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Five different phosphorothioate insecticides, methyl parathion, fenthion, methidathion, azinphos-methyl, and malathion, and a phosphoramidothioate, acephate, were exposed as thin films to irradiation by sunlight and ultraviolet light. With the exception of acephate, measurable amounts of different trimethyl phosphorothioate and dithioate esters, some of them having delayed toxic activity, were formed from these insecticides as a result of irradiation. The extent of photoalteration to these low molecular weight trialkyl phosphorothioates had a strong link with the UV extinction coefficient, as well as absorption maxima of the various insecticides. On the basis of known oral toxicities of the trimethyl phosphorothioate and dithioate esters and the recommended field use rates of the test insecticides, the levels of the photoalteration products formed are not expected to pose toxic hazards to farm workers.Previous reports from this laboratory and the Medical Research Council Toxicology Unit (Carshalton, England) have described the unusual delayed toxic properties of 0,0,S-trimethyl phosphorothioate (1) (Mallipudi et al., 1979;Umetsu et al., 1981) and 0,S,S-trimethyl phosphorodithioate (2) (Aldridge et al., 1979). These compounds, which have been shown to exist as minor contaminants in malathion and other organophosphorus insecticides, caused delayed death in rats following a single oral dose as low as 15 mg/kg. Moreover, 1, 2, and other related organophosphorus impurities also were found to potentiate the acute mammalian toxicity of carboxylic acid ester containing organophosphorus insecticides such as malathion and phenthoate (Pellegrini and Santi, 1972; Umetsu et al., 1977). These impurities are introduced as side products formed during the various synthetic steps leading to the technical material.

The half-life of [14C]4‘‘-deoxy-4‘‘-(epi-methylamino)avermectin B1a (MAB1a) benzoate (1 ppm) photodegradation in buffer (pH 7), natural pond water, and sensitized buffer (1% acetone in pH 7 buffer) determined at Three Bridges, NJ (latitude ∼40° N) during the fall season under natural sunlight was 22, 7, and 1 days, respectively. The half-life of [14C]MAB1a benzoate (10−12 ppm) photodegradation in buffer (pH 7) containing 1% (v/v) acetonitrile, ethanol, or acetone as cosolvent under continuous exposure with a xenon lamp was 64.5, 8.5, or 0.5 days, respectively. The photoisomer 8,9-Z-MAB1a, 8a-hydroxy-MAB1a, and unknown polar residues were found in light-exposed samples of MAB1a in buffer and natural pond water. In light-exposed sensitized buffer samples, 8a-oxo-MAB1a and MAB1a-10,11-14,15-diepoxide were additional products. Very polar residues found in the organic and aqueous phases after extraction increased with time, and their formation followed the order sensitized buffer ≫ natural pond water > buffer. Keywords: MK-0244; emamectin benzoate; photodegradation; residues

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The persistence and residual contact toxicity of foliar dislodgeable emamectin benzoate residues to Apis mellifera (L.) and Diglyphus isaea (Walker) were studied following applications to foliage at maximum anticipated labeled use rate (0.0168 kg of active ingredient/ha). Bee mortality from contact exposure to dislodgeable alfalfa residues aged for 3, 8, and 24 h was 100, 46, and 3%, respectively. This contact toxicity was directly correlated with the magnitude of dislodgeable residues (9.1, 3.6, and 1.3 ng/cm2, respectively), with a dissipation half-life of about 10 h. Similarly, D. isaea mortality from contact exposure to dislodgeable residues aged for 0.5, 1.0, 3.5, 35, and 60 h was 75, 65, 50, 20, and 15%, respectively; these residues (5.2, 5.0, 4.5, 1.4, and <0.2 ng/cm2, respectively) had a dissipation half-life of about 15 h. Therefore, these beneficial insects are expected to survive and colonize treated crops within relatively short intervals (≤24 h) after applications of emamectin benzoate. Keywords: Avermectin; foliar dislodgeable residues; beneficial insects; residual contact toxicity; emamectin benzoate