Oleander aphids (Aphis nerii) mediate a shift in monarch butterfly (Danaus plexippus) host plant selection

Monarch butterflies (Danaus plexippus) have been in decline for the past couple of decades. Milkweed (Asclepias spp.) is a critical resource for ovipositing monarchs; however, monarch preference for milkweed species varies and may be influenced by the presence of other insects, like the oleander aphid (Aphis nerii). The current literature documents monarch preferences for some species of milkweed over others, but studies are largely conducted in exclusion of the natural insect community and within a brief timeframe. The objective of this study is to examine the milkweed species preference of ovipositing monarchs in natural open field plots over the entire season. We established replicated open field plots of swamp (A. incarnata) and common (A. syriaca) milkweed in 2018. In 2019 we assessed monarch oviposition on swamp and common milkweed on a weekly basis throughout the summer while also accounting for natural colonization of plants by oleander aphids. In the early season, monarchs demonstrated an oviposition preference for swamp milkweed over common milkweed. Aphid colonization started mid-season with densities peaking in late summer. These aphids were found primarily on swamp as opposed to common milkweed. Following the colonization of aphids, preference switched and significantly more monarch eggs were found on common milkweed than swamp. Our results suggest that oleander aphids mediate monarch host plant selection, and therefore, host use may be different in the field than the greenhouse or closed studies. Understanding the impact of the insect community on host plant use will help inform habitat management for monarch conservation.