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Revised June 1996
Azinphos-methyl
Trade and Other Names:
Common names include azinphos-methyl and metiltriazotion. Trade
names include Azimil, Bay 9027, Bay 17147, Carfene,
Cotnion-methyl, Gusathion, Gusathion-M, Guthion, and
Methyl-Guthion.
Regulatory Status:
All azinphos-methyl liquids with a concentration greater than
13.5% are classified as Restricted Use Pesticides (RUP) by the
U.S. Environmental Protection Agency (EPA) because of the
inhalation hazard and acute toxicity they present, as well as
their potential adverse effects on mammalian species, birds, and
aquatic organisms. RUPs may be purchased and used only by
certified applicators. The EPA has imposed a 24-hour reentry
interval for this material. It is toxicity class I - highly
toxic. Products containing azinphos-methyl bear the Signal Words
DANGER and POISON.
Chemical Class: OP
insecticide
Introduction:
Azinphos-methyl is a highly persistent, broad spectrum
insecticide. It is also toxic to mites and ticks, and poisonous
to snails and slugs. It is a member of the organophosphate class
of chemicals. It is nonsystemic, meaning that it is not
transported from one plant part to another. It is used primarily
as a foliar application against leaf feeding insects. It works as
both a contact insecticide and a stomach poison. Azinphos-methyl
is registered for use in the control of many insect pests on a
wide variety of fruit, vegetable, nut, and field crops, as well
as on ornamentals, tobacco, and forest and shade trees. Outside
of the U.S., azinphos-methyl is used in lowland rice production.
Azinphos-methyl is available in emulsifiable liquid, liquid
flowable, ULV liquid, and wettable powder formulations.
Formulation:
Azinphos-methyl is available in emulsifiable liquid, liquid
flowable, ULV liquid, and wettable powder formulations.
Toxicological Effects:
- Acute toxicity: Azinphos-methyl is one
of the most toxic of the organophosphate insecticides
[2,23]. It is highly toxic by inhalation, dermal
absorption, ingestion, and eye contact [2]. Like all
organophosphate chemicals, azinphos-methyl is a
cholinesterase inhibitor. It damages normal functioning
of cholinesterase, an enzyme which is essential to proper
nervous system function. Individuals with a history of
reduced lung function, convulsive disorders, or recent
exposure to other cholinesterase inhibitors are at
increased risk from exposure to azinphos-methyl [2,8].
There is wide variation in the recorded LD50 values for
azinphos-methyl depending on the route of exposure and
the test animal. The oral LD50 for azinphos-methyl is 4.4
to 16 mg/kg in rats, 80 mg/kg in guinea pigs, and 8 to 20
mg/kg in mice [2,8,13]. The dermal LD50 is 88 to 220
mg/kg in rats, and 65 mg/kg in mice [2,8,13]. The 1-hour
inhalation LC50 for azinphos-methyl in rats is 0.4 mg/L
[13]. For humans, ingestion of azinphos-methyl in amounts
above 1.5 mg/day can cause severe poisoning with symptoms
such as dimness of vision, salivation, excessive
sweating, stomach pain, vomiting, diarrhea,
unconsciousness, and death [2,23]. Inhalation of the dust
or aerosol preparation of azinphos-methyl may cause
wheezing, tightness in the chest, blurred vision, and
tearing of the eyes. Complete symptomatic recovery may
occur within 1 week after sublethal poisoning; i.e.,
poisoning from an exposure that is just below the amount
necessary to be fatal [23]. Pure azinphos-methyl is
easily absorbed by the skin and lethal amounts can build
up in the body after dermal exposure. Symptoms of illness
caused by this type of exposure include nausea, vomiting,
blurred vision, and muscle cramps [2,23]. Eye contact
with concentrated solutions of azinphos-methyl can be
life-threatening. Within a few minutes of eye exposure,
azinphos-methyl may cause pain, blurring of distant
vision, tearing, and other problems. Symptoms of
cholinesterase inhibition may also occur, such as
respiratory difficulties, gastrointestinal problems, and
central nervous systems disturbances [23]. Some
organophosphates may cause delayed symptoms beginning 1
to 4 weeks after an acute exposure which may or may not
have produced immediate symptoms. In such cases,
numbness, tingling, weakness, and cramping may appear in
the lower limbs and progress to incoordination and
paralysis. Improvement may occur over months or years,
and in some cases residual impairment will remain [2,23].
- Chronic toxicity: Long term exposure to
azinphos-methyl, above the average 8-hour standard set by
the Occupational Safety and Health Administration (OSHA),
can impair concentration and memory, and cause headache,
irritability, nausea, vomiting, muscle cramps, and
dizziness [23]. Cholinesterase inhibition from exposure
to azinphos-methyl may persist for 2 to 6 weeks [1].
