The refuge-mediated apparent competition hypothesis (RMACH) posits that a plant species can indirectly reduce growth of potential competitors, and thus invade, by providing refuge for herbivores of more palatable surrounding plants. Lespedeza cuneata is a non-native legume rapidly invading midwestern US. If Lespedeza cuneata’s dense and chemically defended foliage provides refuge and foraging habitat for herbivorous insects, I predict (a) higher abundance of arthropods in areas where L. cuneata is present, indicating that it provides more suitable habitat, (b) higher percent herbivory on the plants surrounding L. cuneata than on L. cuneata itself, indicating it is not a food source and (c) reduced herbivory on neighboring plants where L. cuneata has been removed. Last summer, I tested prediction (a) in a field experiment, in Hudson, Illinois, to determine how removing L. cuneata would affect the diversity and abundance of arthropod species. I removed L. cuneata, monthly from 20 1-m2 plots and left another 20 plots unmanipulated. Arthropods were collected monthly using pan traps, observations, pitfall traps, and sweep nets. Identification of samples to order and functional group is ongoing, but here I present results on Japanese beetles (Popillia japonica), an easily identifiable invasive beetle.
Results/Conclusions
Summer abundances of P. japonica found in pan traps were combined and analyzed in SAS 9.4 using Proc Glimmix. Results indicate a significantly higher abundance (P = 0.015) of P. japonica in the presence of Lespedeza cuneata. There was no significant interaction (P>0.5) between plant cover (not including L. cuneata) and abundance of P. japonica. Visual observations of little herbivory damage on L. cuneata suggest that P. japonica, and possibly other herbivores, might not be feeding on the leaves and stems. The statistical test and the visual observations described support prediction (a), indicating that L. cuneata provides more suitable habitat. Two more predictions, (b) and (c), of the refuge-mediated apparent competition hypothesis will be tested in the 2018 field season. Support for this hypothesis will decrease the rarity of these types of studies and lead to a better understanding of refuge-mediated apparent competition and its impact on invasions. Furthermore, a better understanding of the effects of L. cuneata on native communities can inform management decisions intended to limit the spread of L. cuneata, which could then increase the arthropod and plant diversity.