Food-availability dependent premature metamorphosis in the bean blister beetle Epicauta gorhami (Coleoptera: Meloidae), a hypermetamorphic insect that feeds on grasshopper eggs in the larval stage

摘要

Larvae of the bean blister beetle, Epicauta gorhami, feed on only grasshopper eggs and undergo hypermetamorphosis with pseudopupal diapause in the fifth instar. Whether E.gorhami larvae enter pseudopupal diapause or pupate directly from the fourth instar is controlled by temperature and photoperiod. In nature, larvae are confronted with a significant variation in the availability of food, suggesting the possibility that feeding conditions may also affect the diapause incidence. Here, we addressed this issue by changing the feeding conditions in the fourth instar under conditions of 16h light:8h dark (LD16:8) at 25 degrees C. Food deprivation reduced the length of instar and increased the tendency to pupate, leading to the early eclosion of a small adult. Even non-feeding fourth-instar larvae pupated. Regardless of the timing of food deprivation, the post-feeding larval period was constant and equivalent to that of ad libitum-fed larvae, suggesting that premature exhaustion of the food supply triggers the initiation of pupation. In agreement with these results, when larvae were fed on intact grasshopper egg pods of various sizes from four species, those that fed on smaller egg pods had a decreased tendency to pseudopupate (i.e., to enter diapause). Food-deprived larvae showed a clearer photoperiodic response and had a shorter critical day-length. Thus, in E.gorhami, feeding conditions do not affect pupation success, but do affect the tendency to pupate or pseudopupate. This is the first report of the occurrence of premature pupation in carnivorous insects. We discuss our findings in the context of the natural history and behavioral ecology of E.gorhami.