Structure and dynamics of a parasitoid community attacking larvae of Psilocorsis quercicella (Lepidoptera : Oecophoridae)

This study investigated the temporal patterns of parasitoid attack and dive rsity within and among years and the potential ecological factors influenci ng the incidence of parasitism experienced by the larvae of a bivoltine mot h, Psilocorsis quercicella Clemens. To do this, field-collected larvae were reared throughout both generations during 1995-1997 in eastern Missouri. A nnual variation in percentage parasitism was extremely low, ranging from 16 .2 to 18.1%. Low annual variation in parasitism may contribute to host popu lation stability. In contrast, intergenerational differences in percentage parasitism were highly significant in 2 of the 3 yr of study, with higher r ates in the Ist generation (G1) than in the 2nd generation (G2). Instar at collection was a significant predictor of percentage parasitism during G1 b ut not during G2: this difference was likely caused by variation in the fre quency of attack by idiobiont parasitoids in the 2 generations. Ten species of parasitoids were positively identified from the rearings. Of these, 3 w ere new records and 1 was a new species. A consistent increase in parasitoi d species richness from G1 to G2 was found for all 3 yr, with several gener alist parasitoids emerging only from larvae collected during G2. Host-switc hing behaviors related to seasonal differences in the abundance of a congen er, 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 indepen dent of the density of both conspecifics and heterospecifies occupying a le aftie. A field experiment examining the effects of prior tie occupancy on p arasitism 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%).