Seasonal changes in the population of the pine needle gall midge, Thecodiplosis japonensis UCHIDA et INOUYE (Diptera: Cecidomyiidae)

Abstract

The life cycle and population dynamics of two generations of the pine needle gall midge, Thecodiplosis japonensis UCHIDA et INOUYE, were studied from 1975 to 1978. This species is univoltine with its life cycle divided into three stages; the stage of adult emergence to the formation of galls by hatched larvae from May to July, that of the 1 st to 3rd instar larvae in the galls from June to October, and that of the 3rd instar larvae and pupae overwintering in the soil. The patterns of the survivorship curves estimated for the two generations were similar and characterized by drastic numerical decrease before the formation of galls and during overwintering in the soil, but by low mortality of larvae in the galls. The failure to achieve maximum fecundity and the high mortality of newly hatched larvae explain the population decrease before the formation of galls. The mortality rate of the overwintering population was about 85 percent, of which 25 percent were accounted for by the parasitism of Inostemma sp. and Platygaster sp. The changes in the distribution patterns throughout the stages are examined and discussed along with the mortality factors. The characteristics of the survivorship curve of this species are compared with those of other gall-forming insects.