic degradation
Under environmental conditions abiotic degradation of
diflubenzuron represents a very minor route of breakdown, owing to
the stability of the substance.
4.3.1.1 Photolysis
On the basis of results from a 15-day photolysis experiment, a
photolytic half-life of 40 days was calculated for diflubenzuron by
regression analysis (Boelhouwers et al., 1988a,b). After one week of
storage at 50°C or after one day at 100°C, there was no significant
decomposition (< 2%). The solid is stable to sunlight.
4.3.1.2 Hydrolysis
Abiotic hydrolysis of diflubenzuron in solution does not occur at
normal pH values. At pH 9 the hydrolytic half-life is 32.5 days,
4-chlorophenyl urea (4-CPU) and 2,6-difluorobenzoic acid (2,6-DFBA)
being the degradation products (Boelhouwers et al., 1988a).
High temperature (121°C) increases the degradation of
diflubenzuron in aqueous media at levels greatly above its solubility
in water and result in its rapid degradation to as many as seven
identified products: 4-CPU, 2,6-DFBA, 2,6-difluorobenzamide,
4-chloroaniline, N,N'-bis (4-chlorophenyl) urea, 1-(4-chlorophenyl)-
5-fluoro-2,4 (1H,3H)-quinazolinedione and 2-[(4-chlorophenyl) amino]-
6-fluorobenzoic acid. 4-Chloroaniline, N,N'-bis (4-chlorophenyl)
urea and 2[(4-chlorophenyl) amino]-6-fluorobenzoic acid were not
detected at lower temperatures (0.1 mg [14C]-diflubenzuron/litre
water or buffer at 36°C). 4-Chloroaniline was a major degradation
product of diflubenzuron in heat-treated samples, but it was not seen
at lower temperatures (Ivie et al., 1980).
The heat-induced degradation of diflubenzuron increased with
increasing pH (Schaefer & Dupras, 1976). Nigg et al. (1986) found
that high temperature and humidity significantly decreased the half-
life of diflubenzuron residues on citrus fruit.
4.3.2 Biodegradation
4.3.2.1 Water
a) Laboratory studies
Degradation in water can also occur through microbial action,
since in sterile water no breakdown or hydrolysis occurs (Boelhouwers
et al., 1988a). In freshly sampled ditch water, Nimmo & De Wilde
(1975a) demonstrated 50% degradation in 1-4 weeks. The breakdown
products were the same as the primary soil metabolites (4-CPU and
2,6-DFBA). Ivie et al. (1980) reported the same metabolites. Anton
et al. (1993) calculated the half-life of diflubenzuron in aerated and
unaerated tap water to be less than half a day and less than one day,
respectively.
When diflubenzuron (1.3 mg/litre) was added to an anaerobic silt
loam/water system, disappearance from the water phase showed a half-
life of 18 days and from the total system a half-life of 34 days.
The metabolites were 4-CPU and 2,6-DFBA, and almost no bound residue
was formed (Thus et al., 1991). After 90 days less than 2% of added
diflubenzuron remained in the system (Thus & van Dyk, 1991).
In another study, van der Laan-Straathof & Thus (1993) calculated
the half-life of diflubenzuron in water to be 2.5 days. Of the two
degradation products, 4-CPU underwent no further degradation but
2,6-DFBA was mineralized.
b) Outdoor models
Schaefer et al. (1980) reported that, in pasture water with a pH
of 8.2 and afternoon temperatures as high as 38-40°C, there was a
decline from an initial nominal concentration of 30 µg/litre to a
one-hour measured concentration of 20.3 µg/litre and subsequently to
21.6, 13.6, 4.4, and 2.4 µg/litre on days 1, 2, 3, and 4 respectively.
Schaefer & Dupras (1976) applied two formulations of
diflubenzuron (a wettable powder and a flowable formulation) to
artificial ponds of 1 m2 surface area containing 318 litres of pond
water. An initial concentration of 80 µg/litre decreased to 50%
within about 2 days. The diflubenzuron residue level after one week
was 2-3 µg/litre.
The half-life of diflubenzuron (1 µg/litre) in the aqueous
fraction of sludge experiments was 4-15 h (Booth et al., 1987), and
the half-life in sea water was reported to be less than 4 days
(Schimmel et al., 1983). Cunningham & Myers (1986) estimated a half-
life of less than 1 day for residues of diflubenzuron in water
following three applications of 0.4% granules and three applications
of 25% WP at a rate of 45 g a.i./ha to a supra-tidal salt marsh.
Madder & Lockhart (1980) studied model ponds (20 m2) to which
Dimilin WP-25 was applied at 56 g/ha (equivalent to 11.2 µg/litre).
For an unexplained reason, the measured concentration reached a
maximum value of about 17.5 µg/litre, 4 days after treatment. It
decreased by around 50% during the next 5 days. A residue of
2 µg/litre remained 2 weeks after application. On the basis of a
bioassay, a diflubenzuron half-life of about 3 days was calculated.
Collwell & Schaefer (1980) applied diflubenzuron to five
experimental ponds (each 100 m2) at a mean concentration of
13 µg/litre. The residue levels in water declined to an average of
7.2 µg/litre after 24 h.
In a study by Sarkar (1982), a 3 × 1 × 0.3 m open tank containing
water was sprayed with a dispersion of Dimilin WP-25. Three
subsequent applications were made, giving diflubenzuron concentrations
of 25, 35 and 50 µg/litre, respectively. These concentrations
decreased to 50% in about 3-4 days.
