RESISTANCE OF CHLORPYRIFOS TO ENHANCED BIODEGRADATION IN SOIL

Laboratory studies were conducted to determine whether the organophosphorus insecticide chlorpyrifos undergoes enhanced microbial degradation in soil. Repeated treatment of soils in the laboratory with chlorpyrifos did not alter the chlorpyrifos degradation rates or product distributions observed in four soils. Likewise, soils from three plots that received annual field applications of chlorpyrifos for 2-4 years did not develop an enhanced rate of chlorpyrifos degradation in laboratory degradation studies as compared to soils from untreated plots. Soils from fields in which a number of insecticides failed to control the target insect pests displayed short chlorpyrifos half-lives of between 4 and 9 days. The degradation of chlorpyrifos in these “problem” soils, which were highly alkaline (pH ≥8), was not microbially mediated and appeared to be a hydrolytic process. Accumulation or mineralization of the major chlorpyrifos hydrolysis product, 3,5,6-trichloro-2-pyridinol, was unrelated to the rate of chlorpyrifos hydrolysis observed. Results indicate that chlorpyrifos is not susceptible to enhanced microbial degradation and repeated chlorpyrifos application should have no effect on its persistence or efficacy. © 1990, American Chemical Society. All rights reserved.