The Effects of Moisture and Microorganisms on the Persistence and Metabolism of Some Organophosphorous Insecticides in Soils, with Special Emphasis on Parathion12

Abstract

Parathion, methyl parathion, and malathion were applied at 5 lb/acre to Carrington silt loam plots. Residue levels of approximately 0.1 ppm (3.1% of the applied dosage) were reached under field conditions with malathion within 8 days, with methyl parathion within 30 days, and with parathion within 90 days after the insecticidal application to the soil. Under laboratory conditions (30° C), 95% of the applied methyl parathion and 30% of the parathion were lost within 12 days after application to a loam soil. The effects of moisture and microorganisms on the persistence and metabolism of parathion in Carrington silt loam soil were tested under laboratory conditions at 30° C. The insecticide was most persistent in dry soil and least persistent in soils with a high moisture content. Parathion was not lost through volatilization, as is the case with some insecticides of the chlorinated hydrocarbon group. Degradation of parathion was either by hydrolysis or by reduction to its amino form depending on populations of soil microorganisms. In soils of low numbers of microorganisms (autoclaved soil) or of low microorganism activity (dry soils) , parathion persisted for a relatively long time. No amino parathion was formed in autoclaved soils. Yeast was primarily responsible for the reduction of parathion in soil to aminoparathion. Bacteria apparently had no effect on this reduction. Para-oxon was hydrolyzed within 12 hours after its application at 20 ppm to loam soil. During that time the amount of para-oxon constantly decreased while the amount of p -nitrophenol produced increased. Amino parathion or p .nitrophenol, added to soils, disappeared within 2 and 16 days, respectively.