FAO/PL:1967/M/11/1
WHO/Food Add./68.30
1967 EVALUATIONS OF SOME PESTICIDE RESIDUES IN FOOD
THE MONOGRAPHS
The content of this document is the result of the deliberations of the
Joint Meeting of the FAO Working Party of Experts and the WHO Expert
Committee on Pesticide Residues, which met in Rome, 4 - 11 December,
1967. (FAO/WHO, 1968)
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
WORLD HEALTH ORGANIZATION
Rome, 1968
DEMETON
This pesticide was evaluated toxicologically by the 1965 Joint meeting
of the FAO Committee on Pesticides in Agriculture and the WHO Expert
Committee on Pesticide Residues (FAO/WHO, 1965). Since no additional
information on the toxicology of this compound has become available,
the following monograph addendum in confined to the evaluation for
tolerances and a review of methods of analysis.
EVALUATION FOR TOLERANCES
USE PATTERN
Pre-harvest treatments
Demeton is a widely used insecticide on over 30 different fruit,
vegetable, nut and forage crops applied primarily as a foliar spray to
combat against aphids, thrips, mites, leafhoppers and leafminers. See
Table I for typical dosages being recommended and pre-harvest
intervals for the various crop classes.
TABLE I
Recommended dosage
Crop lb/100 gal Pre-harvest
or lbs/A period
apples, pears 0.25 "full coverage" 21
peaches, apricots, plums 0.25 "full coverage" 30
citrus 0.25 "full coverage" 21
grapes, strawberries, 0.4 lbs/A 21
melons
nuts 0.25 " 21
leafy vegetables 0.25-0.5 " 21 (28 for celery)
legumes and root veg. 0.25-0.5 " 21
tomatoes and peppers 0.25-0.4 " 3
grains 0.25 " 45
alfalfa and clover 0.25 " 21 day pre-grazing
and cutting interval
Post-harvest treatments
No use is known for application on stored products. Demeton is
recommended for use on some fruit trees after the harvesting of the
fruits.
Other uses
Demeton is used on several ornamental flowers, shrubs, and trees for
garden pests such as various scales, mites, aphids, etc.
RESIDUES RESULTING FROM SUPERVISED TRIALS
A large number of supervised field trials have been conducted. These
data (mostly unpublished) are incorporated in pesticide petitions
submitted to the U.S. Food and Drug Administration, 1955-1962. The
trials were made in various geographical locations throughout the
U.S.A. and represent differing weather conditions, amounts of
application, stage of crop growth, pre-harvest intervals, etc.
A summary of the results of those trials reflecting the recommended
usage given in Table I is shown in Table II.
The actual active residue resulting from treatment with demeton is a
variety of isomers and oxidative alteration products, therefore values
for the residues shown in this table represent a measure of the total
anticholinesterase compounds expressed as equivalents of a mixture of
65 parts of the thiono and 35 parts of the thiol isomers of demeton.
TABLE II
Typical initial Pre-harvest Residue at end Estimated
Crop residues, ppm period, days of pre-harvest half-life
period, ppm days
Tree fruits
Apples 0.6-1.5 21 0.2-0.5 12
Pears - 21 < 0.75 15
Peaches 0.3-5.0 30 0.2-0.7 7
Apricots 1.0-2.0 30 0.3-0.6 20
Plums 0.7-3.0 30 < 0.2 8
Oranges - 21 < 0.1 -
TABLE II (cont'd)
Typical initial Pre-harvest Residue at end Estimated
Crop residues, ppm period, days of pre-harvest half-life
period, ppm days
Lemons - 21 0.3-0.5 -
Grapefruit - 21 0-0.5 -
Other Fruits
Grapes - 21 0.2-1.0 -
Strawberries - 21 < 0.1 -
Melons 0.2-0.5 21 < 0.1 -
Nuts
Walnut, almond - 21 0-0.3 -
and pecan (meats)
Pecan Hulls - 21 0-0.4 -
Almond Hulls - 21 1.9-3.4 -
Vegetables
Broccoli 1.5-2.5 21 0-0.7 14
Brussels Sprouts - 21 0-0.5 -
Cabbage 9-35 21 0-0.7 4
Cauliflower - 21 0-0.5 -
Celery 9-21 28 0.2-0.4 6
Lettuce - 21 < 0.7 8
Green beans 0.2-0.3 21 < 0.1 -
Lima beans 0-0.2 21 0-0.3 -
Peas - 21 0-0.6 -
Potatoes 0-0.2 21 0-0.2 -
Peppers 0.2-0.6 3 0-0.5 7
TABLE II (cont'd)
Typical initial Pre-harvest Residue at end Estimated
Crop residues, ppm period, days of pre-harvest half-life
period, ppm days
