ABSTRACT
Stable isotope (δ2H, δ13C) measurements of wing tissue have been used to determine the natal geographic origin of migrant monarch butterflies that overwinter in Mexico. This study examines the possibility of using δ13C and δ15N to identify the milkweed habitat used by monarchs in their natal region. Milkweeds were common in corn and soybean fields before herbicide use led to their extirpation around 2006, and the loss of those milkweeds has been proposed as a reason for the monarch population decline. If crop-field monarchs can be identified, then historical samples of monarchs could be examined to test that hypothesis. The δ15N and δ13C values of leaves from milkweeds growing in corn fields, soybean fields and non-agricultural habitats were examined as well as monarchs that were raised on those leaves. There were no δ15N values for leaves or monarchs that were distinctive for crop fields. Milkweeds in corn fields, and monarchs that were raised on those milkweeds, were found to have δ13C values distinctly lower than those of other habitats and unlike those of locations within the summer breeding range. Thus, it should be possible to identify monarchs that came from cornfields in samples of overwintering monarchs made before ca. 2006.
Acknowledgements
The author thanks Keith Hobson for advice and feedback on the manuscript.
Author contribution
The author is the sole person responsible for the work.
Disclosure statement
No potential conflict of interest was reported by the author(s).