Pritchard & Bourquin (1981) studied the environmental fate of
diflubenzuron under simulated estuarine conditions in a laboratory
continuous-flow estuarine system and a static test system. The
hydrolytic half-life of diflubenzuron was 17 days in the static test
system, whereas the biological half-life was 5 days. 4-Chloroaniline
was not detected in either of the systems.
Thus & van der Laan-Straathof (1994) studied the fate of
diflubenzuron in two model ditch systems. Diflubenzuron was added at
a concentration of 0.94 mg/kg to two sediments (sandy loam and silt
loam), both of which were covered with aerated surface water. It
disappeared rapidly from the water phase through degradation and
adsorption to the sediment, the half-lives being 1.9 and 1.1 days,
respectively. Dissipation of diflubenzuron from the complete sandy
loam and silt loam systems occurred with half-lives of 25 and 10 days,
respectively. The metabolites (> 1% of the added diflubenzuron)
consisted of CO2, 4-CPU and 2,6-DFBA.
c) Field studies
Apperson et al. (1978) described the treatment of three farm
ponds with diflubenzuron levels of 2.5, 5 and 10 µg/litre, and a lake
with 5 µg/litre. Shortly after the application, a rapid decline in
diflubenzuron residues occurred, resulting in half-life values of only
a few days. In the lake no residues were found in the sediment
samples, suggesting that diflubenzuron was rapidly dissipated before,
or upon reaching, the bottom sediment.
Hester (1982) applied diflubenzuron at 0.045 kg a.i./ha to
specially constructed estuarine ponds. The water residue levels
decreased rapidly from 7.5 to 2 µg/litre in 2-3 days (study II) and
from 3.3 to 0.6 µg/litre in 7 days (study I).
d) Discussion and appraisal
The rate of decrease in diflubenzuron concentration after
application of the formulated product to natural waters depends on the
combined action of many environmental factors. Factors affecting the
degradation rate of diflubenzuron include the acidity (pH), the
relative local abundance of soil and organic debris, and the water
depth.
Half-life values vary from less than 4 days to 4 weeks in
laboratory experiments.
The use of artificial ponds or basins, preferably outdoors,
yields more relevant data and fairly consistent results. Dissipation
half-life values vary from 1-5 days after diflubenzuron has been
applied at recommended rates.
The dissipation half-life of diflubenzuron in the aquatic
environment is between one day and one week in most cases, depending
on the properties of the applied formulation and on the
characteristics of the application site. The presence of organic
sediments (hydrosoil, plant debris) and a relatively high local
temperature are factors that particularly accelerate the disappearance
of diflubenzuron.
4.3.2.2 Soil
a) Mobility in soil
Diflubenzuron is immobile in soil, as demonstrated by Helling
(1985) in column leaching experiments and Booth et al. (1987) in
adsorption-desorption studies with eight soil types.
The work of Carringer et al. (1975) suggests that soil organic
matter is an important parameter in soil adsorption. Due to its
immobility in soil, diflubenzuron is not likely to contaminate
groundwater by vertical movement in soil or to contaminate open water
by lateral movement in groundwater.
This has been confirmed in studies carried out in field soils
with growth of citrus fruits (Verhey, 1991a; Kramer, 1991), apple
(Kramer, 1990, Verhey, 1991b), soybean (Kramer, 1992b) and cotton
(Kramer, 1992a). After three applications of diflubenzuron (Dimilin
25W) at normal rates, most residue was found in the top 15 cm of soil
and no residue was encountered below 30 cm.
b) Degradation in soil
The rates of disappearance of technical diflubenzuron applied at
10 mg/kg on quartz sand to natural sandy loam and muck soils were
significantly greater than for the corresponding sterilized soils
(e.g., 2-12% and 80-87% diflubenzuron, respectively, remaining at
12 weeks), demonstrating that soil microorganisms play a major role in
their degradation (Chapman et al., 1985).
Diflubenzuron is very rapidly hydrolysed in soil. The half-life
time is 2 days to one week. The primary metabolites are 2,6-DFBA and
4-CPU. The process is microbial, since in sterilized soil no breakdown
occurs. The rate of breakdown is strongly dependent on the particle
size of diflubenzuron (see Fig. 1) (Nimmo et al., 1984, 1986).
The half-life in water in alkaline pastures is 1 day and in
neutral lake water it is from 10 to 15 days (Nimmo & de Wilde, 1975a).
Metabolic routes other than 4-CPU and 2,6-DFBA are virtually
irrelevant. Both primary metabolites are further metabolized,
2,6-DFBA with a half-life of about 4 weeks and 4-CPU with a
dissipation time of 1 to 3 months. Radiolabelling of both primary
metabolites and of a carbon atom in the ureido bridge shows carbon
dioxide development from mineralization. However, both the benzoic
acid ring carbon and the ureido bridge carbon are mineralized
much faster than the aniline moiety carbon, suggesting that
para-chloroaniline (PCA) is a major secondary metabolite that is
virtually irreversibly bound to soil (Bollag et al., 1978; Mansager et
al., 1979; Nimmo et al., 1984, 1986, 1990).
