Arlene Koziol
Monarch butterflies at the Madison Audubon’s Goose Pond Sanctuary in Arlington.
Each fall, those wings carry Wisconsin’s migratory monarchs more than 1,500 miles to fir trees scattered over a few mountain tops in central Mexico. Their winter home has never been a very big area, according to Karen Oberhauser, director of the UW Arboretum and co-chair of Monarch Joint Venture, a national coalition of partners ranging in size from the U.S. Fish and Wildlife Service to local nature centers.
During the winter of 1996-97, monarchs covered as much as 45 acres, but during the winter of 2013-14, all the monarchs from east of the Rockies fit in the space of a football field.
Since that frightening winter, their numbers have climbed back to what they were a decade and a half ago.
“There have been a couple of factors in their favor,” says Jennifer Thieme, regional monitoring coordinator of Monarch Joint Venture. Last winter, roosting monarchs covered almost 15 acres — more than have been recorded in over a decade. “That gave them a good kick-off this year,” she says.
Good weather during the past two breeding seasons has helped. The monarch you see in your backyard in September is the great-great grandchild of the previous spring’s first returning monarchs. If this youngster survives its journey and the winter in Mexico, it will head north next spring, and may get as far as Kansas, laying eggs along the way. The next generation moves farther north before laying its eggs.
“In 2018, especially March and April, we had really warm temperatures in the southern U.S.,” says Thieme. “The returning monarchs reproduced well down there, and we’ve had similar beneficial weather patterns for reproduction this year. There is a lot of mortality during fall migration, so spring and summer is their chance to recover.”
Monarchs face their worst peril when one bad year follows another. If monarch numbers are low when a storm hits their wintering colonies, or if there is a summer drought in the upper Midwest, they are at great risk.
Modern agricultural practices have greatly reduced most monarch habitat including the milkweed plants that monarchs need to survive. Several individuals and organizations are working to protect and expand that kind of environment to help the butterflies survive.
“That’s helping them,” says Oberhauser. “If we have plentiful habitat here, in bad years when their numbers are low, it’s more likely they will encounter milkweed, which allows them to lay more eggs. Monarchs co-evolved with milkweed, and it’s the only thing their caterpillars recognize as food.”
Oberhauser urges people who want to help monarchs to check out the Wisconsin Monarch Collaborative. It maintains a list of recommended plants for people looking to make monarch-friendly changes to their landscape, as well as providing information on supporting organizations that are preserving habitat or coordinating citizen science data collection.
“What we want,” says Oberhauser, “is for the monarch numbers we’ve been seeing this summer to be the long-term average and not just a temporary peak.”
