OMETHOATE JMPR 1975
Explanation
Omethoate was evaluated by the Joint Meeting in 1971 (FAO/WHO,
1972). Since the previous evaluation, additional residue data have
become available, and information on tolerances and pre-harvest
intervals has been obtained from several countries.
At the eighth session of the Codex Committee on Pesticide
Residues, several delegations were of the opinion that the proposed
temporary tolerances were too high. The Committee requested the Joint
Meeting to consider residues of omethoate arising from the use of
omethoate, dimethoate or formothion (Alinorm, 76/24, para. 150).
Further data available since the previous review are summarized
in this monograph addendum.
EVALUATION FOR ACCEPTABLE DAILY INTAKE
TOXICOLOGICAL STUDIES
Special studies on mutagenicity
Mouse
A dominant lethal study was performed with male mice to evaluate
the potential effect of omethoate on spermatogenesis. Groups of male
mice (20 mice/group) were administered omethoate at doses of 0 and 5
mg/kg orally and mated to groups of 3 virgin females weekly for 8
weeks over the period of spermatogenesis. Omethoate gave no suggestion
of mutagenic hazard by this test at this acute dose as evidenced by
pre- and post-implanatation mortality (Machemer, 1974).
Special studies on neurotoxicity
Chicken
Groups of hens (10 hens/treatment group and 5 hens/positive
control) were administered omethoate or ToCP orally in a study to
evaluate the potential for delayed neurotoxicity. Omethoate was
administered at the LD50 level (92 mg/kg) with atropine and ToCP was
administered at a dose of 350 mg/kg orally. Although several hens died
following omethoate, none showed clinical signs of delayed
neurotoxicity. Clinical signs were observed with the ToCP group
(Kimmerle, 1972). Histological examination of nervous tissue (using
the H and E stain) showed degeneration in the ToCP treated group. No
such evidence was noted with omethoate (Newman et al., 1972).
Special studies on teratogenicity
Groups of pregnant rats (20-24 rats/group) were orally
administered omethoate at dosages of 0, 0.3, 1.0 and 3.0 mg/kg from
day 6 to 15 of gestation. On the 20th day of gestation the animals
were sacrificed and the foetuses examined for skeletal and tissue
abnormalities. The foetuses and placentas of the 3.0 mg/kg group
weighed less than the control. The other reproductive parameters were
unaffected. No teratogenic effect was observed with respect to
malformations of foetuses (Machemer, 1975).
COMMENTS
Omethoate was reviewed by the Joint Meeting in 1971 and a
temporary ADI was estimated. It was recognized that omethoate is a
metabolic of a common pesticide, dimethoate, for which an ADI had been
allocated and on which several significant studies had been performed.
It was also recognized that while animals exposed to dimethoate were
also exposed to omethoate that long-term studies were needed to give
an assurance of safety to the ADI. Studies were reported on
neurotoxicity, mutagenesis and teratogenesis testing. No adverse
effects were noted in these studies. No long-term studies on omethoate
were reported, although information was received of appropriate
studies in progress. Although the temporary ADI is currently based on
short-term studies, the further studies reported gave the Meeting
added assurance of safety to maintain the temporary ADI until
long-term studies are complete.
TOXICOLOGICAL EVALUATION
Level causing no toxicological effect
Rat: 1.0 ppm in the diet equivalent to 0.05 mg/kg body weight
Dog: 1.6 ppm in the diet equivalent to 0.12 mg/kg body weight
Estimation of temporary ADI for man
0-0.0005 mg/kg body weight
RESIDUES IN FOOD AND THEIR EVALUATION
RESIDUES RESULTING FROM SUPERVISED TRIALS
Additional residue data, summarized in Table 1, are available on
orange, hop, sugar beet, potato and apple (Bayer, 1975). Some of these
are data required by the 1971 Joint Meeting (FAO/WHO, 1972).
