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

Abstract

Abstract. 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.

Monarch butterfly preference and use of nine Midwestern milkweed species

“…A. incarnata and A. syriaca were consistently preferred for oviposition when egg counts were combined across sites within each year. This observed preference for A. incarnata and A. syriaca in the field matches that of a prior laboratory study (Pocius et al 2018). We saw more eggs on all species in 2015, which could be due to the young plant age (Zalucki and Kitching 1982) throughout the first season but more likely due to higher egg production throughout the Midwest in that year.…”

“…incarnata and A. syriaca in the field matches that of a prior laboratory study in which females were naïve to milkweed before the start of the study (Pocius et al 2018). We saw more eggs on all species in 2015, which could be due to the young plant age .…”

“…We counted the number of monarch eggs and larvae on each block of five milkweed plants and noted the number of surviving plants and blooming period at each location between June and August 2015-2017. We predicted that monarchs will use all nine species, and that the oviposition patterns that we observed in the field would follow those observed in prior laboratory studies (Pocius et al 2018); we expected higher egg totals on A. incarnata than on A. tuberosa and A. verticillata (Pocius et al 2018).…”

“…Malcolm and Brower (1986) studied the difference in oviposition preference between A. curassavica and A. incarnata in mixed stands in southern Florida, while assessed female oviposition preference based on cardenoide content in the field and lab in the southeastern U.S.; and evaluated both larval survival and oviposition in the laboratory on leaf cuttings, and trimmed milkweed stems, respectively. Pocius et al (2018) examined oviposition preference on nine native milkweed species in both no choice and oviposition preference tests. Few studies have focused on monarch oviposition preference across multiple native milkweed species; none have replicated a design on a large scale in the field.…”

Pesticide contamination of milkweeds across the agricultural, urban, and open spaces of low elevation Northern California

“…It is important to note that we specifically planned our timing to be during the period that a larval monarch could be present in the Central Valley. A final point of uncertainty worth noting is behavior: monarchs are known to express oviposition preferences among different species of milkweed (Pocius et al, 2018), but it is currently unknown whether pesticide contamination can be a factor in this decision. Despite these uncertainties, we think that these reported LD 50 concentrations offer compelling evidence that certain compounds are being found at biologically meaningful concentrations, with possible regicidal (or sub-regicidal) implications for larval monarchs in the Central Valley.…”