CLINE IN FREQUENCY OF AZINPHOSMETHYL RESISTANCE IN LIGHT BROWN APPLE MOTH (LEPIDOPTERA, TORTRICIDAE)

A rapid and highly significant drop in frequency of the azinphosmethyl -resistant phenotype of Epiphyas postvittana (Walker) was detected alo ng the coastline away from the center of a small group of apple orchar ds. This decline was detected by three assessment methods: (1) topical application of males attracted by sex pheromone, (2) bioassays of the offspring from tethered susceptible females mated to wild males, and (3) bioassays of the offspring from mated, field-collected females. Fr equencies of susceptible-susceptible crosses were not significantly di fferent in (2) and (3) assuming Hardy-Weinbe-Tg equilibrium. The estim ated size of the selection coefficient favoring the susceptible phenot ype was 0.004; the slope of the cline was estimated to be 0.7 x 10(-3) units of susceptible phenotype frequency per M. We used electrophores is to study allelic frequencies from four sites in the Nelson district and one in Canterbury. Five polymorphic loci were detected for phosph oglucomutase (PGM), phosphoglucose isomerase (GPI), isocitrate dehydro genase (IDH A and B), and mannose phosphate isomerase (MPI). Samples f rom four sites in the Nelson district (three at Mariri) showed no diff erence in allelic frequencies with PGM, GPI, and IDH, although a diffe rence between sites 20 km apart was detected for MPI. The average hete rozygosity per locus fit the expected values for an interbreeding popu lation, indicating that Hardy-Weinberg criteria were met at scrub and orchard sites at Mariri (where the localized insecticide-resistant pop ulation is present).