TABLE 1. Omethoate residues in crops resulting from supervised trials
a.i. kg per No. of Days after Residue (mg/kg)
Crop hectare applications last applic.
Fruit Peel Total
Orange 0.75 1 7 0.04 0.7 0.25
ca50 0.12 0.07 0.11
66 0.03 0.05 0.03
Apple 0.75 1 39 0.10 0.14 0.11
1.28 1 0 \¾0.1
1 <0.1
2 <0.1
3 <0.1
Plum 1.28 1 0 <0.1
1 <0.1
2 <0.1
3 <0.1
Total
Hop 1.5 1 0 54.4, 45.8
(dried) 7 44.3, 23.5
14 10.4, 25.0
21 0.6, 2.3
28 0.6, 2.1
10 1 161 0.05, 0.08
Root Leaves
Sugar beet
0.4 2 21 n.d. 0.18, 0.48
49 n.d. 0.08, 0.025
72 n.d. n.d., 0.03
93 n.d. n.d.
Total
Potato
0.6/0.5/0.4 3 16 n.d. - 0.01
21 n.d. - 0.01
54 n.d. - 0.01
63 n.d.
76 n.d.
Witloof chicory
1.28 1 0 <0.1, 0.1
7 <0.1
FATE OF RESIDUES
In plants
As mentioned previously (FAO/WHO, 1972) omethoate is relatively
persistent in plants although oxons are usually hydrolyzed more
readily than thionates. This fact should be borne in mind when the
residues of omethoate derived from dimethoate and formothion in plants
are evaluated. Radio-tracer studios showed that the residues arising
from the use of dimethoate and formothion are identical.
Data on residues in various crops resulting from treatment with
formothion (Environment Agency, Japan, 1974) or dimethoate (Wit, 1972,
Broeke and Dornseiffen, 1973) showed that formothion residues were
always very low <0.01 mg/kg) and the main residue was generally
dimethoate. Residues of omethoate derived from either insecticide were
lower than those of dimethoate except in tomato and witloof chicory
(Tables 2 and 3).
Following the treatment of grapes with dimethoate (Steller and
Brand, 1974), residues of dimethoate decreased from a maximum of 18
mg/kg to about 0.5 mg/kg during a seven-week period, with no
accumulation of omethoate (maximum of 0.3-0.4 mg/kg)
In soils
The persistence of omethoate applied to soils of three localities
at a rate of 3 kg a.i./ha was measured. Half-life values of omethoate
ranged from 3.8 to 25 days. A close correlation was found between the
dissipation of omethoate and the rainfall during the experimental
period, indicating that the water solubility of omethoate was one of
the factors affecting the dissipation of omethoate in soils. (Bayer,
1975).
In processing and cooking
Witloof chicory treated with dimethoate was sampled 36 days after
treatment and analysed before and after washing and cooking (Broeke
and Dornseiffen, 1973). Washing alone removed about one third of both
the dimethoate and omethoate, and washing and cooking together removed
about two thirds of both residues (Table 4).
NATIONAL TOLERANCES REPORTED TO THE MEETING
Further information on changes in and additions to tolerances and
pre-harvest intervals for omethoate was obtained from several
countries (Table 5). Tolerances and pre-harvest intervals vary
considerably from country to country for the same crop, owing to the
differences in agricultural practice, food consumption patterns,
climatic conditions, etc., all of which influence the rate of
dissipation of omethoate residues.
