How does the presence of exotic plants effect monarch butterfly oviposition on their native host, common milkweed?
The primary host plant of monarch butterfly caterpillars is common milkweed (Asclepias syriaca), which thrives in habitats that are also heavily invaded by non-native plant species. In these habitats, common milkweed can grow in close association with non-native plant species. Based on this observation, I asked does monarch butterfly oviposition change when milkweed grows with non-native plant species. Below is a published abstract from the 2012 Ecological Society of America Annual Meeting in Portland Oregon that reports on the results of an experiment that I conducted.
The identity and diversity of co-occurring exotic plant species influence consumption of the native forb Asclepias syriaca by monarch butterfly larvae
Exotic plant invasion is a major threat to ecological communities and may alter interactions between existing native species. Because many native species persist in invaded communities, the co-occurrence of native and exotic plants may increase (associational susceptibility) or decrease (associational resistance) the susceptibility of a native plant to herbivory. To explore the effect of exotic plant neighbors on native plant-herbivore interactions, we manipulated the identity, density, and diversity of two exotic plants (Leucanthemum vulgare and Trifolium pratense) and examined their effect on the consumption of a native forb, Asclepias syriaca, by monarch butterfly larvae. We documented associational resistance for A. syriaca in the presence of L. vulgare, but not in the presence of T. pratense. When both exotic species grew with A. syriaca, the effect on monarch larval abundance was additive, while the effect on consumption of A. syriaca leaves was interactive. Independent of neighbor identity, neighbor density did not influence A. syriaca herbivory or monarch density when one exotic species was present. However, when both exotic species were present monarch larval abundance and A. syriaca herbivory was reduced at high, but not low, neighbor densities. When both exotic species were present, the impact of monarch larvae on the consumption of A. syriaca leaves was greater compared to A. syriaca growing with conspecifics or with one exotic species. Results from this experiment demonstrated that exotic plant species indirectly alter native plant-herbivore interactions by influencing herbivore abundance and the intensity of native plant herbivory. This research highlights the importance of accounting for the influence of co-occurring exotic plant species on interactions between native species.