Aphid abundance and monarch butterfly oviposition in urban gardens

Abstract Monarch butterflies ( Danaus plexippus ) have suffered dramatic declines and, as of 2025, are being considered for listing as a threatened species under the U.S. Endangered Species Act. Monarch declines have been explained, at least in part, by the loss of milkweeds ( Asclepias spp.), their larval host plant. Increasing milkweeds in urban gardens may aid in monarch conservation efforts but concerns have been raised regarding negative interactions between monarchs and aphids co-occurring on milkweeds in these gardens. We made 934 observations on approximately 150 Asclepias syriaca (common milkweed) stems in garden beds in a highly urbanized landscape in Chicago, IL, USA over two summers. Using generalized linear mixed-effects models, we found that the number of monarch eggs and larvae was either not correlated or positively correlated with aphid abundance, indicating that aphid infestation did not deter oviposition by monarch butterflies on milkweed stems. Implications for Conservation: Planting milkweeds in urban gardens may aid in monarch population recovery, but aphid milkweed specialists often occur at high densities in urban gardens. We did not find that stems with high aphid abundance had fewer monarch eggs and larvae, suggesting that aphids may not reduce the value of native urban milkweeds for monarch oviposition as previously suggested.