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in 1996. EXTOXNET no longer updates this information, but it may be useful
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EXTOXNET primary files maintained and archived at Oregon State
Revised June 1996
Trade and Other Names:
Trade names for products containing methidathion include Somonic,
Somonil, Supracide, Suprathion and Ultracide. The compound may be
found in formulations with many other pesticides.
Methidathion is an highly toxic compound in EPA toxicity class I.
Labels for products containing it must bear the Signal Word
DANGER. Methidathion is a Restricted Use Pesticide (RUP), except
for use in nurseries, and on safflower and sunflowers.
Methidathion is a non-systemic organophosphorous insecticide and
acaricide with stomach and contact action. The compound is used
to control a variety of insects and mites in many crops such as
fruits, vegetables, tobacco, alfalfa, and sunflowers, and also in
greenhouses and on rose cultures. It is especially useful against
scale insects. It works by inhibiting certain enzyme actions in
the target pests. It is available in emulsifiable concentrate,
wettable powder, and ultra-low volume (ULV) liquid.
Formulation: It is
available in emulsifiable concentrate, wettable powder, and
ultra-low volume (ULV) liquid formulations.
- Acute toxicity: Methidathion is highly
toxic via the oral route, with reported acute oral LD50
values of 25 to 54 mg/kg in the rat, and 18 to 25 mg/kg
in the mouse [2,13]. Other reported oral LD50 values
include 25 mg/kg in guinea pigs, 80 mg/kg in rabbits, and
200 mg/kg in dogs . It is highly toxic via the dermal
route as well, with reported dermal LD50 values of 85 to
94 mg/kg in the rat . Methidathion is only a mild skin
irritant and is nonirritating to the eyes (in rabbits).
Via the inhalation route, it may be slightly toxic, with
a reported 4-hour inhalation LC50 of 3.6 mg/L in rats
. Effects due to acute methidathion exposures are
similar to those caused by other organophosphate
pesticides, and may include nausea, vomiting, cramps,
diarrhea, salivation, headache, dizziness, muscle
twitching, difficulty breathing, blurred vision, and
tightness in the chest . High acute exposure may cause
intense breathing problems, including paralysis of the
- Chronic toxicity: Beagle dogs fed small
doses of the compound for 2 years experienced no compound
related effects at or below the dose of 0.10 mg/kg/day
[2,8]. At doses of 0.4 mg/kg/day and above, the dogs
experienced enzymatic changes and liver alterations.
Inhibition of red blood cell cholinesterase, an enzyme,
was observed only at the highest dose tested (1.6
mg/kg/day) . Rats also have a low tolerance for the
compound. Compound related effects were first noted in
the rats at doses of 2 mg/kg and above and included
cholinesterase inhibition in the blood and brain and some
nerve related effects. At the highest dose of 5 mg/kg,
the rats ate more food but had less body weight gain.
They also developed skin lesions and foam in their lungs
[2,8,81]. Rhesus monkeys fed small amounts of the
compound developed changes in blood cholinesterase
activity at doses of 1 mg/kg/day and above. Humans
ingesting very small amounts of the compound at doses of
0.11 mg/kg/day for 6 weeks had no noticeable clinical
effects . A study of exposure levels of mixer/loaders
of methidathion (Supracide applications) in California
showed that the greatest exposure potential to the
compound was through the skin (dermal) .
- Reproductive effects: Moderate amounts
of methidathion caused a number of adverse reproductive
effects. When male and female rats were fed moderate
amounts of methidathion over two successive litters, the
parents experienced tremors, decreased food consumption
and decreased ovary weights at 1.25 mg/kg/day [2,8]. The
low dose of 0.25 mg/kg/day disrupted mating behavior and
also affected nursing offspring. At 2.5 mg/kg/day (the
highest dose tested), stillbirths and decreased pup
survival were observed . Reproductive effects in
humans as a result of methidathion exposure are unlikely
under normal circumstances.
- Teratogenic effects: Small to moderate
amounts of methidathion administered to pregnant rats and
rabbits produced no birth defects in the offspring. The
pregnant females experienced several compound related
effects, most of which were typical of cholinesterase
inhibition . The compound is unlikely to pose a
developmental risk to humans.
- Mutagenic effects: Methidathion did not
induce any genetic changes in a number of tests for gene
mutation, chromosomal aberrations, and DNA damage. The
various gene mutation studies were conducted on hamster
bone marrow cells, in mammalian cells, and on several
species of bacteria . These data indicate that
methidathion is not mutagenic.
