Diapause Responses to Photoperiod and Night Interruption in Mamestra brassicae (Lepidoptera : Noctuidae)

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Abstract
The pupae of M. brassicae underwent 2 distinct types of diapause in response to larval photoperiod. A long winter diapause was induced in 13L:11D [light:dark] or longer night lengths, and a short summer diapause in 15L:9D or shorter night lengths. Nondiapause development was prevalent in 14L:10D. A 1 h light break reversed the long-night effects of 4L:20D to 13L:11D only when it occurred .apprx. 9 h after dusk. In the early scotophase, a light pulse of 3 h reversed the long-night effect, but a 2 h and shorter ones were ineffective. Short pulses modified the responses to later night interruptions. In 14L:10D, night interruptions induced winter diapause at hour 4-6 from dusk and summer diapause at hour 8. In 15L:9D and 16L:8D, the incidence of summer diapause varied with the time of night interruption reciprocally. These results suggest various actions of an interrupting light pulse on the time-measuring dark reaction: resetting; exerting a coincidence effect and modifying the rate or shifting the phase. The involvement of a light reaction in the time-measurement was suspected, particularly from the summer-diapause response.