Orange-hued with black and white wing tips, the painted lady (Vanessa cardui) is one of the planet’s most widespread butterflies, living on every continent except Antarctica and South America. Populations reach tens of millions in Europe alone. Like the monarch butterfly, the painted lady undertakes impressive annual migrations; its round-trip journeys of some 12,000 to 14,000 kilometers reach from sub-Saharan Africa to Scandinavia and back again. It is one of the longest known annual insect migrations. But this exodus is erratic, with the number of immigrant insects arriving in Europe sometimes varying 100-fold year over year. The migration “is a wonder of the natural world, but one that has perplexed naturalists for generations,” says ecologist Richard Fox of the U.K. nonprofit Butterfly Conservation, who was not involved in the new study.
Adult painted ladies only live about 2 weeks, so the butterflies’ migrations are multigenerational affairs. Experts have long suspected variations in spring numbers in the Mediterranean occur because conditions farther south have affected the breeding success of an earlier generation.
To see whether that’s true, movement ecologist Jason Chapman of the University of Exeter and colleagues collected 21 years’ worth of butterfly observations spanning West Africa to Western Europe, along with corresponding data on environmental conditions and satellite measurements of vegetation growth. The researchers found that the butterflies’ spring numbers in Europe are heavily influenced by the amount of monsoon rainfall in western sub-Saharan Africa in the previous summer and fall.
More rain there, they found, leads to flooding, which fuels vegetation that emerging larvae feast on in the winter. Wet years appear to have produced European butterfly booms in 2009, 2015, and this year, the team reports today in the Proceedings of the National Academy of Sciences. Spring vegetation levels in northwestern Africa can also affect painted lady numbers; the butterflies make pit stops in the region en route to Europe.
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The findings help scientists envision what a changing climate could mean for the insects, says paper author and ecologist Constantí Stefanescu of the Granollers Museum of Natural Sciences. “Changes in [African] climate and precipitation regimes,” he says, “may have drastic consequences on the European populations of this butterfly.”
The painted lady hasn’t revealed all of its secrets, says ecologist Chris Thomas of the University of York. Now that researchers have taken some mystery out of the insects’ northward trek, Thomas says, one remaining puzzle is “how on Earth they manage to navigate and survive” their fantastic return journey southward at the end of the summer.
