Bacillus thuringiensis (B.t.) delta-endotoxins provide an alternative to chemical insecticides for controlling many species of pest insects. Recent biotechnological developments offer the promise of even greater use of B.t. toxins in genetically transformed pest-resistant crops. However, the discovery that insects can adapt to these toxins raises concerns about the long-term usefulness of B.t. toxins. Several methods for managing the development of resistance to B.t. toxins have been suggested, but none of these approaches offer clear advantages in all situations.

Two Bacillus thuringiensis (Bt)-resistant strains of the Indianmeal moth, Plodia interpunctella, lack a major gut proteinase that activates Bt protoxins. The absence of this enzyme is genetically linked to larval survival on Bt-treated diets. When considered with previous data supporting the existence of receptor-mediated insect resistance to Bt, these results provide evidence that insect adaptation to these toxins occurs through multiple physiological mechanisms, which complicate efforts to prevent or manage resistance to Bt toxins in insect control programs.

Receptor binding studies show that resistance of a laboratory-selected Plodia interpunctella strain to a Bacillus thuringiensis insecticidal crystal protein (ICP) is correlated with a 50-fold reduction in affinity of the membrane receptor for this protein. The strain is sensitive to a second type of ICP that apparently recognizes a different receptor. Understanding the mechanism of resistance will provide strategies to prevent or delay resistance and hence prolong the usefulness of B. thuringiensis ICPs as environmentally safe insecticides.

Resistance to the spore-crystal protein complex of Bacillus thuringiensis, the most widely used and intensively studied microbial insecticide, has been presumed to be unlikely to occur. In this study it was found that Plodia interpunctella, a major lepidopteran pest of stored grain products, can develop resistance to the insecticide within a few generations. Resistance increased nearly 30-fold in two generations in a strain reared on diet treated with Bacillus thuringiensis and after 15 generations reached a plateau 100 times higher than the control level. Resistance was stable when selection was discontinued. The resistance was inherited as a recessive trait. Plodia interpunctella strains collected from treated grain bins were more resistant than strains from untreated bins, indicating that the resistance can develop quickly in the field.