Pesticides and Herbicides Corner 

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Pesticide: ABAMECTIN
Also known as Avermectin B1 and MK-936. Trade names include Affirm, Agri-Mek, Avid, Dynamec, Vertimec and Zephyr.
INTRODUCTION
Abamectin is a mixture of avermectins containing > 80% avermectin B1a and < 20% avermectin B1b ( 1 ). These two components, B1a and B1b have very similar biological and toxicological properties (5 ). The avermectins are insecticidal or anthelmintic compounds derived from the soil bacterium Streptomyces avermitilis (2 ). Abamectin is a natural fermentation product of this bacterium (5 ).

Abamectin is used to control insect and mite pests of a range of agronomic, fruit, vegetable and ornamental crops, and it is used by homeowners for control of fire ants (5 ). Doses of 50 to 200 ug/kg of ivermectin, a similar member of the avermectin family of comounds, is widely used to treat humans in the World Health Organization onchocerciasis (river blindness) program (2 , 8 ).
TOXICOLOGICAL EFFECTS
ACUTE TOXICITY
Abamectin is a highly toxic material, however most formulated products containing abamectin are of low toxicity to mammals (5 , 7 ). Emulsifiable concentrate formulations may cause moderate eye irritation and mild skin irritation ( 1 ). Symptoms of poisoning observed in laboratory animals include pupil dilation, vomiting, convulsions and/or tremors, and coma (5 ).
Abamectin acts on insects by interfering with neural and neuromuscular transmission. It acts on a specific type of synapse located only within the brain and is protected by the blood-brain barrier. However, at very high doses, the mammalian blood-brain barrier can be penetrated, causing symptoms of CNS depression such as incoordination, tremors, lethargy, excitation and pupil dilation. Very high doses have caused death from respiratory failure (2 ).
Abamectin is not readily absorbed through skin. Tests with monkeys show that less than 1% of dermally applied abamectin was absorbed into the bloodstream through the skin (5 ). Abamectin does not cause allergic skin reactions (7 ).

Abamectin is relatively non-toxic to birds

Abamectin is highly toxic to fish and aquatic invertebrate

Effects on Other Animals (Nontarget species)
Abamectin is highly toxic to bees.

ENVIRONMENTAL FATE
Breakdown of Chemical in Soil and Groundwater
Because abamectin is nearly insoluble in water and has a strong tendency to bind to soil particles, it is therefore immobile in soil and unlikely to leach or contaminate groundwater (6 , 7 ). Compounds produced by the degradation of abamectin are also immobile and unlikely to contaminate groundwater (6 ).
Abamectin is rapidly degraded in soil. At the soil surface, it is subject to rapid photodegradation, with half-lives of 8 and 21 hours (6 ) or 1 day ( 7 ) reported. When applied to the soil surface and not shaded, its soil half-life was about 1 week. Under dark, aerobic conditions, the soil half-life was 2 weeks to 2 months (7 ).

The half-life for avermectin B1a in fine sandy loam, clay and construction grade sand was 20 to 47 days. Loss of abamectin from these soils is thought to be due to microbial degradation because abamectin remained undegraded in sterile soil. The rate of degradation was significantly decreased under anaerobic conditions ( 6 ).
Breakdown of Chemical in Surface Water
Abamectin is rapidly degraded in water. After an initial distribution, its half-life in artificial pond water was 4 days. Its half- life in pond sediment was 2 to 4 weeks (6 ). It undergoes rapid photodegradation, with a half-life of 12 hours in water (6 , 7 ).

When tested at pH levels common to surface and groundwater (pH 5, 7, and 9), abamectin did not hydrolyze ( 6 ).
Breakdown of Chemical in Vegetation
Plants do not absorb abamectin from the soil (6 ). Abamectin is subject to rapid degradation when present as a thin film, as on treated leaf surfaces. Under laboratory conditions and in the presence of light, its half-life as a thin film was 4 to 6 hours

 

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