Repeated exposure to small amounts may result in an
unexpected inhibition of cholinesterase, causing symptoms
that resemble other flu-like illnesses, including general
discomfort, weakness, and lack of appetite [1,2]. The
effects of azinphos-methyl exposure may be greater in a
previously exposed person than in an individual with no
previously exposure. Rats tolerated dietary doses of 0.25
mg/kg/day for 60 days without cholinesterase inhibition,
1 mg/kg/day resulted in questionable growth effects and a
slight inhibition of brain and red blood cell
cholinesterase. In chronic oral toxicity studies, rats
and dogs were fed doses of 0.125, 0.5, 1, or 2.5
mg/kg/day. The 2.5 mg/kg/day dose was increased to 5
mg/kg/day after 47 weeks. At 0.125 mg/kg/day,
cholinesterase was not affected in rats and dogs. At 1
mg/kg/day, the plasma and red blood cell cholinesterases
in the rat were initially inhibited, but returned to
normal after 65 weeks. Five mg/kg/day doses produced
convulsions in some animals. In dogs, 0.5 mg/kg/day
produced a slight, irregular decrease in red blood cell
cholinesterase [24,25]. Rats fed about 5 to 10 mg/kg/day
azinphos-methyl for 2 years, had depressed red blood cell
counts and brain cholinesterase activity. Dietary levels
of about 0.5 mg/kg/day or less had no negative effects
[2].
- Reproductive effects: In a two
generation reproduction study, there were no observed
reproductive and maternal effects in rats at 0.25
mg/kg/day [25]. However, at oral doses of 20 mg/kg to 8
day pregnant mice, Guthion was toxic to the fetus [25].
These data indicate that reproductive effects in humans
are unlikely at expected exposure levels.
- Teratogenic effects: In a teratology
study, no maternal or developmental effects were observed
in rats at doses of 2 mg/kg/day [25]. A 16 mg/kg oral
dose to 8-day pregnant rats caused specific development
abnormalities in the muscles and bones. It appears the
teratogenic effects are not likely in humans under
expected exposure conditions.
- Mutagenic effects: No mutagenic effects
were observed in the Ames test on bacteria and a test on
human cell cultures [25]. These data suggest that
azinphos-methyl is not mutagenic.
- Carcinogenic effects: Although one
carcinogenicity study on rats suggested that tumors of
the pancreas and selected thyroid cells may have been
associated with azinphos-methyl [2], two other studies at
doses up to 10 mg/kg/day did not show an increase in the
incidence of tumors in mice from azinphos-methyl [2,17].
The carcinogenicity of azinphos-methyl is not clear from
current evidence.
- Organ toxicity: Toxicity from
azinphos-methyl is primarily manifested in cholinesterase
inhibition which affects the nervous system. Dogs fed 9
mg/kg/day showed tremors, weakness, abnormal quietness,
and some weight loss [2].
- Fate in humans and animals: One study
suggests that Guthion is rapidly broken down into
nonpoisonous forms in the body [24]. Azinphos-methyl is
eliminated in feces and urine of mammals within 2 days of
administration.
Ecological Effects:
- Effects on birds: Azinphos-methyl is
slightly to moderately toxic to birds. Acute symptoms of
azinphos-methyl poisoning include regurgitation, wing
drop, wing spasms, diarrhea, and lack of movement [26].
Chickens fed azinphos-methyl at doses of 40 mg/kg
developed leg weakness. The oral LD50 for azinphos-methyl
is 136 mg/kg in young mallards, 74.9 mg/kg in young
pheasant, 84.2 mg/kg in young chukar partridges, 262.0
mg/kg in chickens, and 32.2 mg/kg in bobwhite quail
[13,17,27]. The dietary LC50 for azinphos-methyl is 639
ppm in Japanese quail, 1821 ppm in ring necked pheasant,
and 1940 ppm in mallard duck [13,17].
- Effects on aquatic organisms: Azinphos-methyl is
moderately to very highly toxic to freshwater fish. For
most species, the LC50 values are less than 1 mg/L. The
96-hour LC50 for azinphos-methyl in rainbow trout is
0.003 mg/L [8,13]. Guthion-poisoned fish exhibit central
nervous system impairment, including erratic swimming
accompanied by uncontrolled convulsions. Rapid gill
movements, paralysis, and death follow in rapid
succession [8]. Azinphos-methyl is highly toxic to
aquatic invertebrates, shellfish, frogs, and toads [8].
