Garlic mustard as an ecological trap for falcate orangetip butterflies.

Background/Question/Methods

Introduced plants can have positive, neutral, or negative effects on native herbivores depending on the degree to which native species can recognize, consume, and grow on them. Garlic mustard, Alliaria petiolata, is an invasive understory herb that ranges throughout the northeastern and midwestern United States and southern Canada. It has known negative effects on two Pierid specialist butterflies by attracting adult butterflies that acquire nectar and oviposit on the plant, but whose larvae generally fail to survive and grow on it. In this study, we examined whether the falcate orangetip butterfly, Anthocharis midea, a related species, meets the same fate. We examined patterns in oviposition by this butterfly on garlic mustard in the field in Ohio, and patterns in larval feeding, survival, and host plant choice in the laboratory. We also examined whether stress on the host plant influenced larval success. These studies were examined in relation to the use of cut leaved toothwort, Cardamine concatenata, one of the primary host plants for this butterfly in Ohio. This plant is smaller and flowers earlier than garlic mustard.

Results/Conclusions

In the field, falcate orangetip butterflies commonly acquire nectar on garlic mustard, and readily oviposit on it. Early in the season a higher proportion of eggs were observed on the native host than on garlic mustard, but within a week, this pattern shifted in favor of garlic mustard as it began flowering. In no-choice assays, no larvae were observed to survive on garlic mustard, while a significant proportion of those on the native host survived to pupation. Whether eggs were initially recovered from garlic mustard or the native host in the field did not influence the outcome. Older larvae were no better able to feed and survive on garlic mustard than younger larvae when transferred among host plants. Drought stress of the host plants, whether garlic mustard or the native host, slightly improved larval feeding and survival. When given a choice, falcate orangetip larvae almost exclusively chose the native host over garlic mustard. Our results indicate that, at present, garlic mustard serves as an ecological trap for falcate orangetip butterflies in ways similar to its effects on other Pierid butterflies.