Even as a bound residue PCA is metabolized. Apparently, the
breakdown of 4-CPU in soil is a complex process in which PCA is a
transient metabolite or intermediate. The breakdown process leads to
products beyond the aniline structure. If PCA is applied to soil,
6 weeks incubation at 25°C yields 60% breakdown products of a
different nature (Bollag et al., 1978). The aniline itself is firmly
bound to soil and immobilized (Hsu & Bartha, 1974; Moreale & van
Bladel, 1976; Bollag et al., 1978; Simmons et al., 1989).
Fig. 2 shows metabolic pathways in soil.
The main metabolic pathway (over 90%) is hydrolysis, leading to
2,6-DFBA and 4-CPU. The second site of cleavage occurs at CœN bonds 2
and 3. Both reactions lead to the formation of 2,6-difluorobenzamide
(DFBAM), which readily hydrolyses to 2,6-DFBA (Verloop & Ferrell,
1977; Nimmo et al., 1984).
The major metabolite in an activated sludge system is 4-CPU.
This is the major metabolite reported in most soil metabolism
experiments (Booth et al., 1987). 4-CPU was found to be converted
into bound residues with a half-life of 5-10 weeks. In the bound
residues, 4-CPU and PCA were present in roughly equal amounts after
2 months (Verloop & Ferrell, 1977). Free PCA was not found in soil
(Nimmo et al., 1986). The soil type and characteristics appear to
have no influence on the rate of degradation (Nimmo et al., 1984).
Metcalf et al. (1975) found no significant degradation of
diflubenzuron in a silty clay loam after incubation at 26.7°C for
periods of 1, 2 and 4 weeks. However, the authors did not take into
account the particle size of the soil, and used techniques that have a
negative influence on breakdown.
The rate of degradation of 14C- or 3H-diflubenzuron applied to
a mushroom growth medium (dose 2 g/m2) was between 30-50% in one
month. The main degradation products, 4-CPU and 2,6-DFBA, were
absorbed from the growth medium by the mushrooms, resulting in
residue levels of 0.1-0.6 mg/kg and 1-3 mg/kg, respectively (Nimmo &
de Wilde, 1977a). Free PCA or its further possible degradation
products were not present in the extractable residues (Nimmo & de
Wilde, 1975a; Verloop & Ferrell, 1977). Organic matter in soil
significantly contributed to the adsorption of chloroaniline compounds
and their immobilization (Hsu & Bartha, 1974; Moreale & van Bladel,
1976; Bollag et al., 1978).
Nimmo & de Wilde (1975a) found a degradation half-life of
0.5-1 week at a diflubenzuron concentration of 1 mg/kg (corresponding
to an application dose of approximately 300 g/ha). 2,6-DFBA was
degraded with a half-life of approximately 4 weeks, and 4-CPU with a
half-life of 2-3 months.
Walstra & Joustra (1990) applied 0.69 mg diflubenzuron/kg to
sandy loam. When incubated in the dark at 24°C, they obtained a half-
life for diflubenzuron of 50 h.
Diflubenzuron was found to be rapidly degraded by four soil fungi
( Fusarium sp., Cephalosporium sp., Penicillium sp. and Rhodotorula
sp.), the half-lives being 7, 13, 14 and 18 days, respectively
(Seuferer et al., 1979).
Several degradation studies on diflubenzuron (Dimilin 25 W) in
field soils have been conducted (Kramer, 1990, 1991, 1992a,b; Verhey,
1991a,b). Most of the degradation half-lives were between one and two
weeks, except in the case of the two Verhey studies, which yielded
half-lives of more than two months. In all studies, the metabolites
were 4-CPU and 2,6-DFBA.
No degradation of diflubenzuron by the soil microorganism
Pseudomonas putida was observed (Booth & Ferrell, 1977).
4.4 Bioaccumulation and biomagnification
Metcalf et al. (1975) studied the fate of 14C-diflubenzuron in a
laboratory model ecosystem. Diflubenzuron was clearly persistent in
some organisms, such as algae (Oedogonium cardiacum), snails
( Physa sp.), caterpillars (Estigmene acrea) and mosquito larvae
(Culex pipiens quinquefasciatus). The fish Gambusia affinis was
able to degrade diflubenzuron more efficiently. Diflubenzuron did not
biomagnify in the fish through food chain transfer. The biomagnifi-
cation was about 40-fold greater in mosquito larvae than in Gambusia
affinis.
When the bluegill sunfish (Lepomis macrochirus) was exposed to
10 µg diflubenzuron/litre for 24 h the tissues contained an average of
264 µg/kg. After 24 to 48 h of exposure, fish degraded and eliminated
the diflubenzuron. The excretory products were neither the parent
compound nor 4-CPU. The amount of diflubenzuron remaining in fish
tissues at various times was dependant on the reduction of residue
concentration in water. However, the potential for degradation and
elimination was very great (Schaefer et al., 1979).
