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While analyzing the mountains of data harvested from three decades of butterfly counting in the Midwest, a Michigan State University Ph.D. candidate had seen enough studies to be braced for bad news.

But, as reported in Proceedings of the National Academy of Sciences, finding that none of the 136 species of butterflies' populations increased over that time took Wendy Leuenberger aback.

"We expected to find that at least some species had done well over the past 32 years," she said.

Leuenberger and colleagues report the results after they combined more than 4.3 million observations in the U.S. Midwest from 1992–2023 to characterize changes in butterfly biodiversity. The results: 59 of the 136 species declined in abundance over the study. That means for every 10 species and 100 individuals present at a county at the beginning of the study period, there are now only nine species and 60 individuals.

Both common and rare butterflies are waning.

"You would have such a different experience taking a walk to look at butterflies in 1992 as compared to 2023," said Leuenberger, who is in the MSU Zipkin Quantitative Ecology Lab in the department of integrative biology.

"While most of the common species like monarchs and cabbage whites are still present, you wouldn't see nearly as many of them as in 1992. You're less likely to spot rare species as well. These are all changes that have occurred during my lifetime, which is humbling."

The publication comes on the heels of another big data analysis published in March in Science, which documented a national butterfly decline across the United States from 2000 to 2020.

This latest study includes more butterfly surveys by going back a decade further, most collected by volunteers working more than 90,000 hours. Although the team did not evaluate the drivers of decline, the time frame is parallel to the widespread use of the insecticide class of neonicotinoids, which were introduced in 1994 and rapidly increased in use after 2003. The last several decades have also seen changes in key weather variables because of global climate change.

The level of detail in the analysis will be crucial to understanding the best ways to conserve butterflies, which fall into different trait groups—including monarchs, swallowtails and skippers. Some migrate vast distances while others stay put. Some are abundant and found in a variety of habitats, while others can only live in a few specific places or in small numbers. They have different strategies to overwinter—as eggs, caterpillars or adults.

"Traits affect how resilient butterflies are to change," Leuenberger said. "For instance, ones that only have a single generation a year don't do as well as multi-generational butterflies."

This diversity is why it's important to have a detailed understanding about how each type of butterfly is faring. Knowing specifically how a species is changing—or not—is the first step in creating effective conservation plans.

"We are lucky to be working in the midwestern U.S., an area with the highest density of butterfly surveys of anywhere in North America," said the study's senior author, Elise Zipkin, Red Cedar Distinguished Professor and director of MSU's Ecology, Evolution, and Behavior Program.

"Volunteer scientists use the same protocols week after week and year after year. Without their invaluable efforts, we would not know how butterflies are faring and what we can do to reverse declines."

Butterflies are the most commonly counted insects and the butterfly is a likely poster species for how other insects and animals crucial to ecosystems are faring. As caterpillars, butterflies are the primary food for many fledgling birds. Butterflies and other insects are also important pollinators for plants and crops.

"Humans rely on insects more than we realize," says Leuenberger. "We need to figure out how we can protect them before it's too late."