Entomologia Experimentalis et Applicata 96: 281–287, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 281 Fitness costs of resistance to Bacillus thuringiensis in the Indianmeal

stored grain, insect, biocontrol Genetic changes in insects that result in insecticide resistance can also affect their fitness. Here, we report measure-ments of development time and survival of the Indianmeal moth, Plodia interpunctella (Hübner), to compare the relative fitness of Bacillus thuringiensis (Bt)-susceptible and-resistant colonies. Measurements of larval develop-ment time and survival indicated that a fitness cost was associated with resistance to Bt in some Bt-resistant colonies but not others. Comparisons of geographically different populations revealed inherent differences in development time and survival. In most cases, Bt-resistant moths suffered no disadvantage when feeding on a Bt-treated diet. In many cases, the development of Bt-resistant moths on Bt-treated diet was slower than the unselected moths on untreated diet, but it is unclear whether these differences would affect the successful mating of susceptible and resistant moths