Eastern monarch butterflies are on the rebound after a strong winter showing

Published: February 11, 2019

Monarch Butterflies on tree branch in blue sky background, Michoacan, Mexico Photo by JHVEPhoto/Shutterstock

Good news for lovers of the monarch butterfly: a recent survey of the monarch’s Mexican wintering grounds shows the butterfly’s eastern North American population is on the rebound—at least for now.

In January, roosting monarchs covered 6.05 hectares in Mexico’s Oyamel fir forest, up from just 2.48 hectares a year ago. It’s the Monarch’s best showing since 2006. University of Guelph biology professor Ryan Norris attributes the increase to “optimal” weather conditions during the 2018 migration and breeding season. Monarchs stage a 4,000-kilometre, multi-generation journey north to Canada every year. Good spring weather in Texas helps them get off to a good start.

The result was a good hatch of larvae and a bumper crop of butterflies. Norris saw the results around his cabin near Algonquin Park. “It was such a pleasure this past summer to see monarchs everywhere. It hasn’t been like that for a number of years,” he says.

Val Deziel, a conservation biologist for the Nature Conservancy of Canada, says crews spotted 170 monarchs during their annual survey on Ontario’s Rice Lake Plains last June—by far the best showing since the count began in 2014. (The worst survey was in 2015, when crews failed to find a single monarch.)

Insect populations are inherently variable, Norris says, cautioning “this population can go down as fast as it went up.”

For proof, consider the western monarch: While the larger eastern population is on the rebound, monarchs west of the Rockies are in a tailspin. In November, a survey of their winter roosts in California tallied just 28,429 monarchs at 213 sites, down from 4.5 million in the 1980s.

Norris suspects western monarchs are struggling with the same problems affecting their eastern kin: the loss of habitat on wintering and breeding grounds, and widespread herbicide use that kills wildflowers and the milkweed caterpillars rely on. Although Canadian cottagers are at the northern end of the monarch’s range, they can help by planting milkweed, leaving more natural areas around the cottage, and naturalizing lawns to promote wildflowers.

Even in Mexico, “this is the low end of where we want to be in monarch numbers,” Norris says. If monarch numbers can continue to build, “that would be really good.”

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