Northerly shift in voltinism watershed in Hyphantria cunea (Drury) (Lepidoptera: Arctiidae) along the Japan Sea coast: Evidence of global warming?

The effects of climate change on the northern limit of the trivoltine area were clarified in the fall webworm, Hyphantria cunea (Drury) (Lepidoptera: Arctiidae). The Takaoka and Kanazawa populations used in the present study occur in locations to the north of Fukui (36.07 degrees N), where H. cunea has recently shifted from a bivoltine to a predominantly trivoltine life cycle. The life-history traits of these populations were investigated and compared to those in the Fukui population. The lower threshold temperature for development and the thermal constant for one generation were 11.3 degrees C and 674.5 degrees d in the Takaoka population, and 11.2 degrees C and 680.7 degrees d in the Kanazawa population. The critical photoperiod for diapause induction at 25 degrees C was 14 h 34 min in the Takaoka population and 14 h 28 min in the Kanazawa population. These critical photoperiods at 25 degrees C were longer by 24 min and 18 min than the Fukui population, respectively. These results, together with climate data, suggest that the Takaoka population maintains a bivoltine life cycle, and a small part of the Kanazawa population has three generations per year, a predominantly bivoltine life cycle. Thus, the present northern limit of the trivoltine area lies around Fukui in districts along the Japan Sea.