Editor’s note: Parts of this article are featured in our Chesapeake Uncharted podcast, available from your podcast service or at bayjournal.com/podcasts. This podcast season is a companion to our new film, Chesapeake Rhythms, which explores wildlife migrations in the Bay region.
ANNAPOLIS, Md. – In a forest in the Sierra Madre Mountains of Mexico in 2016, James Diffendorfer saw the bark of pine trees moving. Monarch butterflies rustled along the tree trunks, clustered on branches and filled the sky.
“You’re talking while you’re hiking, and then you get up there and everyone’s just quiet, and you’re just in this place of a million monarchs,” said Diffendorfer, a research ecologist with the U.S. Geological Survey. “And it’s unbelievable.”
The overall eastern population of monarch butterflies, which visits the Chesapeake Bay region, has been declining since the 1990s, but there are still plenty of opportunities to interact with — and help — the species. Their complex, multigenerational and cross-country migration makes it difficult for researchers to understand all the factors contributing to the decline, and some are better known than others.
Wendy Caldwell, executive director of Monarch Joint Venture, said taking care of monarchs and the habitats they depend on helps address multiple environmental issues. While not as effective as bees, monarchs also act as pollinators as they feed on flowers during their journey.
Monarch habitat includes parts of the Chesapeake Bay watershed. They visit Pennsylvania in the late summer and early fall as they begin heading south. People can also see them en masse along Virginia’s eastern shore and the Chesapeake Bay Bridge in early September and October.
In the early spring, monarchs that weathered the winter in Mexico fly north and produce offspring, primarily in southern Texas. Arter the first generation of offspring emerge from their chrysalises, they head back north in the spring and lay eggs along the eastern United States. Those new eggs hatch in the summer and lay eggs around the Bay region and farther north. They become the great-grandchildren of the butterflies that started in Mexico.
The fourth and final generation, or great-great grandchildren, of this migration cycle have a unique and strenuous role. They must abstain from mating to travel all the way back to Mexico from August to October. Their reproductive organs remain in an immature state in response to the shortening, cooler days. It’s not until they arrive at the Sierra Madre Mountains that they produce offspring. Then, the following year, those offspring somehow know how to repeat the journey of their predecessors.
It’s normal for insect populations to go up and down or to have boom-and-bust cycles. But the population that overwinters in Mexico has gone down overall. According to the U.S. Fish & Wildlife Service, the eastern migratory population of monarchs has declined by about 80% since 1980.
Milkweed is the only plant on which monarchs lay their eggs. Diffendorfer said the decline in monarchs, at least in part, is likely due to the loss of milkweed from pesticide use on Midwest farms.
Caldwell said that pesticides on those farms also can harm butterflies as they interact with it. She cites climate change as another factor changing when milkweed is available.
“We’re talking about this changing climate and just how much that has the potential to impact populations like the monarch that are utilizing a lot of different landscapes, but they’re really reliant on the right resources at the right time,” she said.
Evidence suggests that climate can affect monarch populations, but exactly how remains unclear, according to Diffendorfer. Studies suggest climate conditions in one part of the life cycle can affect the overwintering population size in Mexico. For instance, a drought along the migratory path could make less milkweed available for one of the butterfly generations.
Beyond nationwide tagging and monitoring programs for monarchs, people also can create waystations to give monarchs resources along the way. Waystations are areas of land filled with milkweed and other plants that monarchs use to refuel. There are more than 5,800 waystations ranging from 200 to 5,000 square feet in Maryland, Pennsylvania and Virginia, according to the community science program Monarch Watch.
The Virginia Department of Transportation also began reducing mowing medians in 2023 to encourage milkweed growth.
In December, the U.S. Fish & Wildlife Service proposed listing monarchs as “threatened” under the Endangered Species Act. The listing would give the species certain protections, such as prohibiting the sale or killing of the species and the development of protection plans by the Fish & Wildlife Service. The proposed listing would also provide exceptions under section 4(d) of the act, such as hitting a butterfly while driving.
The Fish & Wildlife Service is currently reviewing input from a public comment period on the proposal, which ended May 19.
The Pennsylvania Farm Bureau said in a statement that 77 million privately owned farms in the state are enrolled in voluntary monarch conservation programs and that pesticides are already heavily monitored.
“While we respect the agency’s goal of identifying ways to assist the monarch butterfly, we are concerned at the potential impact of what could be a wide-ranging review of an unknown number of crop protection tools that are important to farmers,” the bureau said in a statement.
If finalized, Caldwell hopes the listing will bring additional awareness to the monarch’s decline. She is also hopeful that many groups will be consulted to avoid regulatory concerns that disincentivize conservation work. To help conserve monarchs, Caldwell recommends people plant milkweed and other wildflowers native to their region, reduce pesticide use and help spread the word.