The LC50 values are below 1 ug/L for many of the species
[8,17].
- Effects on other organisms: Several
studies have indicated that azinphos-methyl causes
adverse effects in wildlife. Wild mammals and aquatic
organisms appear to be more vulnerable than birds to
hazards created by this material [29]. The EPA requires
endangered species labeling for certain azinphos-methyl
uses [17]. Azinphos-methyl is toxic to honeybees and
other beneficial insects [8,24]. It will cause severe bee
losses if used when bees are present at treatment time or
within a day thereafter [30]. A 90% mortality rate is
seen in pollinating leaf cutting bees after a 9-day
exposure to greenhouse alfalfa treated with
azinphos-methyl [27].
Environmental Fate:
- Breakdown in soil and groundwater:
Persistence of azinphos-methyl in soil is quite variable
but is generally low under field conditions [19,31]. The
half-life in sandy loam soil is 5 days. Its half-life in
nonsterile soil is 21 days when oxygen is present or 68
days under oxygen-free conditions. In sterile soil, the
half-life is reported to be 355 days. Azinphos-methyl is
fairly immobile in soil because it adsorbs strongly to
soil particles and has low water solubility. It has low
leaching potential and is unlikely to contaminate
groundwater [19,31]. It was not detected in 54
groundwater samples collected in New York state [32].
Azinphos-methyl is one of 118 synthetic organic chemicals
that the state of Florida has designated for groundwater
monitoring [33]. It was detected in only 5 out of 1628
wells sampled in 10 states from 1983 to 1991 [34]. The
disappearance of azinphos-methyl from soil is more rapid
in the surface layers (0 2.5 centimeters deep) than it is
in the next deeper layer (2.4 to 7.5 cm). Biodegradation
and evaporation are the primary routes of disappearance
for azinphos-methyl. Azinphos-methyl is also subject to
degradation by ultraviolet (UV) light from the sun and
hydrolytic decomposition. Photodecomposition is
particularly rapid at high levels of soil moisture and in
the presence of UV light [31]. Rapid degradation of
Gusathion was observed at temperatures higher than 37 C
[29].
- Breakdown in water: In general,
organophosphates, such as azinphos-methyl, are dissipated
rapidly in water [35]. In pond water, it is subject to
degradation by sunlight and microorganisms, with a
half-life of up to 2 days. Volatilization from water is
unlikely. Chemical hydrolysis is important in alkaline
waters [12]. Azinphos-methyl is very stable in water
below pH 10.0. Above pH 11.0, it is rapidly hydrolyzed to
anthranilic acid, benzamide, and other metabolites.
Azinphos-methyl has a low to medium tendency to adsorb to
sediments or suspended solids [12].
- Breakdown in vegetation: Residue levels
of azinphos-methyl in crops are dependent on the rate and
frequency of application, nature of the plant surface,
and weather conditions such as rainfall, temperature,
sunlight, humidity, and wind [24]. The half-life on
vegetable and forage crops is 3 to 5 days under field
conditions [24]. It gives effective protection for 2 or
more weeks [36]. On treated apple trees, the half-life of
this pesticide was about 2.6 to 6.3 days [28]. Hawthorn
and American Linden trees have been injured by this
material. It has also caused russeting on certain
varieties of fruit [24].
Physical Properties:
- Appearance: Pure azinphos-methyl is a
white crystalline solid. Technical azinphos-methyl is a
brown waxy solid [13,37].
- Chemical Name:
S-(3,4-dihydro-4-oxobenzo[d]-[1,2,3]-triazin-3-ylmethyl)
O,O-dimethyl phosphordithioate [13]
- CAS Number: 86-50-0
- Molecular Weight: 317.33
- Water Solubility: 30 mg/L @ 25 C [13]
- Solubility in Other Solvents:
dichloromethane v.s.; toluene v.s. [13]
- Melting Point: 65-68 C (technical) [13];
73-74 C (pure form) [13]
- Vapor Pressure: <1 mPa @ 20 C [13]
- Partition Coefficient: Not Available
- Adsorption Coefficient: 1000 [19]
Exposure Guidelines:
- ADI: 0.005 mg/kg/day [38]
- MCL: Not Available
- RfD: Not Available
- PEL: 0.2 mg/m3 (8-hour) (skin) [39]
- HA: Not Available
- TLV: Not Available
Basic Manufacturer:
Miles, Inc.
P.O. Box 4913
8400 Hawthorn Road
Kansas City, MO 64120
- Phone: 816-242-2429
- Emergency: 816-242-2582
References:
References for the information in this PIP can be found in
Reference List Number 5
DISCLAIMER: The
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
product labeling.