Persistence, Degradation, and Bioactivity of Phorate and its Oxidative Analogues in Soil1

The persistence of phorate and its 5 oxidative analogues in soil was determined with thin-layer chromatography, gas-liquid chromatography, and insect bioassay. Phorate and its oxygen analogue were rapidly oxidized to their respective sulfoxides and sulfones. Phorate and its sulfoxide and sulfone persisted beyond 16 weeks in a siltloam at 25°C. The thiolate analogues degraded to low levels within 2 to 8 days. Phorate was highly toxic to a cricket, Achela pennsylvanicus (Bmweister), in direct soil bioassay tests. A lack of contact and fumigant toxicity against crickets suggested that phorate sulfoxide and phorate sulfone were tightly absorbed by soil constituents. The toxicity of soil treated with phorate sulfoxide was greater several weeks after application than it was immediately following treatment because small amounts of the sulfoxide were converted to phorate. The conversion of phorate sulfoxide to phorate in soil was verified with gas chromatography, column chromatography, and insect bioassay.