TABLE 2. Residues of formothion, dimethoate and omethoate in crops treated with formothion
Crop Formulation Application Number of Pre-harvest Residues (mg/kg)
rate, % treatments interval (days) formothion dimethoate omethoate
Cabbage EC 0.036 8 0.004 0.332 0.053
3 15 <0.003 0.068 <0.003
22 0.007 0.153 0.014
8 <0.003 0.119 0.022
5 15 0.008 0.216 0.038
22 0.005 0.147 0.023
Tomato EC 0.036 6 <0.005 0.046 0.027
3 13 <0.005 0.006 0.021
20 <0.005 <0.005 0.015
6 <0.005 0.081 0.042
5 13 < " 0.016 0.035
20 < " <0.005 <0.005
Eggplant EC 0.036 7 <0.005 0.006 <0.005
3 14 < " < " < "
21 < " < " < "
7 <0.005 0.028 0.008
5 14 < " < " < "
21 < " < " < "
Orange EC 0.045 49 <0.005 0.013 0.014
1 62 < " 0.023 0.008
2 49 <0.005 0.016 0.016
62 < " 0.007 0.008
APPRAISAL
Additional data on the residues of omethoate on sugar beets,
potatoes, hops, witloof chicory, oranges, apples and plums have become
available. Residues in dried hops remain high for two weeks following
application, but by three weeks have declined considerably.
TABLE 3. Residues of dimethoate and omethoate in fruit and witloof
chicory treated with dimethoate
Pre-harvest
Application, No. of interval, Residues, mg/kg
Crop kg a.i./ha treatments days dimethoate omethoate
Apples 0.3 1 0 0.19, 0.20 <0.1
1 0.09, 0.10 <0.1
2 0.07, 0.29 <0.1
3 0.07, 0.14 <0.1
Plums 0.3 1 0 0.03, 0.19 <0.1
1 0.01, 0.02 <0.1
2 <0.01, 0.11 <0.1
Witloof 0.5 g/m2 1 36 0.14 0.15
chicory sprayed on 50 0.15 0.19
top of roots
immediately
before forcing
TABLE 4. Effect of washing and cooking on residues of dimethoate and
omethoate in witloof chicory
Residue level, mg/kg, in
Washed and cooked
Residue Raw chicory Washed chicory chicory
Dimethoate 0.14 0.09 0.05
Omethoate 0.15 0.12 0.06
TABLE 5. National tolerances and pre-harvest intervals
Tolerance, Pre-harvest
Country Crop mg/kg interval, days
Australia tomatoes, peppers 1.0
("recommended") all other vegetables,
tree fruit 2.0 21
pastures, forage crops
cereal crops, oil seed
crops 0
citrus fruit, pome fruit - 21
Federal beets
Republic of fruit and vegetables 0.4
Germany all other plant-food 0.05
Finland general 21
France fruit, vegetables 0.4
general 21
Israel citrus fruit 0.4 30
Netherlands fruit and vegetables 0.6
(dimethoate + potatoes 0.05
omethoate from
the use of
dimethoate)
(omethoate from witloof chicory 0.4
direct other vegetables 0.1
application) fruit 0.1
South Africa apricots - 28
peas, alfalfa, clover - 7
USA ("dimethoate corn fodder and forage 1.0
and its oxygen
analogue") corn grain (n.r.) 0.1
eggs and meat, fat and
meat byproducts of 0.02
cattle, goats, horses,
poultry, sheep and
swine
TABLE 5. (Cont'd.)
Tolerance, Pre-harvest
Country Crop mg/kg interval, days
alfalfa 2.0 10
cottonseed 0.1 14
grapefruit, tangerines 2.0 15
grapefruit, tangerines
(dried citrus pulp for 5.0
cattle feed)
grapes 1.0 28
lemons, oranges (dried
citrus pulp for cattle 5.0 15
feed)
safflower (seed) 0.1 14
sorghum (grain) 0.1 28
sorghum (forage) 0.2
UK apples, hops 21
Residues were below 0.2 mg/kg in the other commodities except in
sugar beet leaves and orange peel, where they reached 0.5-1 mg/kg. The
residue data on various crops following treatment with formothion or
dimethoate showed that the main residue was generally dimethoate.
Omethoate residues derived from either of these insecticides were
always lower than those of dimethoate except in tomato and witloof
chicory. Formothion residues were invariably below 0.01 mg/kg. The
effect of cooking on omethoate residues was reported for witloof
chicory, indicating that washing and cooking remained about two thirds
of the original residue. Dissipation of omethoate in soils seems to be
related to rainfall, indicating that leaching from soils takes place
owing probably to high solubility in water.
