Adaptive significance of precocious pupation in the bean blister beetle, Epicauta gorhami (Coleoptera: Meloidae), a hypermetamorphic insect.

Abstract

Larvae of the bean blister beetle, Epicauta gorhami Marseul (Coleoptera: Meloidae), feed on grasshopper eggs in soil and undergo hypermetamorphosis. They normally enter diapause as a pseudopupa at the fifth instar, a form characteristic of hypermetamorphosis for meloid beetles. However, fourth-instar larvae exposed to long days and high temperature avoid pseudopupal diapause and pupate directly from the fourth instar. Fourth-instar larvae also tend to pupate precociously with a smaller body size if they are deprived of food. In these larvae, the critical day-length controlling induction of pseudopupal diapause becomes shorter than that for fully-fed larvae. In this study, we examined how the reaction norm of food-deprived E. gorhami larvae functions in nature by rearing insects from the egg stage outdoors in different seasons with manipulation of the food supply. The results indicated that most fully-fed larvae entered pseudopupal diapause, whereas food-deprived larvae tended to pupate precociously without entering diapause, especially early in the season. The resulting smaller adults reproduced early in the autumn and their progeny attained the pseudopupal stage before winter, indicating that the reaction norm may have an adaptive role in controlling seasonal development in the face of food shortages, producing a bivoltine life cycle.