Comparative Fitness of Malathion-Resistant and Susceptible Indianmeal moth (Lepidoptera: Pyralidae)2

Abstract

Three factors contributing to fitness of a species (fertility, fecundity, and development time) were measured in the susceptible Savlab strain and the > 100-fold malathion-resistant Statesboro strain of the Indianmeal moth, Plodia interpunctella (Hübner). Both fertility (percent egg hatch) and fecundity (eggs/female) were significantly lower in the resistant strain. Percent egg hatch in the two strains was 96.2% and 73.1% for Savlab and Statesboro respectively. Fecundity was 444 eggs/female for Savlab and 321 eggs/female for Statesboro. Development time did not differ significantly (24.2 ± 0.3 and 23.6 ± 0.2 days for 50% emergence). There was no evidence that these differences arose from non-lethal effects caused by malathion treatment to maintain resistance in the Statesboro strain. The differences, while associated with malathion resistance, cannot be unequivocally attributed to the presence of the resistance gene. When the intrinsic rate of increase (rm) was calculated using an iterative computer program, Statesboro had a significantly lower rate (0.1734 ± 0.0016) than did Savlab (0.2048 ± 0.0014). Similar measurements were performed with progeny from reciprocal F1 crosses. While some parameter's values were closer to Statesboro or Savlab an apparent mating incompatibility suggests that using the data from F1 crosses is premature until this question is resolved.