A dynamic 42-day study was conducted by Burgess (1989) in order
to evaluate the bioconcentration of 14C-diflubenzuron by bluegill
sunfish (Lepomis macrochirus). A flow-through proportional diluter
system was used for a 28-day exposure period. Radioanalysis of
fillet, whole fish and visceral portions was performed throughout the
exposure period. Daily bioconcentration factors ranged from 34 to 200,
78 to 360, and 100 to 550 for fillet, whole fish and viscera,
respectively. Uptake tissue concentrations of 14C-diflubenzuron
ranged from 0.25 to 1.7 mg/kg for fillet, 0.58 to 3.3 mg/kg for whole
fish, and 0.75 to 4.7 mg/kg for viscera. To measure the elimination
of 14C-diflubenzuron, the test fish were placed in clean water for 14
days. Radioanalysis throughout the depuration period indicated 99%
depuration for each of fillet, whole fish and viscera. The fillet
concentration of 14C-diflubenzuron decreased from 1.6 mg/kg on day 28
of exposure to 0.012 mg/kg by day 14 of the depuration period. Whole
fish levels decreased from 3.3 mg/kg on day 28 of exposure to
0.038 mg/kg by the end of the study; whereas, viscera concentrations
dropped from 4.4 mg/kg on day 28 of exposure to 0.056 mg/kg by day 14
of depuration. BIOFAC modelling estimated the uptake rate constant
(K1) to be 370 (± 57) mg/kg fish per mg/litre water per day, the
depuration rate constant (K2) 1.2 (± 0.18) day-1, the time for 50%
depuration 0.60 (± 0.09) days, the bioconcentration factor (BCF) 320
(± 70), and the time to reach 90% or steady state 2.0 (± 0.31) days.
The BIOFAC-calculated BCF value was the same as the observed mean
whole fish BCF of 320 for days 3, 7, 14, 21 and 28. Fig. 3 shows the
accumulation, plateauing and depuration in this study.
During the study, no mortality or abnormal behaviour was observed
in the test fish. This appeared to indicate that the test fish were
in good health and would provide acceptable data for defining the
uptake/depuration potential of 14C-diflubenzuron. Analysis of fish
revealed parent compound (80%), 2,6-difluorobenzamide (10-13%) and
three other minor metabolites (one of which probably was 4-CPU). PCA
was demonstrated to be absent (sensitivity limit 0.01 mg/kg).
White crappies (Pomoxis annularis) contained residues from
355.1 to 62.2 µg/kg at 4 and 21 days, respectively, following
treatment of a lake with 5 µg diflubenzuron/litre (Apperson et al.,
1978).
Channel catfish (Ictalurus punctatus) did not bioaccumulate
diflubenzuron residues (less than 0.05 mg/kg) from treated soil in a
simulated lake ecosystem (Booth & Ferrell, 1977).
Assuming a biomagnification of 50-160, and that fish are capable
of rapidly depleting residues from the body, the likelihood of fish
accumulating significant residues of diflubenzuron is low (Apperson et
al., 1978; Schaefer et al., 1980).
4.5 Interaction with other physical, chemical or biological factors
Schaefer & Dupras (1976) reported that application of the
technical grade compound in an ethanol carrier or as a flowable liquid
formulation resulted in higher concentrations in the upper water
levels of mosquito ponds for a period of 3 days following spray
treatment than in the case of spray treatment with wettable powder
formulation (the actual formulation used for mosquito control
spraying).
Seuferer et al. (1979) reported that the soil microorganisms
Rhodotorula sp., Penicillium sp. and Cephalosporium sp. cannot
utilize diflubenzuron as a sole carbon and energy source. However,
accelerated breakdown of diflubenzuron occurred in the presence of
these organisms.
4.6 Ultimate fate following use
It appears that after direct spraying diflubenzuron is persistent
on foliage, it remains almost completely at the site of application on
the surface, and it does not penetrate the plants.
Diflubenzuron is readily degraded in soils of various types and
origin under aerobic or anaerobic conditions with a half-life in the
range of 0.5 to 1 week. It is metabolized by microorganisms
principally to 4-CPU and 2,6-DFBA. The latter is unstable with a
half-life of 3-5 days (Nimmo et al., 1984) to 4 weeks (Verloop &
Ferrell, 1977). The half-life of 4-CPU is about 6 weeks (Nimmo et
al., 1984). Free PCA has not been detected in soil.
In spite of rapid degradation in soil, small amounts of residue
(up to 1 mg/kg, depending on ageing time and growth stage of plants)
may be taken up by crops in treated soil (Thus et al., 1994).
Field applications of diflubenzuron produce soil residues which
might possibly lead to residues in rotational crops by re-uptake from
soil.
Studies with direct applications to field water show a moderate
persistence of diflubenzuron in water. Half-life values average one
week or less. This rapid rate of loss may be more dependent on
adsorption to organic matter than on microbial degradation.
5. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE
5.1 Environmental levels
5.1.1 Air
No information is available on air concentrations of
diflubenzuron.
5.1.2 Water
A total of 1160 ha of insect-infested forest in Finland was
sprayed with diflubenzuron (25% WP) from a fixed-winged aircraft at an
application rate of 75 g a.i. in 50 litres water per ha. The residues
in "run-off" water (gathered in specially dug pits adjacent to the
sprayed area) decreased from 5 µg/litre one day after spraying to
0.1 µg/litre after 2 months. The concentration in water in open pits
was 0.1 µg/litre 1 and 7 days after application and 0.2 µg/litre
1 month after application. After 2 months no residues were detected.
All water samples taken from outside the treated area contained less
than 0.1 µg/litre (the limit of sensitivity) (Mutanen et al., 1988).
Diflubenzuron was found in the water of the Fraser River, Canada,
up to 71 days following application with diflubenzuron (1% granular
formulation) at a rate of 4.5 kg/ha (45 g a.i./ha). The peak value
was 1.8 µg/litre 8 days after treatment (Wan & Wilson, 1977).
