Esterase Polymorphism and Response to Insecticides During Larval Development of the Western Spruce Budworm12

Abstract

Dose-mortality regressions for the insecticides acephate and carbaryl and their relationships to esterase polymorphism during larval development of Choristoneura occidentalis Freeman were exammed to estimate which instar might be most suitable for prespray electrophoretic monitoring. Such monitoring is being developed as a means of forecasting relative population tolerance to either toxicant. Although dosemortality regressions for instars 2 to 6 to acephate sprays applied to instars 2 to 6 were not significantly different, those for carbaryl sprays differed significantly both in response thresholds and change in response per unit dose. No consistent changes in esterase allele frequencies widi advancing stages of larval development were noted in untreated insects. Changes in allele frequency resulting from treatment of an instar and relative toxicological response of the instar appeared to be independent for both chemicals. In all instars treated with acephate, and in 6th instars treated with carbaryl, a marked reduction in frequency of the silent allele and associated increases in frequency of active alleles at the second esterase locus occurred. The 6th instar may be the most suitable for examination before carbaryl application; instars 2 to 6 may be equally useful for electrophoretic monitoring before application of acephate.