Differences in C and N values for monarch butterflies raised on milkweeds from within and outside of crop fields: implications for inferences about natal origins.

Stable isotope (H, C) 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 C and N 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 N and C 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 N 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 C 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.