Abstract
The timing of diapause induction and overwintering success in the cotton bollworm Helicoverpa armigera were studied outdoors in Chiba (35°N, 140°E) and Kyoto (35°N, 135°E) prefectures, Japan, using local populations collected from Chiba and Mie (34°N, 136°E) Prefectures, respectively. For comparison, a similar experiment was performed in Chiba using a closely related species, H. assulta. Diapause induction in H. armigera was stimulated by low temperatures in autumn, and the incidence of pupal diapause increased sharply with little change in photoperiod. In Kyoto, pupae of H. armigera overwintered successfully in a state of diapause. However, larvae that had hatched from eggs deposited in late autumn suffered high mortality, presumably from the cold; therefore, the pattern of temperature change in autumn credibly affects overwintering success in H. armigera. In contrast, a considerable number of H. assulta pupae entered diapause well before autumn, even though those individuals had hatched from eggs deposited during the mid-summer. Thus, H. assulta appeared to prepare for overwintering earlier and more successfully than did H. armigera in this temperate zone. Differences in diapause induction between the sexes revealed in laboratory experiments with H. armigera were not detected under outdoor conditions.
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