Geographic Variation in Photoperiodic Response for Induction of Pseudopupal Diapause in Epicauta gorhami (Coleoptera: Meloidae).

Geographical variations in the threshold of environmental cues for diapause induction are important in understanding the life history strategy of insects. Larvae of the bean blister beetle, Epicauta gorhami (Coleoptera: Meloidae), feed on grasshopper eggs and undergo hypermetamorphosis. They normally enter diapause as a pseudopupa (fifth instar). However, when the larvae are reared under long-day and high-temperature conditions, they do not enter pseudopupal diapause but pupate directly from the fourth instar. In addition, this insect is known to modify its photoperiodic response for induction of pseudopupal diapause depending on food availability for larvae. In this study, the larval photoperiodic responses for diapause induction were examined for three populations of E. gorhami collected from the northernmost (Morioka, 39.7°N), central (Kazo, 36.1°N), and southernmost (Takanabe, 32.1°N) parts of its distribution range, and the responses were examined under both fully fed and food-deprived (FD) conditions. Diapause incidence decreased in FD larvae for all populations in long-day conditions, and the critical day-lengths for the diapause incidence were calculated as 14.81, 13.97, and 13.99 h in the Morioka, Kazo, and Takanabe populations, respectively. The results indicate the presence of a geographical variation in larval photoperiodic response, in which diapause is induced earlier in higher latitudinal areas. From these results, together with data for developmental threshold temperature and thermal constant, the Morioka and Kazo populations were suggested to have a partial bivoltine life cycle under the conditions of extremely early oviposition season, a sufficient summer temperature, and poor larval food availability.