Monarch butterfly (Danaus plexippus) oviposition preference on native milkweeds (Asclepias) of the northern Great Plains

Milkweed (Asclepias ssp.) has faced population declines in the Northern Great Plains for several decades due primarily to agricultural conversion. Monarch butterflies (Danaus plexippus), which rely on milkweed as the host plant for their larva, have also declined in population throughout the past several decades in North America. Amidst ongoing efforts to protect the monarch butterfly by restoring milkweed populations, the relationship and preferences that monarchs may have for specific milkweed must be better understood. The objective of this study was to investigate monarch oviposition preferences on 18 species of milkweed native to the Northern Great Plains. In a common garden plot, 18 species of milkweed were planted in a randomized block design and assessed throughout the growing season for the presence of monarch eggs and larva. Additional data were collected on plant morphological characteristics to examine whether ovipositing monarchs preferred certain traits over others. The total number of eggs and larva found on plants within each species were used as response variables to determine monarch oviposition preference between species. Monarchs laid the most eggs on Asclepias incarnata throughout all three years of data collection, followed by Asclepias syriaca, Asclepias speciosa, and Asclepias sullivantii in 2022 and A. syriaca, A. sullivantii, and Asclepias subverticillata in 2023. Oviposition did not necessarily correlate to larval presence, suggesting higher survival rates on some species than others, such as A. subverticillata, which may serve as an ecological trap for monarchs, and A. sullivantii, which had a higher ratio of larvae to eggs, suggesting better chances of survival. Our site did not provide ideal conditions for every species of milkweed in this study, so species such as Asclepias exaltata may be more appealing to monarchs in different locations.