- Carcinogenic effects: Methidathion
caused malignant and benign liver tumors (adenomas) in
male mice fed 2.5 mg/kg/day for 2 years. Additional
tumors (carcinomas) were found in the male mice fed 5
mg/kg/day over the same period. This higher feeding level
also produced numerous other signs of toxicity [2,8].
Since these results apply to only one sex in one species,
the carcinogenic potential of methidathion is unclear.
- Organ toxicity: Target organs in animal
studies include the nervous system, liver, gall bladder,
- Fate in humans and animals: Methidathion
is rapidly absorbed, broken down, and eliminated in
animals . Following absorption of the compound, the
majority is lost as a breakdown product through the lungs
[2,6]. Between 30 and 50% of the ingested amount is
eliminated (as breakdown products) in urine [2,6]. Half
of the initial amount of the compound is removed from
mammals within 6 hours [2,6]. The breakdown products of
the parent compound are not of toxicological concern .
Only very small amounts of various metabolic products of
methidathion have been detected in milk from cows  and
in chicken eggs .
- Effects on birds: Methidathion is highly
toxic to birds following acute exposure. The reported
oral LD50 values for the compound are 23 mg/kg to 33
mg/kg in mallards, 8.41 mg/kg in Canadian geese, 33.2
mg/kg in the ring-necked pheasant, and 225 mg/kg in the
chukar partridge [8,6].
- Effects on aquatic organisms: The
compound is very highly acutely toxic to aquatic
organisms (both vertebrate and invertebrate); the
reported LC50 values of the compound are 10 to 14 ug/L in
rainbow trout and 2 to 9 ug/L in bluegill sunfish
[13,72]. Tests on lobsters indicated that the combination
of methidathion and another organophosphate insecticide,
phosphamidon, was more toxic than either compound
separately or than would be expected if the toxicities
were added together . Studies with bluegill sunfish
indicate that there is only a slight potential that the
compound would accumulate in fish tissues . Maximum
levels of the residues of the pesticide after 1 month of
exposure to very low concentrations in the water, 0.05
ug/L, were 1.0 ug/kg in the edible tissue, 3.9 ug/kg in
nonedible tissue, and 2.4 ug/kg in whole fish. These
concentrations indicate a low bioconcentration factor of
46 for whole fish. After 2 weeks in water without
methidathion, the concentration in whole fish fell by
nearly 80% .
- Effects on other organisms: Methidathion
is slightly toxic to bees .
- Breakdown in soil and groundwater:
Methidathion is of low persistence in the soil
environment; reported field half-lives are 5 to 23 days,
with a representative value of about 7 days .
Breakdown of the compound in soil occurs through the
action of soil microorganisms . Under alkaline
conditions, methidathion is rapidly degraded by chemical
action . Methidathion and its breakdown products are
poorly bound by soils, and so may be mobile [19,81].
However, they have not been detected in any groundwater
sources. This is probably due to the short half-life of
the compound and its degradates.
- Breakdown in water: No data are
- Breakdown in vegetation: In plants,
methidathion is rapidly metabolized . Oranges sprayed
with Supracide at a rate of nearly 2 pounds per acre had
residues of the compound of about 0.1 ug/ml . Within
2 days over 60% of the compound was removed from the
outside of the fruit, and within 1 week, less than 1% of
the compound remained .
- Appearance: Methidathion is a colorless
crystalline compound at room temperature .
- Chemical Name:
- CAS Number: 950-37-8
- Molecular Weight: 302.33
- Water Solubility: 240 mg/L @ 20 C 
- Solubility in Other Solvents: v.s. in
octanol, ethanol, xylene, acetone, and cyclohexane 
- Melting Point: 39.5 C 
- Vapor Pressure: 186 mPa @ 20 C 
- Partition Coefficient: 4.7243 
- Adsorption Coefficient: 400 (estimated)
- ADI: 0.001 mg/kg/day 
- MCL: Not Available
- RfD: 0.001 mg/kg/day 
- PEL: Not Available
- HA: Not Available
- TLV: Not Available
P.O. Box 18300
Greensboro, NC 27419-8300
- Phone: 800-334-9481
- Emergency: 800-888-8372
References for the information in this PIP can be found in
Reference List Number 5
information in this profile does not in any way replace or
supersede the information on the pesticide product labeling or
other regulatory requirements. Please refer to the pesticide