Pupal diapause of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) mediated by larval host plants: pupal weight is important.

Facultative diapause, a strategy that allows insects to initiate additional generations when conditions are favorable or to enter diapause when they are not, has a profound effect on the ecology and evolution of species. Most previous studies have concentrated on the role of photoperiod and temperature in inducing facultative diapause in insects. In contrast, here we studied pupal diapause mediated by larval host plants in the cotton bollworm Helicoverpa armigera, and confirmed that pupal weight is a critical factor. Two groups of third instar H. armigera larvae, kept at 25 °C with L:D=8:16 and 20 °C with photoperiod of L:D=8:16, respectively, were fed on six host plants and on artificial diet (as a control) to determine how larval host plants affect diapause incidence and related traits (such as pupal weight and developmental duration). The data showed larval host plants affected diapause incidence significantly and the effects could be masked by low temperature. Further analysis showed that pupal size, not the length of the sensitive stage, affected the decision to enter diapause. In a further experiment, third-instar to final-stage larvae deprived of artificial diet for 2 days demonstrated a direct relationship between pupal weight and diapause incidence. These results suggest that larval host plants, by affecting pupal size, may influence diapause occurrence in H. armigera. This has important adaptive significance for both over-wintering survival and the possibility for completing an additional generation.