ENHANCED MICROBIAL-DEGRADATION IMPLICATED IN RAPID LOSS OF CHLORPYRIFOS FROM THE CONTROLLED-RELEASE FORMULATION SUSCON(R) BLUE IN SOIL

Microbial degradation is considered to have contributed to rapid loss of chlorpyrifos from a controlled-release formulation (suSCon(R) Blue) in Australian sugarcane where control of greyback canegrub has failed in pal ts of the Burdekin canegrowing district since at least 1992. T wo surveys showed that rapid loss of chlorpyrifos from granules is mor e likely to occur at sites where the formulation has been used in prev ious crop cycles, compared to sites where there has been no prior use. Fumigation of soils from two sites with rapid loss of chlorpyrifos re sulted in higher concentrations of chlorpyrifos and its metabolite, 3, 5,6-trichloro-2-pyridinol (TCP), accumulating in soil surrounding the granules, compared with non-fumigated soil. Addition of 10% non-fumiga ted soil to fumigated soil resulted in low concentrations of chlorpyri fos and TCP, with levels similar to those in non-fumigated soil. Granu les exposed in fumigated soil for 6 months had significantly higher co ncentrations of chlorpyrifos compared to granules in non-fumigated soi l, and in fumigated soil to which 10% non-fumigated soil had been adde d. In alkaline soils, chlorpyrifos hydrolyses readily to TCP. It is hy pothesised that microbial activity rapidly degrades TCP. Chlorpyrifos continues to hydrolyse to TCP at a rapid rate as pyridinol is lost fro m the soil, with faster diffusion from the granules due to the low con centration of chlorpyrifos in the surrounding soil. (C) 1998 Elsevier Science Ltd. All rights reserved.