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

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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 loss of breeding and forage habitat in the Southern and Midwestern USA. To support a resilient overwintering population of six hectares of occupied forest canopy, approximately 1.6–1.8 billion additional ramets of milkweed are needed in the summer breeding range. Milkweed establishment that facilitates natural behavior of monarchs is necessary for effective conservation restoration. This study explored the effect of milkweed ramet density on larval search behavior, milkweed utilization, and survival without predation, parasitism, or competition. Under our experimental greenhouse conditions, monarch larvae abandoned their natal ramet, and subsequent ramets, prior to the pre-pupal wandering stage and before all available leaf biomass on a ramet was consumed. This is consistent with previous field observations. Larvae consumed biomass from three or four milkweed ramets that totaled the approximate biomass of single 10–35 cm ramet. Movement behavior suggests that isolated ramets may not support development through pupation, even though an isolated ramet could provide enough biomass. Our results suggest milkweed 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.

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Acknowledgments

This work was supported, in part, by USDA-NRCS/CIG Agreement 69-3A75-16-006 and the Iowa State University, College of Agriculture and Life Sciences. We would like to thank Keith Bidne (USDA, ARS-CICGRU) for maintaining monarch butterfly colonies and providing materials and larvae used in these experiments. We thank Julia Pfeiffer, Signey Hilby, Riley Nylin, Cody Acevedo, Jenna Nixt, and Kara Weber for their help with experimental set up and data collection; without their help twice daily data collection would not have been possible. Additional thanks to ISU Statistics Consulting, Kathleen Rey, Audrey McCombs, and Dr. Philip Dixon. Mention of a proprietary product does not constitute an endorsement or a recommendation for its use by Iowa State University or USDA.

Funding

This work was funded by the Natural Resources Conservation Service (award number 69-3A75-16-006; awarded to Steven P. Bradbury) and by Iowa State University, College of Agriculture and Life Sciences.

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Correspondence to Kelsey E. Fisher.

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The authors declare that they have no conflict of interest.

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This work is original and has not been submitted to any other journal for consideration. This is a complete study and has not been split into several parts to increase quantity of submissions. Results are presented to the best of our knowledge without fabrication, falsification, or inappropriate data manipulation. No data, text, or theories by others were presented as if they are our own work; proper acknowledgements were given. All work here is well intended. The author list is complete and will not change. Raw data or documents will be provided upon request.

Research involving human and animal participants

This research was not conducted on human subjects and was consistent with the United States Animal Welfare Act.

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Fisher, K.E., Hellmich, R.L. & Bradbury, S.P. Estimates of common milkweed (Asclepias syriaca) utilization by monarch larvae (Danaus plexippus) and the significance of larval movement. J Insect Conserv 24, 297–307 (2020). https://doi.org/10.1007/s10841-019-00213-2

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  • DOI: https://doi.org/10.1007/s10841-019-00213-2

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