Photoperiodic Programming of Summer Diapause after Hibernation in Spilarctia imparilis BUTLER (Lepidoptera : Arctiidae)

Abstract

The larva of Spilarctia imparilis regains sensitivity to photoperiod after hibernation and thereby controls the duration of pupal stage in summer. At 20°C, a long photoperiod (16hr) shortened, and a short one (12hr) prolonged, the pupal stage. The prolonged pupal stage showed a Q₁₀ value smaller than that for morphogenesis and thus the intervention of diapause. The duration of diapause varied as a function of photoperiod, and an intermediate photoperiod (14hr) produced pupae with intermediate and highly variable durations. When larvae were reared at 28°C, the photoperiodic response was obscure, the pupal diapause being uniformly short though not entirely eliminated. The larvae were most sensitive to photoperiod soon after hibernation and became less so as they grew. When they were transferred from LD 12 : 12 to 16 : 8 at various stages, the duration of pupal stage was inversely proportional to the number of long days. The observed diapause behaviour seems to be effective for programming the adult eclosion to occur after the long-day season. This ensures the offsrping to enter a larval diapause under the influence of short days before winter. The summer diapause thus functions as a mechanism to stabilize the univoltine life cycle by skipping the long days.