The Rate and Factors of Mortality from the Completion of Overwintering in Larvae to Emergence of Adults in a Nymphalid, Sasakia charonda (Hewitson)

Abstract

We investigated the rate and factors of mortality during the stages between the completion of overwintering in larvae and the emergence of adults in a nymphalid, Sasakia charonda, in Tochigi Prefecture, Japan. The number of larvae wintering in a host tree, Celts sinensis, varies among trees of similar size. The analysis of life tables for two generations of natural populations indicate that the mortality rates are higher during the 5th and 6th larval stages than in other instars. One factor responsible for mortality during these stages was predation by wasps. Predation by birds may also be a significant factor, because a large number of larvae disappeared within a short period of time, and many bird droppings were found on the foliage. Exclosure experiments suggest that predation by birds affects the larval mortality differently depending on the density of larvae in host trees. When the larval density in a tree was increased experimentally, the survival rate of larvae was significantly higher in the tree covered with a net excluding birds than in the control tree. When the density was lowered, however, survival rates did not differ significantly between the treatment and control. In conclusion, overwintering larvae of S. charonda may occur locally in host trees as small groups with various densities. The groups occurring in high density may suffer stronger mortality from factors such as predation by birds, and consequently, the numbers of adults emerging might be similar among host trees with different larval densities.