Changes in life-history traits in the Fall Webworm within half a century of introduction to Japan

1. The Fall Webworm, Hyphantria cunea, introduced into Japan from North America in 1945, was bivoltine until the early 1970s throughout its distribution area, but thereafter shifted to be trivoltine in south-western areas of Japan. 2. To clarify changes of life-history traits in relation to the shift in voltinism and to delineate the boundary between bi- and trivoltinism, the developmental rate and photoperiodic response controlling the induction of pupal diapause were investigated in 13 geographic populations. 3. Geographic variation in the critical photoperiod for diapause induction was not a simple dine, but shifted step-wise. The critical photoperiod was longer in populations north of 36 degrees N than in populations south of 36 degrees N, and the interpopulation variance for the critical photoperiod was small in each area. 4. The duration of the larval stage was significantly shorter in the southern populations than in the northern ones, but there was no conspicuous break point in its dine. 5. Geographic variation in pupal development followed a similar pattern to that observed for larval development, although the variation was comparatively small. 6. Thus, both critical photoperiod and developmental rate decreased when the shift in voltinism from bivoltine to trivoltine occurred as this insect expanded its range. The transitional zone between bi- and trivoltinism was around 36 degrees N.