Host plant choice affects parasitism rates of the tulip tree beauty moth, Epimecis hortaria

Background/Question/Methods

Successful parasitism of caterpillars by parasitoid wasps and flies may be affected by caterpillar host plant choice. Allelochemical signals from host plants may attract parasitoids as a defense from herbivory. Caterpillars may also sequester plant compounds as a defense from parasitoids. Epimecis hortaria caterpillars are known to feed on four host plant species, Liriodendron tulipifera, Asimina triloba, Sassafras albidum, and Lindera benzoin, and are preyed on by several parasitoid species. In this study we collected and georeferenced E. hortaria caterpillars from four host plant species (L. tulipifera, A. triloba, S. albidum, and L. benzoin) at Alapocas State Park in Delaware to determine if there is variation in parasitoid infection rates between caterpillars collected on different host plant species. If parasitoid choice of host caterpillar is influenced by caterpillar choice of host plant, we expect to observe variation in rates of parasitism between caterpillars collected on different host plant species. Caterpillars were raised in the laboratory and monitored daily for evidence of parasitism until pupation. The georeferenced collection locations were imported to a GIS system. Geoprocessing techniques and spatial statistics were used to investigate any correlations between the results and other environmental attributes of the study location.

Results/Conclusions

We collected a total of 82 caterpillars from four host plant species in summer 2012. Twelve of the caterpillars showed evidence of parasitism through direct observation of parasitoid eggs or larvae on the outside of the caterpillars or by the presence of a parasitoid pupa after the death of the caterpillar. All of the parasitoid-infected caterpillars were collected on A. triloba or L. benzoin, showing a significant relationship between caterpillar parasitism and host plant species (χ²=12.676, n=67, p<0.01). Fifteen caterpillars died of unknown causes in the lab, but unexplained mortality was not significantly related to caterpillar host plant species (χ²=6.726, n=70, p>0.05). Variation in rates of infection between host plant species may be due to differences in parasitoid adult reproductive behavior or the inability of parasitoids to successfully parasitize caterpillars ingesting certain host plants. Additional caterpillars will be collected and examined for parasitoid infection in summer 2013 and we plan to complete a spatial analysis of the relationship between caterpillar location, parasitism, and other selected environmental attributes.