As exotic plant invasion accelerates globally, many grassland communities are becoming mixtures of native and exotic plants. Native species interactions are expected to be susceptible to exotic species invasion, yet the influence that native-exotic plant associations have on native species interactions is often overlooked. Danaus plexippus (monarch butterfly) larvae are obligate herbivores of native plants in the milkweed family (Asclepiadaceae). Interactions between D. plexippus and the native Asclepias syriaca (common milkweed) could be especially vulnerable to species invasion because A. syriaca and exotic plant species, e.g. Leucanthemum vulgare (ox-eyed daisy) and Trifolium pratense (red clover), commonly co-occur in grassland and disturbed habitats. This study investigated how exotic plants growing in close association with A. syriaca affect interactions between it and D. plexippus. In a fully factorial field experiment, I explored the effect neighbor composition (A. syriaca, L. vulgare, T. pratense, both L. vulgare and T. pratense) and density (low, high) had on the abundance of D. plexippus larval feeding on A. syriaca.
Neighbor identity, not neighbor density, significantly influenced D. plexippus abundance on A. syriaca. The abundance of D. plexippus larval on A. syriaca was significantly lower in plots that contained L. vulgare – 80% reduction in plots that A. syriaca co-occurred with both T. pratense and L. vulgare and 90% reduction in plots that A. syriaca co-occurred with only L. vulgare. This study provides evidence that exotic species may negatively influence D. plexippus oviposition, potentially imperiling its migration. Understanding how exotic species can influence interactions between D. plexippus and native milkweed species is critical to better manage and conserve D. plexippus populations during a period of accelerating exotic species introduction. Furthermore, this result highlights the importance of accounting for the influence that exotic species exert on interactions among native species.