Is insect herbivory contributing to the threatened status of Agalinis auriculata (Orobanchaceae) in Illinois?

During August-November 1999-2002, we studied herbivory in two populations of the Illinois threatened prairie plant, Agalinis auriculata (Michx.) Raf. (Orobanchaceae). We collected and identified insect herbivores, and measured levels of folivory and granivory. We also tested whether observed levels of folivory influenced the reproductive success of the plant (i.e., fruit set, seed set and seed mass). Major herbivores included the blackhorned tree cricket (Oecanthus nigricornis), the verbena bud moth (Endothenia hebesana), and the buckeye butterfly (Junonia coenia). Significant differences were found between populations for percent leaf damage, although the percent of leaf area per plant removed by insects was relatively low, ranging from 0% to 40% in 2000 and 0% to 12% in 2001. A significant negative correlation between leaf damage and seed set was found, but not with fruit set or seed mass. In addition, E. hebesana was capable of causing up to 100% damage to seeds within individual fruits, although the infestation level of total yearly fruit samples was relatively low in 2000 (21%, N = 72) and 2001 (6%, N = 266), but high in 1999 (89%, N = 18). Insect herbivory, combined with habitat loss and other biotic constraints may hinder the recovery of Agalinis auriculata.