Metabolic pathway
In laboratory studies, emamectin benzoate is relatively stable to aqueous
hydrolysis (acid, neutral and alkaline) under dark conditions. Emamectin
benzoate degrades extensively in soil via microbial action to multiple
degradation products including the 8a-oxidation and 8a-hydroxylation
products. CO2 and soil-bound residues were the major terminal residues
which are incorporated into humic and fulvic acid fractions. In plants,
emamectin benzoate degrades extensively to multiple polar components
including the N-demethylated, N-formylated and conjugated products.
Probably due to rapid elimination in faeces, the metabolic transformation
of emamectin benzoate in animals is minimal. However, N-demethylation
was observed as the major metabolic pathway. The hydrolytic, photolytic
and overall metabolic pathways of emamectin benzoate in soils, plants
and animals are presented in Schemes 1,2 and 3.
Degradation
Emamectin benzoate (1) was relatively stable to hydrolysis in acidic, neutral
and alkaline solutions (pH 5, 7 and 9) at 25 °C, with less than 10%
degradation occurring after 30 days. It hydrolysed most rapidly in acidic
solution (pH 5) at 25 °C with a DT50 of 136 days (Chukwudebe, 1992). Two
polar components (each <10%) eluted close to the void volume under
reversed phase chromatographic conditions.
The photodegradation half-lives of emamectin benzoate (10-12 mg 1-1)
in buffered distilled water (pH 7) containing 1% (v/v) acetonitrile, ethanol
or acetone as cosolvent under continuous exposure to a xenon lamp
were 65,9 and 0.5 days, respectively. In both buffered and natural pond
water, the major photodegradation products included 8,9-MAB1a (2), 8a-hydroxylated
MAB1a (3) and minor unknown polar residues (Scheme 1).
In sensitised buffered water, 8a-oxo-MAB1a (4) and 10,11-14,15-MAB1a
diepoxide (5) were found as additional residual products (Mushtaq et al.,
1997).
