The aerobic soil degradation of spinosad ‐ a novel natural insect control agent

Abstract

Spinosad is a natural product with biological activity against a range of insects including lepidoptera. It is comprised of two major components namely spinosyns A and D. The degradation of spinosad in soil under aerobic conditions was investigated using two U.S. soils (a silt loam and a sandy loam) which were treated with either ¹⁴C‐spinosyn A or ‐spinosyn D at a 2X use rate of 0.4mg/kg soil for spinosyn A and 0.1mg/kg for spinosyn D. Further samples of soil were pre‐sterilised prior to treatment in order to establish whether spinosyns A and D degrade abiotically. Flasks of treated soil were incubated in the dark at 25°C for up to one year after treatment.

HPLC and LC‐MS of soil extracts confirmed that the major degradation product of spinosyn A was spinosyn B, resulting from demethylation on the forosamine sugar. Other dégradâtes were hydroxylation products of spinosyns A and B, with hydroxylation probably taking place on the aglycone portion of the molecule. Half lives were similar for both spinosyns and were in the range 9–17 days, with longer half lives in the pre‐sterilised soils (128–240 days) suggesting that degradation was largely microbial.