Effects of Longhorned Milkweed Beetle Root Feeding on Re-sprouting in Common Milkweed

Common milkweed (Asclepias syriaca) is an essential source of food for monarch butterflies (Danaus plexippus) as caterpillars, and there are concerns about the decrease in the monarch butterfly population. The decline in monarchs has been linked to a decline in common milkweed. Common milkweed plants are also where the longhorned milkweed beetle (Tetraopes tetrophthalmus) spends its larval and adult stages. The larvae eat the roots, and the adults feed on the leaves. This feeding means that the beetles affect multiple parts of the plant throughout their life cycle, suggesting that the beetle could have a greater impact on milkweed than the monarch does. However, not much is known about the impact of the beetle’s feeding on common milkweed. My research project aims to fill this gap.

At high densities rarely seen in the field, there is documentation that the feeding of longhorned milkweed beetles can decrease root biomass and pod production (Erwin et al., 2013, 2014). Through the investigation of beetle feeding at low, medium, and high densities, I will gain a better understanding of the role that beetle density plays in common milkweed bud and sprout production. I will collect the beetles from the field and then introduce the larvae from the collected beetles to milkweed roots at the various (low, medium, and high) density treatments. Throughout the summer, I will inspect each plant daily, and I will use this data on damage and number of buds and sprouts to compare the impacts of beetle feeding at the varying densities. I hypothesize that feeding and some of the resulting damage from beetles at lower densities stimulates bud production and sprouting because common milkweed forms buds from true roots and not underground stems (rhizomes). Research in other plants has shown that root damage to root sprouters, such as milkweed, can increase bud and sprouting (Bartušková and Klimešová 2010).

Literature Cited

Bartušková, A., Klimešová, J.,  2010. Reiteration in the short lived root-sprouting herb Rorippa palustris: does the origin of buds matter? Botany 88, 630–638.

Erwin, A.C., Geber, M.A., and Agrawal, A.A. (2013). Specific impacts of two root herbivores and soil nutrients on plant performance and insect–insect interactions. Oikos 122, 1746–1756.

Erwin, A.C., Züst, T., Ali, J.G., and Agrawal, A.A. (2014). Above-ground herbivory by red milkweed beetles facilitates above- and below-ground conspecific insects and reduces fruit production in common milkweed. J. Ecol. 102, 1038–1047.