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The striking Painted Lady (Vanessa cardui) butterfly has been shown for the first time to be capable of making the 12,000-14,000km round trip -- the longest insect migration known so far -- in greater numbers, when wetter conditions in the desert help the plants on which it lays eggs.
The international research team's findings increase understanding of how insects, including pollinators, pests and the diseases they carry could spread between continents in future as climate change alters seasonal conditions.
Professor Tom Oliver, an ecologist at the University of Reading and co-author of the study, said: "We know that the number of Painted Lady butterflies in Europe varies wildly, sometimes with 100 times more from one year to the next. However, the conditions that caused this were unknown, and the suggestion the butterflies could cross the Sahara desert and oceans to reach Europe was not proven.
"This research shows this unlikely journey is possible, and that certain climate conditions leading up to migration season have a big influence on the numbers that make it. It demonstrates how the wildlife we see in the UK can transcend national boundaries, and protecting such species requires strong international cooperation."
As well as answering long-asked questions about butterfly migrations, the findings could help predictions of the movements of other insects that affect people, such as the locusts currently plaguing East Africa, or by malaria-carrying mosquitoes.
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