After aerial application of diflubenzuron (25% WP formulation) to
two forest ponds in Canada, the maximum residue levels in water,
sediment, aquatic plants and fish were 13.82 µg/litre (at 1 h),
0.24 mg/kg (at 1 day), 0.36 mg/kg (at 1 day) and 0.11 mg/kg
(at 1 day), respectively. The rate of dissipation was rapid, non-
detectable levels being reached in 20 days for water, 5 days in
aquatic plants and 3 days in fish (Kingsbury et al., 1987).
A pond in Salt Lake County, Utah, USA, was treated with three
applications of diflubenzuron at a rate of 280.25 g a.i./ha.
Diflubenzuron was found at less than 0.05 mg/litre 4 days following
treatment (Booth et at., 1987).
Residues in three farm ponds in California treated with
diflubenzuron (2.5, 5 and 10 µg/litre) averaged 1.9, 4.6 and
9.8 µg/litre, respectively, 1-4 h after the applications. They
declined steadily averaging 0.5, 0.3 and 0.2 µg/litre, respectively,
2 weeks later. Residues in a small lake treated at 5 µg/litre
averaged 3.3 µg/litre following treatment and 0.4 µg/litre after
35 days. No residues were found in sediment samples taken post-
treatment (Apperson et al., 1978).
One hour after a single application of 45 g diflubenzuron/ha to
brackish water pools the residues in water and in sediment were
3.6 µg/litre and 80 µg/kg, respectively. The concentration in
sediment increased to 520 µg/kg after 1 day and reached its maximum of
780 µg/kg 4 days following application (Hester et al., 1986). After
6 applications of diflubenzuron at a rate of 145.73 g/ha to Utah Lake,
USA, the residues in sediments were less than 0.05 mg/kg (Booth et
al., 1987).
Other field studies with similar results have been reported by
Anon (1980), Smith & Edmunds (1985), Van Den Berg (1986), Huber &
Collins (1987), Jones & Kochenderfer (1988), Huber & Manchester
(1988), Downey (1990) and Sundaram et al. (1991). It is clear that a
variety of application scenarios will result in measurable residues of
diflubenzuron in water (Table 6).
The overall conclusion is that diflubenzuron residues in stagnant
water dissipate rapidly within days. In flowing water, e.g., in
wooded areas, diflubenzuron residues may peak shortly after rainfall
but such peak concentrations are very transient in nature.
5.1.3 Food and feed
Data on residues in food resulting from treatment with
diflubenzuron have been summarized by FAO/WHO (1982a,b, 1985a,b,
1986a,b).
Residue data obtained from various countries showed residues in
apples below 1.0 mg diflubenzuron/kg at 2 weeks after the last
application at recommended rates. Residues in whole citrus fruit were
below 0.5 mg/kg 1 week after the last application at the recommended
rate. Residues in soybean seed and cottonseed were generally below
the limit of determination (0.05 mg/kg).
Mushrooms have a residue pattern different from other plant
material. In mushrooms growing on diflubenzuron-treated soil, high
levels of the metabolite 2,6-DFBA are taken up from the soil.
Diflubenzuron was found at a level of 0.1 mg/kg, while the 2,6-DFBA
level was around 1 mg/kg (see chapter 4).
Residues in wild mushrooms after aerial application to forests in
Finland were on average 0.07 mg/kg 1 week after spraying with 75 g
diflubenzuron in 50 litre water per ha. In bilberries the residues
decreased on average from 0.2 mg/kg 1 day after spraying to 0.09 mg/kg
after 1 month (Mutanen et al., 1988).
Diflubenzuron applied as a wettable powder spray to growing
alfalfa at 20-100 g/ha showed initial residue levels of 1.8-8.5 mg/kg.
Residues of 0.3-1.5 mg/kg remained 22 days after applications (Lauren
et al., 1984).
Table 6. Summary and comparison of experimental parameters among key studies designed to measure environmental concentrations of
diflubenzuron in water
Medium Formulation a.i.% Method of Application Maximum Time for Minimum Time for References
treated application rate a.i. concentration maximum concentrationa minimum
concentration concentration
Farm ponds 25 WP 25 hand sprayer 2.5-10 µg/litre 1.9-9.8 µg/litre 1-4 h 0.5-0.2 µg/litre 14 days Apperson
(0.06-0.2 ha) from boat et al.
(1978)
Small lake 25 WP 25 hand sprayer 5 µg/litre 3.3 µg/litre 4 h 0.4 µg/litre 35 days Apperson
(18.6 ha) from boat et al.
(1978)
Pond W-25 25 hand-operated 0.28 kg/ha 56 µg/litre 96 h < 0.01 µg/litre 40 days Booth
spray et al.
applicator (1987)
Brackish 25 WP 25 clothes 0.045 kg/ha 7.5 µg/litre 48-72 h < 0.3 µg/litre 25-30 days Hester
pools sprinkler (1986)
Forest ponds 25 WP 25 aircraft 0.07 kg/ha 13.82 µg/litre 1 h < DL 20 days Kingsbury
(25 ha) (four et al.
atomizers) (1987)
Field plot 25 WP 25 fixed-wing 0.075 kg in 50 5.0 µg/litre 24 h < DL 60 days Mutanen
(1160 ha) aircraft litre water/ha et al.
