Abstract
Studies were carried out in four study plots during the years 1982-1985. A. areolatus parasitized the 1st and 3rd generations of P. californiensis and produced two generations in spring. Though the aphid infests maple trees all the year round, winged females appear several times in a year and produce larvae on the trees which provide a large amount of suitable feeding sites. When the winged females reached the trees and produced larvae, the wasps which emerged on other trees also came and parasitized the larvae intensely. Thus, the number of both the aphids and mummies on the trees increased markedly. The number of mummies in the 2nd generation correlated with the number of mummies in the 1st generation of the same year and the aphid density, and increased with increasing aphid density. However, even on the trees where the density of wasps was high and that of aphids low, less than 40% of the aphids were parasitized. Some aphids escaped from parasitism and deposited aestivating dimorphs which never became parasitized. This parasitoid was one of the important mortality factors of the aphids during the years covering the study. However the mortality of the aphid population was inversely proportional to the density.
