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Determining monarch (Danaus plexippus) natal site distribution in Nevada using stable isotope analysis and wing morphometrics
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West of the Rockies, monarch butterfly (Danus plexippus) populations have been significantly declining over the last 30 years. Although the Eastern monarchs have also shown a similar decline, the two populations are independently decreasing both temporally and spatially. Site-specific information for Western monarchs outside of the overwintering areas in California is largely unavailable, yet this information is critical for understanding monarch population biology and movement. Patterns in deuterium isotope ratios produced by local precipitation have been linked to those isotopes in monarch host plants, the Asclepias milkweeds, across the eastern United States. However, little is known about the Western natal sites and migrational patterns. In this study, 16 wild caught monarchs and 150 milkweed samples from across a precipitation gradient in Northern Nevada were analyzed for deuterium isotopes in order to determine larval host plant preference. Results showed that deuterium values in monarch wings were correlated with precipitation deuterium values increasing from south to north. In addition, measurements of monarch wings demonstrated a decrease in wing aspect (length/width) and wing roundness over the season, which are correlated to flying efficiencies of monarchs. These results show that Northern Nevada contains multiple natal ground regions. In addition, plant isotopes data show that different species of milkweed host-plants are correlated with varying monarch wing shape. Therefore, our results suggest that determining the geographic distribution of natal sites of Western monarchs and oviposition preference across a precipitation gradient is essential for developing effective conservation strategies for this threatened species.