PERSISTENCE AND ACTIVITY OF MEXACARBATE AND ITS METABOLITES AGAINST SPRUCE BUDWORM LARVAE

This study was conducted to determine if metabolites of mexacarbate contribute to its residual toxicity to spruce budworm (Choristoneura fumiferana) larvae. Potted white spruce (Picea glauca) trees were treated with 'Zectran' UCZF # 19 at 100 g a.i. ha-1 and kept in a glasshouse. Mexacarbate residues declined by 98-99% within three days and reached non-detectable levels 10 days after treatment. Mortality of larvae fed on buds from these trees declined more gradually and was still 19-27% when exposed 10 days after treatment. The very low levels of mexacarbate (< 0.07 mug g-1) found after three days did not produce such mortality. Gas chromatographic analysis of metabolites in needles revealed that after three days, 4-methylformamido-3,5-xylyl N-methylcarbamate was present at levels 20-30 times higher than the parent compound. This metabolite was about 50 times less toxic than mexacarbate to larvae when applied topically but was only 7 times less toxic when ingested. Two other methylcarbamate metabolites, the amino, and methylamino analogues were detectable for one day following treatment but not at later time points. They were as toxic as mexacarbate both topically and orally. Based on these findings, the methylformamido analogue could contribute to the residual toxicity of mexacarbate treatments of spruce.