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Abamectin (Avermectin) The
active ingredient of many abamectin products, like AvidTM,
ZephyrTM, VertimecTM or Agri-MekTM, is
actually a mixture of 80% avermectin B1a and 20% avermectin B1b
(FCH 2000). These B1a and B1b avermectins are
purified from a chemically complex insecticidal/miticidal toxin produced
by an actinomycete bacterium, Streptomyces avermitilis, found in
soil. Although, abamectin is a natural fermentation product of this
bacterium, the pesticide is classified by the U.S. Environmental
Protection Agency (EPA) as a class II toxicity pesticide on a scale of I
to IV, I being the most toxic. Abamectin has been fully evaluated by EPA
for its effects on human health or the environment. Mode of Action
Like
most other insecticides, avermectins are nerve poisons. They stimulate the
gamma-aminobutyric acid (GABA) system, a chemical “transmitter”
produced at nerve endings, which inhibits both nerve to nerve and nerve to
muscle communication. The affected insect becomes paralyzed, stops
feeding, and dies after a few days. AvidTM, used against mites
and leaf-miners, is said to spare some of the major parasites of the miner
and some predacious mites. When applied to foliage, it is absorbed by the
leaves, where feeding insects encounter the poison. Toxicity
Technical
avermectin is quite acutely toxic, with an oral rat LD50
(lethal dose for 50% of the test rats) of 30 mg/kg. EPA reviewed
toxicological data from the manufacturer in connection with a 1987
petition for establishment of a tolerance in citrus oil and citrus pulp.
EPA’s reviewers found that avermectin does not cause birth defects in
rats and rabbits, but can cause cleft palate in mice. The calculated
“lowest effect level (LEL)” for the latter effect was quite low at
0.10 mg/kg/day. EPA reviewers stated that “studies on mutagenicity
demonstrated an overall negative potential (ETN 1996). Abamectin
has been shown to cause pupil dilation, mild skin irritation, vomiting,
convulsions and/or tremors and coma in laboratory animals.
Because it is a nerve poison, it can also cause nervous system
depression in mammals at very high doses. A study in rats given 0.40
mg/kg/day of abamectin showed decreased lactation, increased stillbirths
and an increased likelihood of producing unhealthy offspring,
demonstrating a strong chance of similar effects in humans at high enough
doses. Abamectin is also very toxic to fish and aquatic invertebrates (FCH
2000). Environmental Fate
Abamectin
is broken down quickly in the soil via photodegradation at the soil
surface and microbial degradation in dark, aerobic conditions. The
chemicals half-life is about 1 week on an unshaded soil surface and about
two weeks to two months underneath the soil surface. It is also rapidly
broken down in water, its half-life being four days in pond water and two
to four weeks in pond sediment (ETN 1996). References: Extension
Toxicology Network (ETN). 1996. Pesticide Information Profiles: Abamectin.
<http://ace.orst.edu/cgi-bin/mfs/01/pips/abamecti.htm.> Farm
Chemicals Handbook 2000 (FCT). 2000. Meister Publishing Co. Willoughby, OH. Hoy,
M. and J. Conley. 1987. “Toxicity of pesticides to western predatory
mite.” California Agriculture 41:12-14. Moar,
W. and J. Trumble. 1987. “Biologically derived insecticides for use
against beet control.” California Agriculture Nov/Dec issue. Parella,
M. 1987. “Pest control.” Greenhouse Manager November: 105-108. Stinson,
S. 1988. “Total synthesis of avermectin achieved.” Chemical &
Engineering News January 4. U.S. EPA. 1987. Pesticide tolerances in food. Federal Register 52:17941. |
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BEYOND PESTICIDES |
