A European butterfly bonanza starts with caterpillars 4,000 miles away

They say a butterfly’s wing could change the weather. How is the weather changing butterflies?

This story is part of Down to Earth, a Vox reporting initiative on the science, politics, and economics of the biodiversity crisis.

Half a century ago, the late meteorologist Edward Lorenz observed that a small change in a complex system can have far-reaching consequences. He coined the phrase “the butterfly effect” to describe this phenomenon, suggesting that the flap of a butterfly’s wing could change the weather.

But sometimes it’s the weather that changes butterflies.

In a study published this week, scientists suggest that rainfall in sub-Saharan Africa, which controls the growth of vegetation, has a dramatic impact on the number of painted lady butterflies that summer in Europe — more than 4,000 miles away.

These butterflies, which are as colorful as their name suggests, complete the longest known migration circuit of any insect, from Scandinavia down to just below the Sahara desert, the authors write. And when vegetation there is abundant during the winter, more butterflies seem to arrive in Europe months later.

Published in the Proceedings of the National Academy of Sciences, the study tells of a remarkable and daring migration. But more importantly, it shows that ecosystems across the world are connected in ways we’re only just beginning to understand. And those connections raise questions about how a changing climate will affect threatened migratory species — and pests.

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A butterfly mystery

With splotchy orange and black wings, tipped with dabs of white, the painted lady butterfly is sometimes confused for the famous monarch that makes an epic migration from the US and Canada to central Mexican forests — though it’s attracted far less attention than its charismatic look-alike. That’s perhaps because it’s so commonplace: The painted lady is the most widespread butterfly on Earth, found on all continents except South America and Antarctica.

Like monarchs, many birds, and hundreds of other insects, painted ladies migrate with the seasons. They can’t withstand the cold, so they head toward the equator when temperatures begin to fall. Unlike migratory birds, however, these winged critters breed year-round and complete their circuit — which can extend for thousands of miles — over several generations.

“It’s not the same generation that makes the whole trip,” said Constanti Stefanescu, a co-author of the new study and a butterfly researcher at the Granollers Museum of Natural Sciences in Spain. “Each generation makes a leg.”

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But while painted ladies are common, their migration has eluded researchers for decades.

Data from surveys in Europe, where many of the butterflies summer, shows dramatic booms and busts that don’t follow a pattern, with their populations sometimes varying 100-fold in successive years. One summer there’s a burst of butterflies; the next, just a few flitting about.

Entomologists have long suspected it has something to do with where they winter — but they weren’t necessarily expecting a far-off desert to matter so much.

Experts of migration

To figure out what was controlling the populations of European butterflies, a team of researchers compared climate and environmental data in parts of Africa, such as estimates of vegetation, to the abundance of butterflies showing up in Europe in the summer months.

When they looked for patterns, they found one: In the months before a huge number of painted ladies arrive in Europe, there’s a spike in vegetation — which caterpillars eat — in the region of West Africa right below the Sahara, likely owing to a surge in rainfall.

This suggests that butterflies as far north as Scandinavia are affected by habitat in countries like Chad and Nigeria. “It’s brilliant, really,” said Karen Oberhauser, a monarch expert and professor of entomology at the University of Wisconsin Madison, who was not involved in the study. “Until you know this, you’d never think that, ‘Wow, what’s going on so far away could have an impact.”

That impact is all about the availability of food, the researchers say.

Typically, the southern edge of the Sahara doesn’t have much vegetation. That means painted lady caterpillars — which are grayish-brown with yellow stripes and spikes — don’t have a lot to eat, and the painted lady population can’t grow. Plus, the adults don’t have much nectar to fuel their long flight ahead.

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On occasion, however, the region is hit with torrential rains, which cause a surge in plant life. The caterpillars gorge on the flora cornucopia, and the butterfly numbers balloon. There are also few parasites and predators to keep them in check, because painted ladies aren’t showing up in abundance from one year to the next, said Jason Chapman, a co-author of the paper and associate professor at the University of Exeter who specializes in animal migration. (Chapman suspects part of the reason they migrate is to avoid parasites and predators.)

Taken together, “that results in enormous numbers of painted ladies arriving in Europe — in the UK, in Sweden and Germany, and other countries that are so far away” from sub-Saharan Africa, he said.

By linking the population in Europe to sub-Saharan Africa, the study also reveals that these butterflies are expert migrators. Over several generations, they travel as much as 8,700 miles round-trip, and unlike many insects, they appear to be crossing one of the largest deserts in the world. “It’s the longest migratory cycle that has been described for an insect,” said Stefanescu. “It’s really exceptional.”

The researchers also found that wind patterns likely play a role in the migration. Unlike birds, butterflies are at the mercy of winds, and depend on them to cross the desert.

The far-reaching impacts of climate change

The study reveals how ecosystems in disparate parts of the world are connected. And that carries important implications for conservation and pest control.

Consider the fall armyworm, a pest native to tropical and subtropical regions of the Americas that feeds on corn. In 2016, it arrived in Africa, and now researchers fear that its adult moth form could make its way to Europe.

A big remaining question is whether and how it might cross the Sahara. According to Keith Cressman, who studies migratory pests at the Food and Agriculture Organization (FAO) of the UN, the fall armyworm’s flight abilities are not that unlike the painted lady’s. “The fall armyworm could do the same thing,” he said, following the same route to Europe.

The study also serves as a reminder that the impacts of climate change will be complicated and far-reaching. It might make a particular region drier or wetter and in doing so affect places thousands of miles away. “We need to think on a big scale, especially for migratory species,” Oberhauser said.

Climate change could, for example, make cyclones more common or severe. That’s frightening, in part because “historically, cyclones have been the originators of locust plagues,” said Cressman, a senior locust forecaster at FAO. “That’s very serious.” An ongoing locust outbreak in Africa and the Middle East, which began in 2018, started due to unusual rainstorms in part of eastern Saudi Arabia, Cressman said.

There are also migratory insects that conservationists are trying to save, not control, such as the monarch butterfly. The population of Eastern monarchs, the most common species in North America, has declined by more than 80 percent in the past 20 years, and Oberhauser said climate change is likely to make things worse. Understanding the migration of species helps scientists figure out which habitats are especially in need of protection.

As for the painted ladies, they seem to be doing just fine, even as the climate changes. In sub-Saharan Africa, rainstorms are becoming more erratic — but also more severe. “These changes will undoubtedly impact painted lady generations south of the Sahara and likely influence immigrations to Europe,” the study authors write. “But exactly how these changes will manifest is difficult to predict.”

A butterfly effect, indeed.