Seasonal adaptations of the fall webworm <Emphasis Type="Italic">Hyphantria cunea</Emphasis> (Drury) (Lepidoptera: Arctiidae) following its invasion of Japan

The fall webworm, Hyphantria cunea (Drury) (Lepidoptera: Arctiidae), invaded Japan from North America about 60 years ago. Immediately after its invasion – and for the first three decades – its life cycle was bivoltine, two generations per year throughout its entire distribution in Japan. Thereafter, its life cycle shifted to trivoltine in the southwestern areas of Japan. In the present study we examined the life-history traits proposed to be implicated in this event with the aim of clarifying the mechanism of this life-cycle shift. The critical photoperiod for diapause induction, as defined by the photoperiod at which 50% of individuals enter diapause, was shorter in the trivoltine populations than in their bivoltine counterparts. The temperature sensitivity of the photoperiodic response, as defined by the difference in the critical photoperiod between 20 and 25°C, was greater in the trivoltine populations than in the bivoltine ones. The geographic variation in larval and pupal periods was positively correlated to the latitude of the original localities of the populations. The change in the number of larval instars would be one of the main factors accounting for the regional differences in the developmental period. These results suggest that some life-history traits of H. cunea have changed following its invasion of Japan as an adaptive response to local climates.