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Katie M. LaPlante, Terry L. Woods, Timothy M. Reinbott and Deborah L. Finke, Division of Plant Sciences, University of Missouri, Columbia, MO
Katie M. LaPlante
Division of Plant Sciences, University of Missouri Columbia, MO, USA
Background/Question/Methods The monarch butterfly (Danaus plexippus) is an iconic insect pollinator currently in decline. Conservation of the monarch butterfly requires a better understanding of interactions between monarchs and other herbivores utilizing their shared milkweed host (Asclepias spp.). The interaction between the oleander aphid (Aphis nerii) and monarch larvae is well-studied. Aphid feeding induces chemical changes in the plant that affect the quality of the larval food, and the presence of aphids can influence the susceptibility of monarch larvae to shared natural enemies. Although much is known about the effects of oleander aphids on larval monarchs, the interactions between oleander aphids and adult monarch butterflies, particularly oviposition success, is largely unexamined. We established 5m x 5m replicated plots of common (A. syriaca) and swamp (A. incarnata) milkweed within a matrix of nectar plant species or at a 6m distance from nectar plants and located in open field crop habitats or along tree lines. From May-August 2019 and 2020, we monitored monarch egg laying and colonization of milkweed by oleander aphids and generalist natural enemies. Path analysis was used to determine the influence of aphids, generalist natural enemies, and landscape context (nectar plant matrix and tree lines) on the abundance of monarch eggs. Results/Conclusions Oleander aphid abundance peaked on milkweed host plants in mid-summer and were present at high densities at the end of the summer when adult monarchs were laying eggs. We documented a seasonal shift in egg abundance from swamp milkweed to common milkweed that corresponded with the arrival of aphids. We also found a positive association between monarch eggs and aphid abundance on swamp milkweed. Lady beetles formed a numerical response to the presence of aphids, but had no direct or indirect effect on monarch egg abundance. Monarch eggs were more abundant in milkweed patches along tree lines than in open agricultural fields, potentially due to adult monarch resting in trees, but were not affected by the proximity of the nectar plants. Our results suggest that monarch preference for and oviposition success on different milkweed species are temporally variable and influenced by the occurrence and seasonal population dynamics of oleander aphids. Interestingly, we found that aphids may be beneficial to monarch egg abundance under certain conditions, which is consistent with previous laboratory work with monarch larvae. Therefore, best management practices for monarch conservation should consider not only milkweed species and availability of nectar plants, but the broader community of milkweed herbivores.