Tomatoes 0.1-0.3 3 0.2-0.3 -
Sugar beets (roots) - 30 0-0.3 -
Hops - 21 < 1.0 -
Grains
Barley - 45 < 0.1 -
Oats - 45 < 0.3 -
Wheat - 45 < 0.1 -
Forage, and Hays
Fresh alfalfa 20-90 21 1-3 3
Fresh clover - 21 1-2 8
Sugar beets 1-2.5 30 0-3.4 -
Pea 8-21 21 0.3 5
Bean 5-7 21 0.3-0.7 5
Alfalfa hay - 21 < 12 -
Barley straw - 45 0-1.0 -
Oats and wheat - 45 0-0.5 -
straw
FATE OF RESIDUES
General considerations
Cook (1954, 1955a, 1955b) reported that the two demeton isomers were
very susceptible to ultraviolet light. It was shown that short
exposure to ultraviolet light produced powerful anticholinesterase
products more hydrophilic than the parents. This could have a very
significant bearing on the "metabolism" of these compounds because of
the similarity of the light-produced and metabolism-produced
compounds.
In plants and animals
The metabolism of demeton involves a complexity of oxidation and
hydrolytic reactions resulting in a number of intermediates. Metcalf
and associates (1954, 1955) demonstrated that the compounds
0,0-diethyl-0-ethyl-2-ethylthio phosphorothionate (demeton
thiono isomer) and
0,0-diethyl-S-ethyl-2-ethylthio-phosphorothiolate (demeton thiol
isomer) are metabolized in plant and animal tissues to the
corresponding sulfoxide and sulfone derivatives, some of which are the
ultimate systemic toxicants. In a series of papers, Fukuto et al.
(1955) and March et al. (1955) described the chemical behavior of the
demeton isomers in biological systems. Comparison of cholinesterase
inhibiting activity, systemic activity, mammalian and insect
toxicities, and relative behavior on paper chromatograms of the
metabolic and synthetic oxidation products showed that each isomer was
first converted to the corresponding sulfoxide by oxidation of the
ethyl-thioethyl moiety and subsequently converted to the sulfones.
Metcalf et al. (1955) concluded that the thiolphosphate sulfoxide
(demeton thiolsulfoxide) and the thiolphosphate sulfone (demeton
thiolsulfone) are probably the principal toxic plant metabolites
resulting from the pesticidal application of Systox. Additional
studies by Metcalf (1956) and Fukuto et al. (1956) have shown that the
thiolisomer metabolites accumulate in plants from 5 to 10 times as
rapidly as the thiono isomer metabolites and are considerably more
persistent. Muhlmann and Tietz (1956) corroborated the finding of the
previous workers utilizing methylisosystox and its sulfoxide and
sulfone. This work further showed that the sulfoxide and sulfone were
the toxic metabolites formed when applied to peas, potted sugar beets,
cucumbers, potatoes and cabbages.
Other related compounds have been studied including the P(S)S analog
Di-Syston, and the dithiomethylene analog, Thimet. The metabolism of
these thioethers has a bearing on the consideration of demeton because
they are so similar chemically and it has been shown that metabolism
results in a number of metabolites, some of which are identical and
some similar to those from demeton. Bull (1965) studied the metabolism
of Di-Syston by insects, plant leaves and rats. He reported that
insects excreted the toxic oxidative derivatives as well as the
hydrolytic products of Di-Syston metabolism, but rats slowly excreted
only the hydrolytic products. As many as four oxidative and nine
hydrolytic metabolites of Di-Syston were found in the biological
systems used. When Di-Syston was supplied to cotton plants, it was
rapidly metabolized. By 24 hours, four metabolites were present,
representing the sulfoxides and sulfones of Di-Syston and its
phosphorothiolate oxidation product. The proportion of these changed
with time. Metcalf's group (1959) studied the effects of temperature
and plant species upon the rates of metabolism of applied Di-Syston.
The metabolism of Di-Syston sulfoxide and the hydrolytic decomposition
of the toxic products occurred from 2-3 times as fast in tomato leaves
at 70°F as in cotton leaves under identical conditions.
