In the forests of central Mexico, the number of monarch butterflies grew for the second year in a row, suggesting the population has stabilized after years of decline.
For the past quarter century, the future of monarch butterflies has looked dire, with these iconic American insects flitting toward extinction. Now, however, there is at least a small reason for hope: New data from WWF Mexico, a large conservation group, offers further evidence that the decline of eastern monarchs — the world’s largest population — has stopped, even as the insects face worsening threats across their range.
Each fall, tens of millions of monarchs that live east of the Rocky Mountains migrate, rather miraculously, to the same forested region of central Mexico. The featherweight insects can be so plentiful there during winter that the tree branches droop under their collective weight.
In December and January, researchers hike into the forest and measure the area of monarch-covered trees to estimate how abundant they are. And this winter, the numbers were up — monarchs aggregated in trees covering about 7.2 acres of forest in Mexico, up substantially from 4.4 acres the year before and from 2.2 acres the year before that.
The new numbers are still way below the average from the first 10 years of monitoring (about 21 acres) and what scientists consider sustainable (about 15 acres). But they still amount to good news, said Karen Oberhauser, a professor emeritus at the University of Wisconsin Madison, and one of the nation’s leading monarch experts.
“We are in a period of relative stability where the population has stopped declining,” said Oberhauser, who was not involved in the new WWF Mexico report.
Oberhauser largely attributes the latest monarch bump to weather — there was plenty of rain last year in the middle of the country, along the butterflies’ migratory path, providing adult monarchs with lots of flowers to feed on. But it’s also a sign, she said, that scattered efforts across the country to restore milkweed are helping monarchs hold on. (Even in the middle of New York City, small private gardens and city parks are fueling monarchs.)
“Our efforts can make a difference,” Oberhauser said.
The crash in U.S. monarch populations is largely rooted in perhaps an unexpected source: genetically modified seeds. A few decades ago, farmers across the Midwest began planting new corn and soybean seeds that were modified to withstand a common herbicide known as glyphosate. That made it easier for farmers to spray their fields and kill the weeds growing in them.
Milkweed, the only plant that monarch caterpillars can eat, was one such weed. And as it vanished in the 1990s, so did monarchs.
Responding to this decline, the Biden administration proposed at the end of 2024 to list monarchs as threatened under the Endangered Species Act, the strongest wildlife law in the country. Before the listing was finalized, however, Donald Trump’s second term began. In September, his administration punted the decision, and indicated it would not make a final rule in the next 12 months. A spokesperson for the U.S. Fish and Wildlife Service confirmed that it does not expect to issue a final rule before late September 2026.
Two environmental groups have since sued the U.S. Fish and Wildlife Service — the federal agency that enforces the Endangered Species Act — in an effort to set a binding date by which it needs to finalize the rule. When that happens, it’s possible that the administration could grant the species protection or reverse course and decide that protection isn’t warranted, said Lori Nordstrom, a retired Fish and Wildlife Service official, who was closely involved in the 2024 proposal to list monarchs as threatened.
“The U.S. Fish and Wildlife Service continues to evaluate the monarch butterfly using the best available science and in accordance with all requirements of the Endangered Species Act,” the agency spokesperson told Vox. “The administration continues to emphasize voluntary, locally driven conservation as a proven tool for supporting species and reducing the need for additional federal regulation.”
Still, however, both eastern and western monarch populations are at historic lows. Good weather can certainly boost their numbers for a year, like we have seen last winter. But bad weather, too, can precipitate future declines — and monarch populations don’t have much room for more loss. Researchers suspect that climate change is likely to worsen weather conditions for monarchs.
To truly stabilize monarch populations — and to make them more resilient in the face of further warming — they will need more than a few patches of milkweed. “We need to regain a lot of habitat to be able to get numbers back up,” Nordstrom said. “We are still a long way from where we need to be.”
—Benji Jones, Vox
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