Abstract
The effects of temperature on the larval developmental traits of the fall webworm, Hyphantria cunea (Drury), were investigated at 18, 20, 23, 25 and 27°C. In Japan, this species has two types of larval instars, the six-instar and seven-instar. The proportion of the seven-instar type was significantly higher in females than in males at all temperatures. In females, the proportion of the seven-instar type was high at higher temperatures. However, the effect of temperature on the incidence of larval instar types was not detected in males. The larval developmental time was significantly longer in the seven-instar type than in the six-instar type for both sexes at all temperatures. The difference in the developmental time between the instar types decreased as temperature increased. The pupae of the seven-instar type were heavier than those of the six-instar type at higher temperatures. These results suggest that the fitness advantage of the seven-instar type is greater in females than in males, and is relatively high at higher temperatures because the disadvantage of the long developmental period in the seven-instar type is reduced by higher developmental rates.
