Monarch butterflies do not place all of their eggs in one basket: oviposition on nine Midwestern milkweed species

Over the past two decades, the population of monarch butterflies east of the Rocky Mountains has experienced a significant decline in overwintering numbers. Habitat restoration that includes planting milkweeds is essential to boost monarch numbers within the breeding range. Milkweeds are the only host plants for larval monarch butterflies, but female oviposition preference for different milkweed species, especially those with overlapping ranges, is not well documented. We examined the relative inclination to lay eggs on nine milkweed species native to Iowa (no choice), and oviposition preference (choice) among the four most commonly occurring Iowa species (Asclepias incarnata, Asclepias syriaca, Asclepias tuberosa, and Asclepias verticillata). In both experiments, eggs were counted daily for four days. The milkweeds tested were Asclepias exaltata (poke milkweed), Asclepias hirtella (tall green milkweed), A. incarnata (swamp milkweed), Asclepias speciosa (showy milkweed), Asclepias sullivantii (prairie milkweed), A. syriaca (common milkweed), A. tuberosa (butterfly milkweed), A. verticillata (whorled milkweed), and Cynanchum laeve (honeyvine milkweed). When females were given only a single species on which to lay eggs, there were significant differences among milkweed species in the average number of eggs laid; A. incarnata had the highest average egg count. When females were given a choice among A. incarnata, A. syriaca, A. tuberosa, and A. verticillata, there were also differences among milkweed species in the number of eggs laid; again, A. incarnata had the highest average number of eggs laid. Additionally, females laid more total eggs when four plants of different milkweed species were available than when there were four plants of a single milkweed species. Our results show that monarch butterflies will lay eggs on all nine milkweeds, but that there are clear preferences for some milkweed species over others