Varying Susceptibility to Methomyl and Permethrin in Widely Separated Cabbage Looper (Lepidoptera: Noctuidae) Populations within Eastern North America1

Cultures of Trichoplusia ni (Hübner), collected during 1980 from 11 eastern states and California, were assessed for susceptibility to methomyl and permethrin. T. ni were more susceptible to permethrin than methomyl, and the ratio of LD50 values (methomyl to permethrin) within each location ranged from 28:1 to 691:1. LD50 values for permethrin varied by a factor of 15 between locations, compared with 8.2 for methomyl. LD50 values for the California culture were similar to those of its eastern counterparts. There was no significant trend indicating that susceptibility to methomyl influenced susceptibility to permethrin. Although susceptibility to methomyl varied between geographic strains, there was no consistent pattern of increased tolerance in northern populations due to migration during a single growing season.