Resistance to Bacillus thuringiensis in Gypsy Moth (Lepidoptera: Lymantriidae): Genetic and Environmental Causes

Second-instar gypsy moths ( Lymantrla dispar L.) from three wild populations and one laboratory population were challenged with Bacillus thurlngiensts subspecies kurstaki (HD-l strain), which was incorporated into synthetic diet at concentrations ranging from 10 to 295 international units (IU) per ml. Susceptibility among the 16-19 families within each of the four populations varied significantly. Families within a population had variable regression coefficients. Significant variation in LC50’s suggested the potential for resistance development through natural selection. Significant variation among populations was observed; the average LC50’s for three wild populations and the laboratory strain were 76, 106, 121, and 180 IU/ml diet, respectively. Variation in B. thuringiensts susceptibility within families was measured by comparing LC50’s of siblings from eggs of an egg mass laid first versus laid last (egg mass position is correlated with timing of maternal provisioning). We found that differential egg provisioning among eggs of a single mother yielded offspring with differential sensitivities to B. thuringiensts; the LC50 of larvae from eggs laid first versus those laid last averaged 401 IU/ml diet and 211 IU/ml diet, respectively. Based on oviposition sequence, qualitative differences among siblings accounted for 42% of the total variation in B. thurlngiensts susceptibility, whereas familial differences (due to genetic and mean maternal effect differences among families) accounted for 16% of the total variation. We hypothesize that variation in susceptibility to B. thuringiensts in the gypsy moth is based on vigor differences in growth and developmental capability, attributes that are the product of both genotype and the maternally determined nutritional status of the egg.