The relationship between variable host grouping and functional response among parasitoids

Authors: LOW, C. - Cornell University; Scheffer, Sonja; Lewis, Matthew; Gates, Michael

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/8/2012
Publication Date: 10/23/2012
Citation: Low, C., Scheffer, S.J., Lewis, M.L., Gates, M.W. 2012. The relationship between variable host grouping and functional response among parasitoids. Proceedings of the National Academy of Sciences. 21(23):5892-5904.

Interpretive Summary: Parasitic wasps are important enemies of insect pests in agricultural and natural environments. Their use as biological control agents against insect pests saves millions of dollars in pest management costs each year. This project uses DNA molecular probes to determine the timing and location of attack by parasitic wasps on a leaf-feeding caterpillar. We found that different species of wasps attack the caterpillars during different life stages. Knowledge of the ecological patterns of wasp-caterpillar interactions will contribute to a better understanding of processes influencing the efficacy of biological control programs. This information will be of interest to scientists, ecologists, and pest management specialists.

Technical Abstract: Our study investigated the patterns of attack by hymenopteran parasitoids in the Tupelo leafminer, Antispila nysaefoliella Clemens, to understand the selective pressures that shape host group size and timing of emergence. A large sample of host larvae was collected from a natural field population of Tupelo, Nyssa sylvatica Marsh, and screened for parasitism using polymerase chain reaction of mitochondrial cytochrome oxidase I using markers designed specifically for amplifying parasitoid DNA. This method was effective for detecting the presence of immature stages of three hymenopteran superfamilies: Chalcidoidea, Ichneumonoidea, and Platygastroidea, which represented 83.4, 16.0, and 0.6 percent of the total detectable parasitism, respectively. Using sequences from identified adult parasitoids that were either reared or field-captured, a cluster analysis revealed 10 distinct clades that differed in patterns of attack with respect to host group size, host mine size, and timing. Collectively, parasitism followed an inverse density-dependent or density-independent pattern. However, when parasitoid taxa were considered separately, Pnigalio maculipes Crawford (Chalcidoidea: Eulophidae) was the only species to increase its rate of attack with host density (or group size). Our results suggest that host grouping provides a general survival advantage by diluting parasitism risk, but this advantage can disappear with specific parasitoids. We discuss our findings from the general perspective of the adaptive strategies of a host species given a highly variable risk environment.