Thimet metabolism (Bowman and Casida, 1958a, 1958b) followed a
substantially similar pattern except that the parent-compound
persisted longer, and the rates of oxidation of two sulfoxides to
sulfones were measurably slower. For example, the half-life of the
phosphordithicate sulfoxide was 200 hours as compared to 100 hours for
the corresponding Di-Syston metabolite.
O'Brien (1960) indicates that the thioether pesticides show
selectivity with respect to toxicity. It was tentatively concluded
that the phosphatases responsible for the degradation of the selective
compounds are more effective in mammals than in insects.
In storage and processing
Since the residues of demeton are within the plants, the fate after
harvest probably depends on the metabolic rate in the product in
storage. Thus it is anticipated that residues in food products from
treating growing plants would not diminish appreciably in storage
because storage conditions are generally designed to retard plant
metabolism processes. The reduction of residues during the milling of
grain may be less marked than with other chemicals which are found on
the outside surfaces of grain being milled; although cooking may lead
to destruction of some residues, little work has been done to confirm
this supposition. The only information available indicates stability
during cooking. Apples from trees treated with demeton were used for a
collaborative methods study. Some observations were made on the
stability of the metabolites (Cook, 1955c). Boiling apple juice
prepared from treated apples did not detectably reduce the
anticholinesterase activity; nor did the activity of the boiled juice
diminish over a short storage period. Painter et al. (1963) have shown
that demeton in grape musts were still present in the finished wine
with only a small loss.
METHODS OF RESIDUE ANALYSIS
There have been two principal methods of analysis for demeton. The
first, an anticholinesterase method, is based on the inhibiting
properties of the thiol isomer and its sulfoxide and sulfone. The
thiono isomer is noninhibiting, therefore the ratio of the two isomers
in an analytical standard is highly important. The inhibition of the
residue products is calculated as equivalents of the standard mixture,
see discussion in Cook, 1955c. This method was used to obtain
essentially all of the residue data presented in this monograph. The
method is nonspecific since any other anticholinesterase agents give
the same response. The second method available is one in which total
phosphorous is measured. Dry samples are extracted with chloroform and
the residues partitioned into acetonitrile. Moist samples are
extracted with acetone and water and the residues partitioned into
chloroform. Naturally-occurring phosphorous compounds are removed by
cleanup on an activated carbon-alumina column utilizing acetone as the
eluting solvent. The column eluant is evaporated and the residues
digested with a mixture of nitric and perchloric acids. The final
determination is based on the phosphomolybdenum blue reaction.
Sensitivity of the method is approximately 0.2-0.4 ppm. This method is
also nonspecific but can be combined with a paper chromatographic
method to provide a qualitative identification of the residue. The
cleaned up residue is spotted on silicone treated paper. Compounds
containing P-S form a red color on the chromatogram when treated with
a series of reagents and 2,6-dibromo-N-chloro-p-quinoneimine (Chemagro
Reports).
The high polarity of the actual toxic residues causes them to behave
very much like other polar compounds from plant materials and makes
them difficult to clean up and gas chromatograph. These factors have
greatly impeded the development of an adequate gas chromatographic
method of analyses for the residues from demeton; however, many of
these problems are fairly well solved and it is hoped that adequate
methods of gas chromatography are nearing completion.
NATIONAL TOLERANCES
Country Tolerance, ppm Crop
Canada 0.75 6 fruits and 8 vegetables
0.5 citrus and strawberries
0.3 Melons, beans, tomatoes,
0.2 potatoes
U.S.A. 0.75 11 fruits, 14 vegetables
0.75 3 nuts and 3 grains
0.3 beans
0.5 sugar beets
1.25 grapes, hops
5 almond hulls
5 fresh alfalfa and clover
5 green fodder or straw of
barley, oats
and wheat.
12 alfalfa and clover hay
RECOMMENDATIONS FOR TOLERANCES
No recommendation for tolerance is made.
When demeton is utilized to protect food products, residues as high as
those shown below may be encountered :
Tree fruits, including citrus 0.75
Grapes 1.25
Melons, strawberries, nuts 0.2
Vegetables :
leafy, brassica, legume 0.75
root 0.2
Grains 0.2
The residues from demeton appear to be fairly persistent. They do not
disappear rapidly during storage and some data are available to show
that residues persist during cooking.
The meeting is of the opinion that the data derived from supervised
trials (shown above) and other data do not give assurance that
residues below the ADI for technical demeton will be present.