(1988)
Fixed plots granular 1.0 aircraft 0.023 kg/ha, 1.8 µg/litre 192 h < DL 60 days Wan &
(3-40 ha) 0.46 kg/ha Wilson
(1977)
a DL = determination limit
After two soil applications of 67.26 g/ha, the residues of
diflubenzuron in the rotational crops (wheat, cabbage and onions) were
less than 0.01 mg/kg (Danhaus & Sieck, 1976).
Mian & Mulla (1983) studied the persistence of diflubenzuron in
stored wheat after applications of 1, 5 and 10 mg/kg. The residue
levels were 0.59, 2.75 and 5.00 mg/kg, respectively, 23 months after
treatment.
5.1.4 Forest plants and litter
The level of diflubenzuron residues in pine needles was on
average 3.0 mg/kg 1 day after application to the forest in Finland at
a rate of 75 g diflubenzuron in 50 litres water per ha. The level had
decreased to 0.2-0.3 mg/kg or was not detectable 2 months later
(Mutanen et al., 1988).
Booth et al. (1987) found diflubenzuron residues of less than
0.05 mg/kg in the forest litter 1, 4, 10 and 21 days after treatment
with 0.28 kg a.i./ha.
Sundaram (1986) studied the residues in a forest in Canada after
simulated aerial spraying of diflubenzuron in acetone and in fuel oil:
Arotex 3470 mixture, each at 90 g a.i. in 18 litre/ha. The residue
levels 1 h after application varied, respectively, from 23.8 to
30.6 µg/g in foliage and from 3.08 to 4.60 µg/g in litter. Forty-five
days after spraying the residue levels in foliage were 0.80 and
3.9 µg/g, respectively, for the above-mentioned formulations.
Spray deposit patterns and persistence of diflubenzuron in white
pine ( Pinus strobus L.) and sugar maple ( Acer saccharum Marsh.)
canopies, forest litter and soil were studied after aerial application
of a 250 g/kg wettable powder formulation (Dimilin WP-25) at
70 g a.i./ha, using three volume rates (2.5, 5 and 10 litres/ha), over
three blocks in a mixed forest near Kaladar, Ontario, Canada, during
1986 (Sundaram, 1991). In the spray block that received 10 litres/ha,
diflubenzuron persisted in foliage as long as 120 days after
treatment, but it lasted for only about a week in forest litter and
soil samples. At 2.5 and 5 litres/ha, diflubenzuron failed to persist
in foliage as long, and residues in litter and soil, which were barely
above the quantification limit, persisted only for a few days.
5.1.5 Aquatic organisms
Residues in fish are given in section 4.4.
5.2 General population exposure
Exposure of the general population to diflubenzuron via food and
drinking-water may occur.
Twelve volunteers with whole body dosimeters were exposed for 4 h
to Dimilin 25 W after simulated indoor treatment of carpets at
0.16 g/m2. Average deposition was 5.3 ± 2.3 µg diflubenzuron/cm2
carpet. Total dermal exposure ranged from 0.053 to 0.25 mg/kg body
weight per day to (average 0.15 ± 0.066 mg/kg body weight per day).
Assuming a dermal absorption of 0.2%, the total exposure via the
dermal route was calculated to be 0.0003 mg/kg body weight per day.
Air concentrations ranged from 10.2 to 32.4 µg/m3 during the first
4 h and were < 1 µg/m3 at 12-16 h. The total respiratory exposure
was calculated to be 0.0011 mg/kg body weight per day. The total
exposure, via the dermal and respiratory route, was calculated to be
0.0014 mg/kg body weight (Honeycutt, 1993).
5.3 Occupational exposure during manufacture, formulation or use
In a US Department of Agriculture report, human exposure via a
variety of exposure scenarios was estimated using standardized methods
and assumptions. The exposure scenarios included mixing and loading by
workers, via aircraft or truck spillage, and general public exposure
via the diet or resulting from occupational aerial spraying. Dermal
absorption of diflubenzuron was assumed to be 10%. Estimated
realistic doses for humans were < 0.003 mg/kg body weight per day
except where aircraft or truck spillages occurred, in which case
exposures were significantly higher. Estimated worst-case doses for
humans were < 0.01 mg/kg body weight per day, except where aircraft
or truck spillages occurred (USDA, 1985).
6. KINETICS AND METABOLISM IN LABORATORY ANIMALS AND HUMANS
6.1 Absorption
Diflubenzuron is absorbed from the digestive tract but only
poorly absorbed through the skin. Willems et al. (1980) found that in
rats the relative intestinal absorption diminished greatly with
increasing dose. Following a dose of 4 mg/kg body weight 42.5% was
absorbed, but only 3.7% of a 900 mg/kg body weight dose was absorbed.
Dermal absorption of 14C-diflubenzuron was only 0.2% when it was
applied to the shaved skin of rabbits as an aqueous micro-suspension
of 150 mg/kg (De Lange, 1979).
When applied dermally to cattle 14C-diflubenzuron was not
absorbed or degraded through the skin to any detectable degree (Ivie,
1978).
6.2 Distribution
Body tissues show little tendency to retain diflubenzuron.
Analysis of tissues for radiocarbon residues, 4 days (for sheep) or
7 days (for cows) after a single oral dose of 10 mg/kg body weight,
indicated that only the liver contained appreciable levels of
radioactivity, ranging from 2 to 4 mg/kg diflubenzuron equivalents
(Ivie, 1977).
