Voltinism of the wild mulberry silkmoth, Bombyx mandarina (Lepidoptera: Bombycidae), is defined by spring temperature and autumn daylength in Japan

Abstract This study aims to elucidate how temperature and photoperiod interact across the annual lifecycle of the wild mulberry silkmoth, Bombyx mandarina (Moore) (Lepidoptera: Bombycidae), to determine the timing of generations and geographic variation in voltinism. We investigated the seasonal prevalence of B. mandarina across eight regions in Japan with a broad latitudinal and altitudinal range. Two peaks of occurrence were observed in cooler areas, while three peaks were found in warmer areas. The timing of the first occurrence was closely associated with spring temperature, leading to delayed emergence in cooler areas. Given that B. mandarina lays diapause eggs when exposed to short daylength conditions during the larval stage, the occurrence of a third generation depends on whether second‐generation larvae encounter daylengths longer than the critical threshold for diapause induction. In warmer regions, this condition was fulfilled, allowing a third generation to occur; in cooler regions, diapause induction in the second generation prevented further development. Our extensive field observations demonstrate that voltinism in B. mandarina is determined by the combination of spring temperature, which governs the timing of the first‐generation occurrence, and autumn daylength, which determines the egg diapause and termination of annual life cycle.