Estimates of common milkweed (Asclepias syriaca) utilization by monarch larvae (Danaus plexippus) and the significance of larval movement

Authors: FISHER, KELSEY - Iowa State University; Hellmich Ii, Richard; BRADBURY, STEVEN - Iowa State University

Submitted to: Journal of Insect Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2019
Publication Date: 1/1/2020
Citation: Fisher, K.E., Hellmich, R.L., Bradbury, S.P. 2020. Estimates of common milkweed (Asclepias syriaca) utilization by monarch larvae (Danaus plexippus) and the significance of larval movement. Journal of Insect Conservation. 24:297-307. https://doi.org/10.1007/s10841-019-00213-2.
DOI: https://doi.org/10.1007/s10841-019-00213-2

Interpretive Summary: The population of monarch butterflies east of the Rocky Mountains has noticeably declined over the past two decades. In order to increase monarch numbers in the breeding range, habitat restoration that includes planting milkweed plants is essential. Milkweeds in the genus Asclepias are the only host plants for larval monarch butterflies. This study aimed to understand monarch caterpillar (larva) use of milkweed and their search behavior when provided two, three or four milkweed stems. Under our greenhouse conditions, and consistent with previous field observations, monarch larvae abandoned their natal ramet (stem), and subsequent ramets, prior to the pre-pupal wandering stage and before all the available leaf biomass on a ramet was consumed. Larvae consumed biomass from three or four milkweed ramets that totaled the approximate biomass of one 10-35 cm ramet. Although an isolated ramet could provide enough food, movement behavior suggests that isolated ramets may not support development through pupation. Our results suggest patches containing at least two to four ramets of closely-spaced common milkweed would provide sufficient biomass for development and increase the likelihood that larvae moving in random directions would encounter non-natal ramets to support development. Larval movement behavior and biomass requirements are critical aspects of monarch larval biology that should be considered in habitat restoration and maintenance plans, monitoring survey designs and protocols, and population modeling. 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 noticeably declined over the past two decades. The decline is due, in part, to land conversion and the adoption of efficient weed management technologies. To support a resilient overwintering population of six hectares of occupied forest canopy, an estimated additional 1.6-1.8 billion ramets of milkweed are needed in the summer breeding range. Milkweed establishment that facilitates natural behavior is necessary for effective conservation restoration. This study aimed to understand milkweed utilization and search behavior of monarch larvae provided multiple milkweed ramets with decreasing degrees of plant isolation without predation, parasitism, and competition. Under our greenhouse conditions, and consistent with previous field observations, monarch larvae abandoned their natal ramet, and subsequent ramets, prior to the pre-pupal wandering stage and before all the available leaf biomass on a ramet was consumed. Larvae consumed biomass from three or four milkweed ramets that totaled the approximate biomass of one 10-35 cm ramet. Although an isolated ramet could provide enough food, movement behavior suggests that isolated ramets will not support development through pupation. Our results suggest patches containing at least two to four ramets of closely-spaced common milkweed would provide sufficient biomass for development and increase the likelihood that larvae moving in random directions would encounter non-natal ramets to support development. Larval movement behavior and biomass requirements are critical aspects of monarch larval biology that should be considered in habitat restoration and maintenance plans, monitoring survey designs and protocols, and population modeling.