Community Structure of Parasitoids Attacking Leaf Galls of Belonocnema treatae on Quercus fusiformis

Abstract

The research presented herein addresses the structure of the parasitoid community centered on the asexual generation of the host-specific phytophage, Belonocnema treatae (Hymenoptera: Cynipidae) which induces leaf galls on Quercus fusiformis. Four specific objectives are examined: 1) determining the diversity and relative abundance of the parasitoid community members, 2) defining the specific niche dimensions of each parasitoid species m relation to size of gall attacked and timing of attack, 3) elucidating the nature of interactions among parasitoid species, and 4) determming the cumulative effect of parasitoids on B. treatae survivorship. The objectives were addressed using an exclosure study which limited access of parasitoids to leaf galls to a series of discrete 30 day window of opportunity. Following B. treatae oviposition (April) but before initiation of gall development, exclosure bags were placed on 210 branches distributed across six of the gall formers host trees. Eight windows (May through November) were established by removing a subset of bags for a one month period to allow active parasitoids access to developing galls. Beginning in July, and monthly thereafter a subsample of galls previously exposed to parasitoids were collected. Just prior to emergence of the gall former (end of November window) all remaining galls were collected. All galls were returned to the lab, measured, stored individually in gelatin capsules, and incubated at seasonally adjusted temperatures and lighting conditions. Emergent parasitoid species were collected daily and identified. The relative frequency, abundance, niche breadth, niche overlap and within gall interactions were determined for each species. A total of 957 individual parasitoids were recovered from 21,690 galls sampled in the study. Four orders of insects (Hymenoptera, Coleoptera, Lepidoptera, and Diptera) and 27 species were represented in the parasitoid community. The relative abundance of species was lognormally distributed with most species being relatively rare and three species making up 49% of all individuals in the community. Diversity for both the overall parasitoid community and the hymenopteran component of the community (24 species) was high (H' = 3.48 out of a theoretical maximum of 4.76 and H = 3.32 out of 4.58 respectively). Diversity varied seasonally across windows though not significantly. Seven species were able to parasitize the gall former prior to gall development. Most parasitoid species attacked the gall early in the season with a peak in July while few species attacked late with only two species attacking in November. Parasitoid use of galls was not random with respect to gall size as the distribution of gall sizes used by the 10 commonest parasitoid species differed significantly from the distribution of gall sizes available. Niche breadth for time of attack had a broader range among the 13 commonest parasitoid species (BA= 0.14 to 0.58) than niche breadth for gall size (BZ = 0.34 to 0.63). Niche overlap for gall size among species (n = 13) range RO = 0.42 to 0.95, was greater than niche overlap for time range RO = 0.07 to 0.088. The most frequent intraspecific interaction was the production of two individuals of the same species from a single gall. For eight of the 13 most common species multiple individuals were capable of emerging from a single gall. The most common interspecific interact10n was the emergence of two different species from a single gall. There were 26 different pairwise co-emergences among the 27 parasitoid species (7.4% of the possible number of coemergence interactions). Belonocnema treatae did not exhibit intraspecific interactions and in virtually all instances of co-occupation (99%) with any parasitoid species B. treatae did not emerge. This indicates that all species attacking the gall former,B. treatae, function as parasitoids in spite of their trophic level. Belonocnema treatae emergence was highest in the May and November windows at the time the number of parasitoid species present were lowest. For the July window which had the highest number of parasitoid species, B. treatae emergence success was lowest.