The last leg of the longest butterfly migration has been mapped at last.

Science News brings tidings from Central Africa, where painted lady butterflies born in Europe spend their winters in the longest migration of any butterfly:

Pinpointing exactly where painted lady butterflies (Vanessa cardui) overwinter and breed was the last unknown piece of their roughly 15,000-kilometer migration, says Gerard Talavera, an entomologist and evolutionary biologist at the Botanical Institute of Barcelona.

Before this study, Talavera and colleagues had predicted that subtropical regions close to the equator might provide suitable habitat. To know for sure, the researchers needed to find butterflies in the field.

From late 2017 through early 2020, Talavera and an international team of researchers conducted fieldwork at a wide variety of sites across sub-Saharan Africa during the months of December and January. Another 15 sites were monitored year-round for about two years. In total, the team looked for adults and their young in nine countries, including Benin, Cameroon, and Kenya.

It was a big bet that the team would even find the butterflies, Talavera says. Their preferred plants in Europe are well-known, making them relatively easy to find. But what plants the insects might settle on in Africa was a big open question. Aside from the previous analysis pointing toward the subtropics and knowing that the insects prefer open spaces over forests, the team was going in blind.

From September to November, the butterflies occupy semiarid savannas. As those areas dry up, the painted ladies shift south to savannas and highlands in slightly humid regions from December to February. Central Africa’s tropical rainforests stop the butterflies from going any farther south, Talavera says. “These butterflies don’t like wet areas.”

Field observations suggest that lots of breeding might happen in these slightly humid savannas and highlands.

You can read more of Talavera’s migratory discovery here, in PNAS.