Monarch butterflies show differential utilization of nine Midwestern milkweed species

Authors: POCIUS, VICTORIA - Iowa State University; PLEASANTS, JOHN - Iowa State University; DEBINSKI, DIANE - Montana State University; Bidne, Keith; Hellmich Ii, Richard; BRADBURY, STEVEN - Iowa State University; BLODGETT, SUE - Iowa State University

Submitted to: Frontiers in Ecology and Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/4/2018
Publication Date: 10/25/2018
Citation: Pocius, V.M., Pleasants, J.M., Debinski, D.M., Bidne, K.G., Hellmich II, R.L., Bradbury, S.P., Blodgett, S.L. 2018. Monarch butterflies show differential utilization of nine Midwestern milkweed species. Frontiers in Ecology and Evolution. 6:169. https://doi.org/10.3389/fevo.2018.00169.
DOI: https://doi.org/10.3389/fevo.2018.00169

Interpretive Summary: The population of monarch butterflies east of the Rocky Mountains has experienced a significant decline over the past two decades due to many factors including loss of habitat. Thus, habitat restoration that includes planting milkweeds is essential to boost monarch numbers within the breeding range. We conducted a suite of experiments, both in the laboratory and in the field to assess larval performance and female egg laying levels on different milkweed species. In the greenhouse larval survival from first instar to adult on young milkweed plants of nine Midwest milkweed species, including Asclepias exaltata (poke milkweed), A. hirtella (tall green milkweed), A. incarnata (swamp milkweed), A. speciosa (showy milkweed), A. sullivantii (prairie milkweed), A. syriaca (common milkweed), A. tuberosa (butterfly milkweed), A. verticillata (whorled milkweed), and Cynanchum laeve (honeyvine milkweed). In the laboratory egg laying preference was examined with the first four of these species and in a field setting with all nine. Eggs were counted daily over four-day trials in the laboratory and were counted weekly at 14 selected experimental sites across the state of Iowa from June through August, 2015-2017. Larval survival (including pupation and eclosion) in greenhouse experiments was statistically equivalent on seven of the nine species but fewer larvae that fed on tall green milkweed and prairie milkweed reached adulthood. The survival probabilities varied from 30% to 75% among the nine milkweeds tested. Egg laying experiments in the laboratory showed that when individual females were given a single milkweed species on which to lay eggs, there were significant differences in the average number of eggs laid among species. When females were given a choice among swamp, common, butterfly and whorled milkweed plants, swamp milkweed received over 50% of all eggs laid; the order of preference in the choice experiment was similar to that of the no-choice experiment. In the field, females preferred to oviposit on swamp and common milkweed in all years. Fewer eggs were counted on poke, tall green, butterfly, and whorled milkweed plants, and honeybine milkweed. Our results show monarch butterflies can use all nine native milkweed species for larval food sources and oviposition, but that there are clear preferences for some milkweed species over others. This information is useful to all groups interested in restoring habitat for monarch butterflies.

Technical Abstract: The population of monarch butterflies east of the Rocky Mountains has experienced a significant decline in overwintering numbers over the past two decades. Habitat restoration that includes planting milkweeds is essential to boost monarch numbers within the breeding range. We conducted a suite of experiments, both in the laboratory and in the field to assess larval performance and female oviposition levels on different milkweed species. In a greenhouse we assessed larval survival from first instar to adult on young milkweed plants of nine USA Midwest milkweed species (Asclepias syriaca, Asclepias tuberosa, Asclepias incarnata, Asclepias verticillata, Asclepias exaltata, Asclepias hirtella, Asclepias speciosa, Asclepias sullivantii, and Cynanchum laeve). We examined oviposition preference in a laboratory setting with the first four of these species and in a field setting with all nine. Eggs were counted daily over four-day trials in the laboratory and were counted weekly at 14 selected experimental sites across the state of Iowa from June through August, 2015-2017. Larval survival (including pupation and eclosion) in greenhouse experiments was statistically equivalent on seven of the nine species but fewer larvae that fed on A. hirtella and A. sullivantii reached adulthood. The survival probabilities varied from 30% to 75% among the nine milkweeds tested. Oviposition experiments in the laboratory showed that when individual females were given a single milkweed species on which to lay eggs, there were significant differences in the average number of eggs laid among species. When females were given a choice among A. incarnata, A. syriaca, A. tuberosa, and A. verticillata, A. incarnata received over 50% of all eggs laid; the order of preference in the choice experiment was similar to that of the no-choice experiment. In the field, females preferred to oviposit on A. incarnata and A. syriaca in all years. Fewer eggs were counted on A. exaltata, A. hirtella, A. tuberosa, A. verticillata, and C. laeve. Our results show monarch butterflies can use all nine native milkweed species for larval food sources and oviposition, but that there are clear preferences for some milkweed species over others.