COS 85-6
The effects of host plant selection on monarch-parasite interactions
The annual monarch butterfly migration from overwintering areas in Mexico to the northern U.S. and southern Canada is a threatened phenomenon, and population numbers are decreasing annually. Possible reasons include habitat loss, reduced nectar and host plant availability, and climate change. Monarchs move through central Oklahoma during their spring and fall migrations, and use Asclepias viridis and Cynanchum leave as host plants in managed prairies. These two species exhibit very different growth forms (herb versus vine), which could influence monarch oviposition behavior and parasitoid searching behavior. Host plant choice also has the potential to influence monarch-parasite interactions and reproductive success. Monarchs are infected by several parasites, including Lespesia archippivora (tachinid fly), Ophryocystis elektroscirrha (spore-forming protist), and Pteromalus puparum (wasp). We evaluated if host plant choice influenced parasite loads and larval survival rates to adulthood. During the monarch butterflies’ late breeding season in Oklahoma (mid-August through mid-late October), we collected late instar larvae from A. viridis and C. leave and reared them in the lab on the same host plant species from which they were collected. We recorded the presence of any parasites and monarch survival to adulthood. Individuals that died from unknown causes were not included in the analysis.
Results/Conclusions
We found three different parasites of monarchs within the study site, each of which parasitized larvae on both host plant species. We found no significant difference between host plant species and overall parasite infection rates (all parasites combined). However, we identified parasite preferences for larvae on particular host plant species. The parasitoid wasp (P. puparum) infected more larvae collected from C. leave, but we observed more parasitism by L. archippivora and O. elektroscirrha on larvae collected from A. viridis. Thus, overall infection rates were not influenced by the selection of A. viridis or C. leave as the larval host plant, but infection by specific parasites does seem to be influenced by larval host plant. While A. viridis is more abundant in prairie sites throughout Oklahoma, C. leave can be locally abundant, and may occur in areas where A. viridis is absent. These two species also represent two very different growth forms which could influence monarch-parasite interactions. Additional research should focus on the effects of land management practices on host plant-monarch-parasite interactions and the implications for the overall monarch population.