POPULATION ECOLOGY Structure and Dynamics of a Parasitoid Community Attacking Larvae of Psilocorsis quercicella (Lepidoptera: Oecophoridae)

This study investigated the temporal patterns of parasitoid attack and diversity within and among years and the potential ecological factors inßuencing the incidence of parasitism experienced by the larvae of a bivoltine moth, Psilocorsis quercicella Clemens. To do this, Þeldcollected larvae were reared throughout both generations during 1995Ð1997 in eastern Missouri. Annual variation inpercentageparasitismwas extremely low, ranging from16.2 to 18.1%.Lowannual variation in parasitism may contribute to host population stability. In contrast, intergenerational differences in percentage parasitismwere highly signiÞcant in 2 of the 3 yr of study,with higher rates in the 1st generation (G1) than in the 2nd generation (G2). Instar at collection was a signiÞcant predictor of percentage parasitism during G1 but not during G2; this difference was likely caused by variation in the frequency of attack by idiobiont parasitoids in the 2 generations. Ten species of parasitoids were positively identiÞed from the rearings. Of these, 3 were new records and 1 was a new species. A consistent increase in parasitoid species richness from G1 to G2 was found for all 3 yr,with several generalist parasitoids emerging only from larvae collected duringG2.Host-switching behaviors related to seasonal differences in the abundance of a congener, P. reflexella Clemens, are hypothesized to contribute to the increased parasitoid diversity in G2. Although parasitoids were more likely to visit high-density ties, the risk of parasitism for individual larvae was independent of the density of both conspeciÞcs and heterospeciÞcs occupying a leaftie. A Þeld experiment examining the effects of prior tie occupancy on parasitism found that larvae placed in previously occupied ties were almost twice as likely to be parasitized (24.4%) as larvae placed in newly created ties (13.6%).