Butterfly populations are disappearing - and it's happening fast

Most people wouldn’t expect a quiet walk in the park today to feel so different from one in the early ’90s – but a butterfly watcher would. Fewer flutter past, and some species don’t show up at all.

That hunch is backed by numbers. Scientists spent decades tracking butterfly populations across the Midwest – recording over 4.3 million sightings in all. The results show a steady, widespread decrease in abundance, with nearly half of the 136 species declining.

These results come from a study led by researchers at Michigan State University. Their findings paint a sobering picture of how butterfly biodiversity is shifting in the heart of the country.

“We expected to find that at least some species had done well over the past 32 years,” said lead researcher Wendy Leuenberger.

Instead, the numbers tell a different story. For every 10 butterfly species in a Midwest county in 1992, only nine remain today. And in terms of individuals, populations have declined by 40 percent. The loss touches rare and familiar species alike.

“You would have such a different experience taking a walk to look at butterflies in 1992 as compared to 2023,” Leuenberger said.

“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.”

Decades of butterfly data

The study is based on a massive dataset – over 90,000 hours of butterfly surveys conducted by volunteers across the U.S. Midwest. The time span of the data stretches from 1992 to 2023, offering one of the most complete overviews of butterfly population trends in North America.

This work builds on a national study published earlier this year in Science, which found similar butterfly declines across the country between 2000 and 2020. But by looking a decade further back, the MSU team had a chance to see how patterns of decline started even earlier.

Though this study didn’t focus on the exact causes of the decline, the timeline overlaps with several key changes in the environment. Neonicotinoid insecticides became widely used starting in the mid-1990s.

At the same time, shifts in weather patterns due to climate change also began to take hold – including warmer winters, drier summers, and more erratic rainfall.

Not all butterflies are the same

Butterflies aren’t one-size-fits-all when it comes to their responses to stress. Some migrate long distances. Others stay in one place their whole lives.

Some butterflies are picky about where they live or what they eat, while others are more flexible. Even how they survive the winter – as an egg, a caterpillar, or an adult – affects their chances of survival.

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

That kind of variation matters. Conservation strategies that help one species might not help another. That’s why this study focused on details – not just overall trends – to understand which types of butterflies are declining the most, and how fast.

A rare, long-term view

According to MSU’s senior author Elise Zipkin, the Midwest plays a key role in butterfly research.

“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 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.”

This consistency gives scientists a rare long-term view of how these insects are doing, and that view is essential. Butterflies aren’t just pretty to look at. As caterpillars, they are prey for many species of birds.

As adults, they pollinate flowers which develop into fruits and vegetables. What happens to them is often a warning sign for what’s happening to other insects – and the ecosystems that depend on them.

“Humans rely on insects more than we realize,” said Leuenberger. “We need to figure out how we can protect them before it’s too late.”

The full study was published in the journal Proceedings of the National Academy of Sciences.

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