Monarch Butterfly (Lepidoptera: Nymphalidae) Migration Monitoring at Chincoteague, Virginia and Cape May, New Jersey: A Comparison of Long-term Trends

Abstract

The recent increase in illegal logging of the eastern North American monarch butterfly population's overwintering site in central Mexico has emphasized the need for accurate long-term population monitoring tools, especially for this population. One way to obtain indices of annual population size is to record the numbers of monarchs passing by fixed points along the migration route each year, using standardized censusing methods. This methodology has been employed at several sites in eastern North America; however, long-term trends obtained from these sites have never been compared. From 1997 through 2004, the numbers of migrating monarchs were censused daily at the Chincoteague National Wildlife Refuge, VA, and 100 km directly north at Cape May, NJ, each fall using similar methodologies. We compared the daily census counts from these sites to determine if each site recorded similar daily and yearly numbers of monarchs, and if the year-to-year trends obtained from each site reflected the same patterns over the eight year period (1997–2004). From comparisons of 210 days during which both sites were operated simultaneously, we found that consistently more monarchs were counted at the Cape May than at the Chincoteague site. This may indicate a greater preference for Cape May as a stopover site or its proximity to the tip of the peninsula where monarchs accumulate in unfavorable weather. However, the year-to-year trends were the same at both sites, with each site showing fluctuating numbers of monarchs from year to year with the lowest numbers in 2004. These results indicate the strengths of migration monitoring as a population monitoring tool, and suggest that future monitoring sites be widely separated to minimize redundancy in data collection.