INSECTICIDAL ACTIVITY AND PERSISTENCE OF PHORATE, PHORATE SULFOXIDE, AND PHORATE SULFONE IN SOILS

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Abstract
Laboratory tests indicated that phorate, phorate sulfoxide, and phorate sulfone were toxic contact insecticides with broad spectrum activity. In general, p. sulfoxide was most toxic to insects > p. sulfone > phorate. When compared with other insecticides using 24–48 h old field crickets, Acheta pennsylvanicus (Burmeister), as test insects, the three compounds were less toxic by direct contact than carbofuran and fensulfothion, but as or more toxic than aldrin. P. sulfoxide and p. sulfone were strongly inactivated in moist Plainfield sand and were not as insecticidally active in soil against crickets as phorate. Nevertheless, the sulfoxide arid sulfone in soil were intermediate in toxicity between carbofuran and fensulfothion. The insecticidal activity of phorate, p. sulfoxide, and p. sulfone in soil was influenced by soil type, moisture, and temperature. Fumigant toxicity tests indicated that the three compounds volatilized from soil. Chemical analysis showed p. sulfone was most persistent in sand and muck > p, sulfoxide > phorate. Oxidation was observed to be a major degradation reaction for phorate and p. sulfoxide in both soils. Parallel bioassay tests indicated that, in sand, the biological action of "phorate" was due to the joint action of the three compounds. In a field study phorate EC, applied as a broadcast application at 3.4 kg AI/ha and incorporated into the top 7–8 cm of soil was slightly more persistent in muck than in sand. More than 99% and 98% of the applied phorate and its oxidation products disappeared from the sand and muck, respectively, within a year of treatment, indicating that the residues were only moderately persistent under climatic conditions encountered in southwestern Ontario. Low residues of p. sulfoxide and p. sulfone were detected in radishes grown on both soils in the first year but none were present in the second year. No residues were detected in carrots grown on either soil.