Monarch butterflies migrate 4000 km between the United States and Mexico every winter.

Monarch butterflies migrate 4000 km between the United States and Mexico every winter.

Richard Ellis/Alamy

More monarch butterflies in Mexico, but numbers still low

MEXICO CITY—For the first time in 4 years, scientists have seen an uptick in the number of monarch butterflies migrating from the United States to Mexico. As of mid-December, the butterflies covered 1.13 hectares of forest in Michoacán and Mexico states, up from an all-time low of 0.67 hectares last year. But despite a nearly 70% increase in forest coverage—the established proxy for butterfly numbers—the 2014 to 2015 number is still the second lowest since recording began in the winter of 1993 to 1994.

The increase is “good news, but the numbers still remain very low,” said Omar Vidal, director general of the World Wildlife Fund (WWF) Mexico, in a press conference here today. WWF Mexico administers the winter colony count, surveying the sites where monarchs are known to gather to wait out the winter before returning to the United States to breed.

This year’s population increase was probably largely due to “good monarch breeding weather” last summer in the upper Midwest, says Karen Oberhauser, a conservation biologist at the University of Minnesota, Twin Cities. The region saw relatively mild temperatures and regular rain to nourish milkweed, on which monarchs lay their eggs. That allowed more caterpillars to survive to adulthood and boosted the number of butterflies attempting the journey to Mexico, Vidal explained.

Still, “we were hoping for a bigger uptick in the population,” Oberhauser says. “There have been several years in which the population more than doubled from one year to the next … so the increase is not as strong as it has been several times in the past.” Others are even more downbeat. “My first response to [the 1.13 hectare number] was sorrow,” says Dara Satterfield, a doctoral student in ecology at the University of Georgia, Athens, who studies the monarch migration. “Such low numbers of monarchs … again. This confirms our fears that last year wasn't just a bad year—it's been a bad decade for monarchs.”

Until the early years of the 21st century, the main threat to the monarch migration was deforestation in Mexico. Illegal logging of the butterflies’ winter habitat now appears to be largely in check, with a mere 5 hectares lost to loggers this year, down from about 8 hectares last year, Vidal reported. “But we can’t let our guard down,” Vidal says, citing the importance of ongoing vigilance from the Mexican government, as well as economic aid to the communities living near the butterflies’ habitat.

Now, however, it’s habitat loss in the United States that poses the biggest threat to the migration. The rise of herbicide-resistant crops has taken a significant toll on the availability of milkweed in agricultural areas. Many people have responded by planting milkweed in their gardens, which may be leading to an increase in habitat for the butterflies, Oberhauser says. The details of that approach are still being worked out, however; Satterfield recently led a study suggesting that planting nonnative milkweed may discourage monarchs in some parts of the United States from migrating, thereby increasing their risk of exposure to a dangerous parasite. “In the U.S. we are working hard to restore milkweeds, but we can’t say that efforts to date have been sufficient enough to make a real difference,” says Chip Taylor, an ecologist at the University of Kansas, Lawrence, and the director of Monarch Watch, which monitors U.S. populations.

Ultimately, “there is progress, because we can say that there are more monarchs this year than last year,” Satterfield concludes. “To be able to say that again next January, and the January after that, we need to protect and plant more milkweed.”