Monarch butterflies are arriving in much larger numbers to their wintering grounds in Mexico than in the past couple of years, when their population shrank to record lows. But the orange-winged insects are not in the clear yet, with unusually abundant rain and cold weather expected in the forests where they settle until the spring.
The number of monarchs, the farthest-migrating butterflies in the world, has been dwindling for years as the habitat along their 3,000-mile journey from Canada to Mexico is degraded by farming, development, and illegal logging.
Now the weather is threatening them too, says Gloria Tavera, central Mexico regional director for the National Commission of Natural Protected Areas (CONANP, for its acronym in Spanish). The butterflies gather in Mexico because of the relatively mild climate. But this year, the weather is expected to be colder than usual, and the rain more frequent, as warmer ocean temperatures intensify the effects of the El Niño weather system.
Although the butterflies can withstand the cold, heavy rain knocks them off the trees where they perch by the thousands. Because their wings are wet, they can’t fly back up to avoid freezing to death.
“Climate change is our main worry,” Tavera says. “We wake up with angst about the weather forecast.”
A bad winter could derail efforts in Mexico, the US, and Canada to increase the butterfly’s roosting territory to six hectares (around 15 acres) by 2020, up from the less than two hectares recorded last year.
In the US, the government is trying to convince farmers to reestablish milkweed, a plant with pinkish clusters of flowers that monarchs use as food and to breed. Monarch Watch, a nonprofit that advocates for butterfly preservation, has been distributing free milkweed plugs for people to plant in their gardens under a program sponsored by Monsanto.
In Mexico, officials and nonprofits are helping farmers develop butterfly tourism as an alternative to cutting down forests—though after illegal logging dropped to virtually zero, felled forest acres have been on the rise the past three years.
Tavera says the rebound in monarch numbers this year is a sign that those measures are working. But others, such as Chip Taylor, an insect ecologist at University of Kansas, say this year’s bigger population is simply the result of good weather conditions during the breeding season.
“The weather was better. That’s all we can say,” Taylor, who runs Monarch Watch, told Quartz.
And now weather will determine how the monarchs do this winter.