Unique group of monarch butterflies discovered in coastal South Carolina

July 11, 2023

In our part of the world, monarch butterflies are best known for a few key traits: they depend entirely on milkweed plants to survive; they migrate each fall to Mexico; and their population is in trouble.

Now, the findings of a five-year study by South Carolina Department of Natural Resources (SCDNR) biologists suggest that coastal South Carolina is home to a unique group of monarchs that takes exception to some of these assumptions.

The recently published research indicates that monarch butterflies live year-round in South Carolina, relying on swamps in spring, summer and fall and sea islands in the winter. While these monarchs rely heavily on aquatic milkweed (Asclepias perennis) as a host plant for their eggs and caterpillars, they were also found to use swallow-wort (Pattalias palustre) – a viney relative of milkweed that grows near salt marshes and was previously unrecognized as an important host plant for monarchs.

"This research adds another layer of nuance to the fascinating story of the monarch butterfly," said SCDNR associate marine scientist and first author Dr. Michael Kendrick.

The peer-reviewed paper, published recently in Scientific Reports, has its roots in the decades-long fascination one man has had for South Carolina’s butterflies. Longtime SCDNR biologist John "Billy" McCord (who retired from the Department as a fisheries biologist in 2010, only to return to focus on monarchs) first began capturing and marking monarch butterflies along the coast in 1996, which is when he began to take note of an intriguing pattern.

"I noticed butterflies at Folly Beach, South Carolina, in November and December when I thought they should be overwintering in Mexico," McCord said. "Once I started tagging some of these butterflies, it became clear they were staying here throughout the winter."

Researchers commonly study butterfly migrations using a capture-mark-recapture method. In this system, biologists or volunteers catch a butterfly using a net and apply a small sticker with an identifying number to its outer wing. If that individual is later recaptured, researchers can use its identifying sticker to determine how long the butterfly has been ‘at large’ since first being tagged and how far the butterfly has traveled.

For this study, McCord began tagging monarchs across both inland swamp and sea island habitats in January 2018. Over the next several years, McCord tagged 18,375 monarchs and documented aquatic milkweed as a host plant in swamps ranging from the Pee Dee watershed south to the Savannah River border with Georgia. At the same time, he worked to recapture tagged butterflies to determine how long they stayed in South Carolina and when they moved each season.

The results showed strong seasonal patterns. McCord captured monarch butterflies in every month of the study period, establishing that many monarchs are overwintering in South Carolina rather than migrating to the well-known sites in Mexico. Monarchs in this study were more concentrated in maritime habitats (i.e., barrier islands directly on the ocean) in winter but were widely dispersed across coastal plain swamps in spring, summer and fall.

McCord found aquatic milkweed and monarchs (either eggs, caterpillars, pupae or adults) in 18 different watersheds in the coastal plain. These shallow, swampy systems can be hot, buggy, and challenging to sample, which may partly explain why their use by monarchs has gone relatively unnoticed. Nonetheless, this new research shows just how important South Carolina’s swamps are to monarch butterflies, a finding that could inform land conservation in coastal South Carolina.

"The extensive use of inland swamps and maritime habitats by monarchs suggests that protections of these habitats may be critical to protecting monarchs in this region," the authors wrote in the paper.

"Many questions remain to be answered, however," said Dr. Kendrick. "More information on population trends and the full extent of the migration patterns of these monarchs will be important to developing effective conservation strategies."

Citation: Kendrick, M.R., McCord, J.W. Overwintering and breeding patterns of monarch butterflies (Danaus plexippus) in coastal plain habitats of the southeastern USA. Sci Rep 13, 10438 (2023). https://doi.org/10.1038/s41598-023-37225-7