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Like many other butterflies, the great purple hairstreak (Atlides halesus) is declining in the western United States.


Butterflies are vanishing in the western U.S.—but not for the reasons scientists thought

Earth is in the midst of an insect apocalypse, with thousands of species dwindling over the past several decades. Scientists have often blamed habitat loss or pesticide use. But a new study of butterflies in the western United States has found that warmer fall weather may be taking as big, if not a bigger, toll.

The findings are a wake-up call—not just for butterflies, but for all insects—says Jessica Ware, an entomologist at the American Museum of Natural History who was not involved with the study. “If humans don’t take dramatic steps to curb global warming,” she says, entire ecosystems could disappear, with untold impacts on biodiversity and human health.

Scientists already know some butterflies are in trouble. Recent studies have shown monarchs are in steep decline, and surveys of insects, in general, show shrinking numbers. Yet most data for these studies come from densely populated or intensively farmed areas.

But butterflies are at risk in open spaces, too. Art Shapiro, an insect ecologist at the University of California, Davis, and colleagues have shown that over the past 35 years, butterflies are disappearing even in pristine protected areas such as the Sierra Nevada mountain range in the western United States.

To see whether that finding held up elsewhere, Shapiro and Matthew Forister, an insect ecologist at the University of Nevada, Reno, gathered data from the North American Butterfly Association, which has coordinated community scientist butterfly counts across the United States for more than 42 years. The duo also incorporated 15 years of data from iNaturalist, a web portal that collects sightings of plants and animals, including butterflies. In all, the researchers tracked the fates of 450 butterfly species from 70 locations in the western United States.

Butterfly numbers have dropped an average of 1.6% per year between 1977 and 2018, the team reports today in Science. Fifty species declined in at least two of the data sets used, including the Edith’s checkerspot (Euphydryas editha), the rural skipper (Ochlodes agricola), and the great copper (Lycaena xanthoides). Some species may completely disappear from parts of their ranges in the coming decades, Forister says.

The declines didn’t seem linked to human development or pesticide use—though such activities can still pose problems. Instead, the insects seemed to be disappearing in areas where fall temperatures had risen significantly more than summer temperatures over the past several decades, as in the U.S. Southwest. The warmer weather may disrupt the butterfly’s breeding cycle or negatively affect the plants they depend on, Forister speculates.

“[The new study] did a good job of working with what is inherently messy data,” says Scott Hoffman Black, an ecologist and executive director of the Xerces Society for Invertebrate Conservation. Given that butterflies are key pollinators, such declines portend bigger problems for plants and even whole ecosystems, Forister adds. “The climate effects will almost certainly affect many other insects, including bees.”

And these effects will “undermine” efforts to protect and restore butterfly habitat, adds Dave Goulson, an ecologist at the University of Sussex who was not involved with the work.

Combined with other studies, including one from 2019 that found butterflies on the decline in Ohio, the new work “is very troubling,” says Leslie Ries, an ecologist at Georgetown University. It’s “further evidence that these declines seem to be pervasive and general.”