Adaptation to the newland: rapid voltinism change in a lepidopteran insect

Takehiko YAMANAKA (NIAES, Japan)

Sadahiro TATSUKI (The University of Tokyo, Japan)

Masakazu SHIMADA (The University of Tokyo, Japan)

The fall webworm, (Hyphantria cunea Drury) invaded Japan at Tokyo in 1945 and expanded its distribution gradually into northern and south-western Japan. All populations in Japan were bivoltine (two generations per year) until the early 1970s, at which time trivoltine (three generations per year) populations appeared in several southern regions. Presently, H. cunea exists as separate bivoltine and trivoltine populations divided around latitude 36^o. In the course of this voltinism change, the mean surface temperature in Japan rose by 1 ^oC.

To determine how this temperature increase might be responsible for the voltinism change, an age-structured model was constructed incorporating growth speed driven by actual daily temperature and detailed mechanisms of diapause induction triggered by both daily photoperiod and temperature. The simulation result suggests that both the acceleration of the growth speed and the prolongation of diapause induction are necessary to cause changes in voltinism, regardless of temperature increase. We concluded that the H. cunea population changed its life-history traits as an adaptation parallel with its invasion into the south-western parts of Japan.