FURTHER WORK
Further work required before tolerances can be recommended :
1. Date on mount of residue appearing in total diet studies.
2. More data on the possible loss during processing and preparation
for consumption.
3. Method of analysis adequate for use in total diet studies.
REFERENCES PERTINENT TO EVALUATION FOR TOLERANCES
Bowman, J.S. and Casida, J.E. (1958a) Systemic insecticides for
THEBROMA CACAO 4, their translocation and persistence in foliage and
residues in cacao beans. J. Econ. Ent. 51 (6): 773 - 780.
Bowman, J.S. and Casida, J.E. (1958b) Further studies on the
metabolism of Thimet by plants, insects and mammals. J. Econ. Ent. 51
(6): 838 - 843.
Bull, D.L. (1965) Metabolism of Di-Syston by insects, isolated cotton
leaves and rats. J. Econ. Ent. 58 (2): 249 - 254.
Chemagro Corporation, Kansas City, Missouri, USA. Analytical methods
report Nos. 3424, 4638, 5339, 6684 and 8544.
Cook, J.W. (1954) Paper chromatography of some organic phosphate
insecticides. III. Effects of light on Systox and IsoSystox. J. Assoc.
Offic. Agr. Chem. 37 (4): 989 - 996.
Cook, J.W. (1955a) Paper chromatography of some organic phosphate
insecticides. IV. Spot tests for in vitro cholinesterase inhibitors.
J. Assoc. Offic. Agr. Chem. 38 (1): 150 - 153.
Cook, J.W. (1955b) Paper chromatography of some organic phosphate
insecticides. V. Conversion of organic phosphates to in vitro
cholinesterase inhibitors by N-bromosuccinimide and ultraviolet
light. J.Assoc. Offic. Agr. Chem. 38 (3): 826 - 832.
Cook, J.W. (1955c) Report on determination of insecticides by
enzymatic methods. J. Assoc. Offic. Agr. Chem. 38 (3) : 664 - 669.
FAO/WHO. (1965) Evaluation of the toxicity of pesticide residues in
food. FAO Meeting Report 1965/10/1; WHO/Food Add./27.65.
Fukuto, T.R., Metcalf, R.L., March, R.B., and Maxon, M.G. (1955)
Chemical behavior of Systox isomers in biological systems. J. Econ.
Ent. 48 (4): 347 - 354.
Fukuto, T.R., Wolf, J.P., III, Metcalf, R.L. and March, R.B. (1956)
Identification of sulfoxide and sulfone plant metabolites of the thiol
isomer of Systox. J. Econ. Ent. 49 (2): 147 - 151.
March, R.B., Metcalf, R.L., Fukuto, T.R., and Maxon, M.G. (1955)
Metabolism of Systox in the white mouse and American cockroach. J.
Econ. Bat. 48 (4): 355 - 363.
Metcalf, R.L. (1956) The role of systemic insecticides in world
agriculture. Plant Protection Conference. Proc. of the 2nd Intern.
Conf. Fernhurst Research Station, England: 129 - 142. Plant Protection
Ltd. Butterworths Scientific Publications, London.
Metcalf, R.L., March, R.B., Fukuto, T.R., and Maxon, M.G. (1954) The
behavior of Systox-isomers in bean and citrus plants. J. Econ. Ent. 47
(6): 1045 - 1055.
Metcalf, R.L., March, R.B., Fukuto, T.R., and Maxon, M.G. (1955) The
nature and significance of Systox residues in plant materials. J.
Econ. Ent. 48 (4): 364 - 369.
Metcalf, R.L., Reynolds, H.T., Winton, M., and Fukuto, T.R. (1959)
Effects of temperature and plant species upon the rates of metabolism
of systemically applied Di-Syston. J. Econ. Ent. 52 (3): 435 - 439.
Muhlmann, R. and Tietz, H. (1956) The chemical behavior of
methylisosystox in the living plant and the problem of residues.
Off-print from Hofchen-Briefs 2/1956 Farbenfabriken Bayer, Leverkusen,
W. Germany.
O'Brien, R.D. (1960) Toxic phosphorus esters; Chemistry, metabolism
and biological effects: 32 New York, N.Y. Academic Press Inc.
Painter, R., Kilgore, W.E., and Ough, C.S. (1963) Distribution in
fermentation products obtained from artificially fortified grape
musts. J. Food Sci. 28 (3): 342 - 346.