More than one third of an oral diflubenzuron dose appeared in the
bile of a cannulated sheep (Ivie, 1977).
The highest 14C-diflubenzuron residue present in pig tissues
after a single oral dose of 5 mg/kg body weight was 0.43 mg/kg in the
gall bladder. All other tissue residue levels were found to be less
than 0.30 mg/kg (Opdycke et al., 1982a).
Twenty-two dairy cows were fed 14C-diflubenzuron (labelled in
both phenyl moieties) in a diet at dose levels of 0.05, 0.5, 5, 25 and
250 mg/kg feed for 28 days. Residues in blood, fat and muscle were
below the detection limit (0.0067-0.04 mg/kg) at all dose levels. They
were only detected following a dose of 250 mg/kg in the liver and
kidney where residues were 6.040 and 1.038 mg/kg, respectively.
Residues in milk were found at dose levels of 5 and 250 mg/kg, where
the highest levels of diflubenzuron were 0.009 and 0.20 mg/kg,
respectively (Smith & Merricks, 1976a).
In a study by Miller et al. (1976a), two dairy cows were fed
diflubenzuron at 0.25 or 1 mg/kg body weight per day for 4 months. A
third cow received an increased dosage of 8 to 16 mg/kg body weight
per day, the highest value being maintained for three months. In the
fat, liver and milk of the third cow, residues were 0.2, 0.13 and
0.02 mg/kg, respectively.
When dairy bull calves (four treated and four controls) received
diflubenzuron at 1.0 to 2.8 mg/kg body weight, residues were detected
only in the tissue samples of one bull (0.02 mg/kg in liver and
kidney, 0.04 mg/kg in the subcutaneous fat, and 0.08 mg/kg in the
renal and omental fat (Miller et al., 1979).
The maximum total residue in eggs 3 days after a single dose of
5 mg/kg 14C-diflubenzuron to hens was 0.248 mg/kg (Opdycke, 1976).
When laying hens were administered 14C-diflubenzuron at dose
levels 0.05, 0.5, and 5.0 mg/kg feed for 28 days, dose-related
residues ranging from 0.007 mg/kg at the lowest to 1.2 mg/kg at the
highest dose level were found in kidney, liver and fat. After 7 days
of withdrawal, residues in all tissues and eggs were below the
detection limit (0.0006-0.032 mg/kg) for all dose levels (Smith &
Merricks, 1976b).
When diflubenzuron was fed to white leghorn and black sex-linked
cross hens at a level of 10 mg/kg feed for 15 weeks, detectable
residues were found in eggs, liver and visceral fat. Residues were
significantly higher in eggs from white leghorn hens than in eggs from
black sex-linked cross hens, the average levels being 0.55 and
0.38 mg/kg, respectively (Miller et al., 1976b).
6.3 Metabolic transformation
The metabolic fate of diflubenzuron has been studied in various
species. Metabolic pathways of diflubenzuron are shown in Fig. 4
In rats and cows the major metabolic pathway involves
hydroxylation of the phenyl moieties of the compound. About 80% of the
metabolites in rat urine were identified as 2,6-difluoro-3-
hydroxydiflubenzuron and 4-chloro-2-hydroxy- and 4-chloro-3-
hydroxydiflubenzuron. About 20% underwent scission of the benzoyl
ureido bridge. The major part was excreted as 2,6-DFBA and
constituted more than half of the urinary metabolites. 4-CPU was not
detected in bile or urine in a significant quantity (De Lange et al.,
1975; Willems et al., 1980).
The major metabolite in cow urine was 2,6-difluoro-3-hydroxy-
diflubenzuron (45%). Relatively small quantities of 4-chloro-2-
hydroxy- (1.6%) and 4-chloro-3-hydroxydiflubenzuron (3.7%) and the
scission products 4-CPU (0.6%), 2,6-DFBA (6.0%) and 2,6-difluoro-
hippuric acid (6.9%) were present (Ivie, 1978).
The major metabolites (approximately 50%) in sheep urine were
2,6-DFBA and 2,6-difluorohippuric acid (Ivie, 1978).
14C-Diflubenzuron uniformly radiolabelled in both rings was
administered to a pig as an oral dose of 5 mg/kg body weight. Of the
administered dose, 82% was eliminated in faeces as parent compound and
5% was recovered in urine. Identification of the metabolic products
in urine revealed 2,6-DFBA (0.28% of the dose), 4-CPU (0.82%), PCA
(1.03%) and 2,6-difluorobenzamide (0.83%). Cleavage of the urea
moiety between the benzoyl carbon and urea nitrogen was shown to be
the primary degradation pathway in pigs (Opdycke et al., 1982a).
In chickens only small quantities of the metabolites 2,6-DFBA,
4-CPU and PCA were found in excreta and tissues (Opdycke, 1976).
Neither induction nor inhibition of mixed-function oxidase activity
altered diflubenzuron metabolism in chickens (Opdycke et al., 1982b).
After 4 days daily doses of 7.8 g diflubenzuron/kg body weight,
De Bree et al. (1977) found PCA at a level of 30 ng/ml in rat plasma
and 323 ng/g in erythrocytes. PCA, estimated by the concentration in
the urine, represented at most 0.01% of the dose actually absorbed.
