A painted lady butterfly in Morocco. Photo credit: Orio Massana
Weather conditions have been shown to have a major impact on migration figures.
Scientists have found that a species of butterfly found in sub-Saharan Africa can migrate thousands of kilometers to Europe and cross the Sahara desert in years of favorable weather conditions.
The showy Painted Lady butterfly (Vanessa cardui) has been shown for the first time to be capable of handling the 12,000-14,000 km round trip – the longest known insect migration – in greater numbers when the wetter conditions in the desert helping the plants on which it lays eggs.
The results of the international research team will improve our understanding of how insects, including pollinators, pests and the diseases they carry, could spread between continents in the future if climate change changes seasonal conditions.
Professor Tom Oliver, ecologist at the University of Reading and co-author of the study, said: “We know that the number of Painted Lady butterflies in Europe fluctuates widely, sometimes 100 times more from one year to the next. However, the conditions that caused this were unknown, and the suggestion that the butterflies could cross the Sahara and oceans to reach Europe has not been proven.
Map showing the vast area that Painted Lady butterflies travel through during their annual spring migration. Source: Gao Hu et al.
“This research shows that this unlikely journey is possible and that certain climatic conditions that lead to train season have a huge impact on the number it creates. It shows how the wildlife we see in the UK can cross national borders and protecting these species requires strong international cooperation. “
In addition to answering long-asked questions about butterfly migrations, the results could also help predict the movements of other insects that infest humans, such as the locusts currently plaguing East Africa or malaria-transmitting mosquitoes.
Professor Oliver said, “We enjoy seeing the beautiful Painted Lady butterflies in our gardens in Europe, but climate change will also shift invasive species, pest plants or spread diseases. The food shortage in East Africa is a reminder that the effects of climate change can be far more dramatic than a few degrees of warming appear at first glance. “
The Painted Lady migrates in the spring after a winter breeding season. Researchers used long-term monitoring data from thousands of trained volunteer recorders as well as climate and atmospheric data in regions of sub-Saharan Africa and Europe to learn more about their movements.
The study, published in the journal Proceedings of the National Academy of Sciences, found that increased vegetation in the African savannah in winter and in North Africa in spring, combined with a favorable tailwind, are the three most important factors in the number of migrations to Europe.
Painted lady caterpillars feed on the leaves of plants that thrive in wetter winter conditions in the savannah and Sahel regions of sub-Saharan Africa, causing population numbers to explode. They migrate through the Sahara, and if there are wet and green spring conditions in North Africa as well, this allows further reproduction and swells the number that cross the Mediterranean to reach Europe.
Simulations by the scientists also showed that there is regularly favorable tailwind between Africa and Western Europe, which offers insects opportunities for transcontinental travel.
The team calculated that the butterflies must fly non-stop during the day and rest at night to cross the Sahara, making stops to feed on nectar. This is similar to the pattern in which night-flying songbirds migrate.
They came to the conclusion that the butterflies have to fly up to 1-3 km above sea level in order to take advantage of the favorable tailwind, since their maximum self-flight speed of about 6 meters per second would make crossing the Sahara extremely difficult.
The researchers used observations of similar butterfly species to calculate that, after metamorphosis, painted ladies have enough body fat to fly 40 hours non-stop, and that they are replenished by nectar feeding whenever possible to cross the Sahara.
The results can help improve predictions about which insect species might be found in different regions in the future due to climate change and how many they might arrive.
Reference: “Environmental Drivers of Annual Population Fluctuation in a Trans-Saharan Insect Migrant” by Gao Hu, Constanti Stefanescu, Tom H. Oliver, David B. Roy, Tom Brereton, Chris Van Swaay, Don R. Reynolds, and Jason W. Chapman, Jan. June 2021, Proceedings of the National Academy of Sciences.
DOI: 10.1073 / pnas.2102762118