Omethoate is used in many situations where dimethoate and
formothion are also used and it is a residue from treatment with
dimethoate or formothion. For this reason, and because omethoate is
determined by the analytical methods used for these parent compounds,
the Meeting recommends that the limits previously recommended for
dimethoate should apply also to omethoate. (The Joint Meeting has
already recommended that residues arising from the use of formothion
should be determined as dimethoate + omethoate and expressed as
dimethoate). Some additional limits covering direct uses of omethoate
are recommended below.
RECOMMENDATIONS
The previously recommended temporary tolerances for omethoate are
deleted and are replaced by temporary maximum residue limits identical
to the tolerances previously recommended for dimethoate (FAO/WHO
1973a, 1974a). The following additional recommendations for temporary
maximum residue limits apply to both omethoate and dimethoate. They
are based on residues likely to be found at harvest following the
application of omethoate or dimethoate according to current use
patterns. Residues of omethoate arising from treatment with dimethoate
or omethoate are to be expressed as dimethoate. The recommendation of
the 1972 Joint Meeting (FAO/WHO, 1973) that residues of formothion
should be determined as dimethoate + omethoate and expressed as
dimethoate remains in force.
TEMPORARY MAXIMUM RESIDUE LIMITS
Commodity Limit (mg/kg, Pre-harvest
expressed interval
as dimethoate) on which
recommendations
are based
Sugar beet (roots),
potatoes 0.05 21
Sugar beet (leaves) 1 21
Grapes 2 21
Hops (dried) 3 21
FURTHER WORK OR INFORMATION
REQUIRED (by 30 June 1978)
1. Long-term; studies in at least one species.
DESIRABLE
1. Information on residues occurring in food in commerce.
REFERENCES
Bayer, (1975) Unpublished report
Environment Agency, Japan. (1974) Unpublished residue data.
Broeke, R. T. and Dornseiffen, J. W. (1973) Residues of dimethoate and
omethoate in Witloof in various stages of household preparation. KvW
173/WRV (73) 35.6.2.1. Food Inspection Service, Amsterdam,
Netherlands.
Kimmerle, G. (1972) Folimat / Akute Neurotoxizitätsuntersuchungen an
Hühnern. Unpublished report from Institue für Toxikologie,
Wuppertal-Elberfeld, submitted to the World Health Organization by
Bayer A.G., FRG.
Machemer, L. (1974) Präparat S 6876/ Dominant-Letal-Test an der
männlichen Maus zur Prüfung auf mutagene Wirkung. Unpublished report
from Institut für Toxikologie, Wuppertal-Elberfeld, submitted to the
World Health Organization by Bayer A.G., FRG.
Machemer, L. (1975) Präparat S 6876/ Untersuchung auf exbryotoxische
und teratogene Wirkung nach oraler Verabreichung an der Ratte.
Unpublished report from Institut für Toxikologie, Wuppertal-Elberfeld,
submitted to the World Health Organization by Bayer A.G., FRG.
Newman, A. J., Cherry, C. and Urwin, C. (1972) Pathology Report of
Folimat Active Substances in Hens (Add. to Rep. No. 3439). Unpublished
report from the Huntingdon Research Centre, England, submitted to the
World Health Organization by Bayer A.G., FRG.
Steller, W. A. and Brand, W. W. (1974) Analysis of Dimethoate-treated
grapes for the N-Hydroxymethyl and De-N-methyl Metabolites and for
their sugar adducts. J. Agr. Food Chem., 22(3): 445-449
Wit, S. L. (1972) Residues of the insecticides dimethoate and
omethoate in apples and plums. (Original in Dutch) Rep. 144/72.
TOX-ROB. National Institute of Public Health, Netherlands.