Developmental polymorphism: a major factor for understanding sublethal effects of Bacillus thuringiensis

Developmental polymorphism in Choristoneura fumiferana (Clemens) (Lepidopte ra: Tortricidae) was induced by Bacillus thuringiensis (Berliner) in the la boratory. The B.t. formulation concentration induced a quadratic response o n the incidence of developmental polymorphism. A double application of Baci llus thuringiensis at fourth and fifth instar respectively resulted in a hi gher occurrence of developmental polymorphism than a single application at fifth instar. Smaller larval weight prior to B.t. exposure resulted in high er incidence of developmental polymorphism. However, larvae exhibiting deve lopmental polymorphism exhibited larger pupae and an extended larval period . Thus, developmental poly morphism and Bacillus thuringiensis ingestion to gether prolonged the development time but induced opposite effects an pupal weight. These findings suggest that the calculation of sublethal effects o n pupal weight and development lime that include developmental polymorphism as an uncontrolled source of variation could lead to an inadequate underst anding of insect response in any stress-induced experiments.