6.3.1 Metabolites - distribution, excretion, retention and turnover
When 14C-PCA was administered orally as single doses of 0.3,
3.0 or 30.0 mg/kg to male Fischer-344 rats, approximately 75% of the
administered radioactivity was excreted in the urine within 24 h,
while approximately 10% appeared in the faeces. Excretion was
virtually complete (92-97%) 7 days after dosing. The highest tissue
levels of radioactivity following a single intravenous dose of
3.0 mg/kg were found in the liver, fat, muscle and skin. Tissue
levels peaked within 5-60 min after dosing. By 3 days, concentrations
in all tissues except the blood had declined to < 0.3% of the dose
(Sipes & Carter, 1988). At this time, the only tissue containing more
than 1% of the dose was the cellular compartment of blood, which
contained 1-2% of the dose. The decline of PCA concentration in all
tissues, except for urine, faeces and intestinal contents, was
biexponential. The t alpha 1/2 for fat, muscle and skin was about
1.5 h, while the tß1/2 was approx. 43-59 h. The t alpha 1/2 for
liver was 3.5 h. Levels of unchanged PCA in all tissues peaked after
5 min following intravenous administration. The highest amount of
unchanged PCA was attained in muscle (15% of radioactivity in the
tissue) followed by skin (6%), fat (3%) and liver (2%). The decline of
PCA in all tissues, except for the liver, followed biexponential kinetics
with an estimated t alpha 1/2 of 8 min and a tß1/2 of 3 to 5 h.
PCA is rapidly metabolized to p-chloroacetanilide (PCAA) as the
initial step in the metabolism and excretion of PCA. The decline of PCAA
was monoexponential, the appearance half-life being approx. 6 min in the
testes and 15 min in the brain. The elimination half-life in the
brain, kidney, testes, muscle, skin and fat was around 1.0 to 2.0 h.
The elimination of PCA does not depend on either the dose or route of
administration. Approximately 4% of the urinary radioactivity in the
0-24 h urine sample was unchanged PCA; less than 1% was found in the
faeces. PCAA was not detected in either urine or faeces over a 3-day
period (Sipes & Carter, 1988).
After a single intravenous dose of 14C-PCA (3 mg/kg), maximal
tissue levels were reached within 15 min in most tissues. At this
time, most of the radioactivity was located in muscle (34%), fat
(14%), skin (12%), liver (8%) and blood (7%). Elimination half-lives
from tissues ranged between 1.5 and 4 h. By 8 h, approximately 90% of
the administered dose had been eliminated into urine and faeces. By
3 days, concentrations in all tissues, except blood, had declined to
< 0.3% of the dose (US NTP, 1989).
6.4 Elimination and excretion
After oral administration to rats of 5 mg diflubenzuron labelled
with 3H in the benzoyl and with 14C in the aniline moiety, 95% of
the 3H and 70-75% of the 14C radioactivity were retrieved in urine
and faeces. 2,6-DFBA was shown to constitute more than half of the
urinary metabolites (De Lange et al., 1977). Up to 1% of an oral dose
of 5 mg 14C-diflubenzuron labelled at the benzoyl moiety was
recovered in the expired air of rats (De Lange et al., 1974; Willems
et al., 1980).
When 14C-diflubenzuron, labelled in the aniline moiety, was
administered by gavage (4, 16, 48, 128, 900 and 1000 mg/kg body
weight) to rats, the urinary excretion was complete after 48-72 h.
Urinary excretion after single oral administration of diflubenzuron
relatively decreased with increasing dose level, being 27.6% of the
dose at 4 mg/kg and 1% at 1000 mg/kg (De Lange et al., 1977).
When 14C-diflubenzuron was administered at single oral doses of
12.5, 63.5, 202.5 and 925 mg/kg body weight to Swiss mice, the
excretion was almost completed within 48 h. The cumulative percentage
of the dose excreted in the urine decreased from 15% at the dose level
of 12.5 mg/kg to approximately 2% at 925 mg/kg (De Lange & Post,
1978).
Hawkins et al. (1980) studied the excretion of radioactivity in
urine and faeces after oral administration of 3H/14C-diflubenzuron
(7 mg/kg) to male cats. The radioactive dose was given on day 10 of a
15-day dosing regime of non-radioactive diflubenzuron (days 1-9 and
days 11-15). The excretion of radioactivity in urine accounted for
9.5 and 9.6% of the 14C and 3H doses, respectively, during 6 days
after dosing. The elimination of radioactivity in faeces accounted
for 77.3 and 71.6% of the 14C and 3H doses, respectively, during 6
days after dosing.
After an oral administration of 14C-diflubenzuron (5 mg/kg) to
female pigs, 82% of the dose was eliminated via faeces and 5% via
urine in 11 days (Opdycke, 1976).
About 85% of a single oral dose of 14C-diflubenzuron (10 mg/kg
body weight) administered to a cow was recovered in the faeces during
the first 4 days after treatment. About 15% was recovered in urine
and only about 0.2% was secreted in the milk (Ivie, 1977, 1978).
Sheep excreted 41% of the dose (10 mg/kg) in the urine and 43% in
the faeces during the 4 days after treatment. Bile-cannulated sheep
eliminated 24% of the dose in the urine, 32% in the faeces and 36% in
the bile. Sheep treated with 500 mg 14C-diflubenzuron/kg as a single
oral dose eliminated a much smaller proportion of the 14C in urine
and bile. This was probably due to reduced absorption