Summer diapause in the polymorphic life cycle of the noctuid moth Mamestra brassicae

Abstract

The southern life cycle of Mamestra brassicae (Lepidoptera: Noctuidae) is characterized by two phenologically contrasting forms of pupal diapause. In the strain from Kagoshima (southern Japan, 31.5 {\textdegree}N) at 25 {\textdegree}C, summer diapause was prevalent in 14.5 h or longer days, and winter diapause in 13 h or shorter days. Nondiapause development occurred only to a limited extent in intermediate days. The direction of changes in daylength affected the number of long days required for 50% induction of summer diapause: the required day number was 10 in transfers from long day (LD, 15 h) to short day (SD, 11 h) but only 5 in the reverse transfer. Conversely, the induction of winter diapause required fewer days in transfers from LD to SD than in the reverse. Summer diapause required a smaller number of inductive light cycles than did winter diapause. The larvae were more sensitive to the summer-diapause-inducing effect of LD experienced from the middle extending into the second half of larval life than in the early or late stages of development.