After the fall webworm, Hyphantria cunea, was introduced into Japan from North America in 1945, a bivoltine life cycle was maintained throughout its distribution in Japan for three decades (MASAKI, 1975). The critical photoperiod for the induction of pupal diapause was 14 h 35 min (25 degrees C) in two local populations at 35.3 and 39.5 degrees N, as in the founder population (MASAKI et al., 1968), and was relatively stable over a range of temperatures from 17 to 25 degrees C (MASAKI, 1977). Recently, a trivoltine life cycle was found in localities south of 36 degrees N, although bivoltinism is still the rule in the northern areas (Gomi and TAKEDA, 1990, 1991). The bivoltine and trivoltine: populations show a significant difference in critical photoperiod for the induction of diapause (GOMI, 1992). This species is a long-day type and develops without diapause under long-day conditions. In this type of photoperiodic response, the critical photoperiod is generally shortened by high temperatures in insects (DANKS, 1987). The present study examines whether H. cunea populations derived from the bivoltine and trivoltine areas exhibit such a temperature-dependent change in critical photoperiod, and discusses the possibility that such a response is involved in the recent shift of the life cycle from bivoltine to trivoltine. In one of the two populations examined, I found a marked effect of temperature on the incidence of diapause. Whereas the stage sensitive to photoperiod for diapause induction is known to occur in early larval instars (MASAKI, 1977), the stage sensitive to temperature has not been studied. Because insects usually experience fluctuating temperatures in the field, such information is important in discussing the life cycle and voltinism in local populations. In die second half of this paper, therefore, I present data on the stage sensitive to temperature for diapause induction in this species.
