Painted ladies undertake the longest known insect migration over several generations – from their winter quarters south of the Sahara to Scandinavia. How many specimens arrive in Europe can vary a hundredfold from one year to the next. Researchers have now investigated which factors cause the large differences in population size. The main factors are therefore precipitation in the African savannah and in North Africa, which provide enough food plants, as well as favorable tail winds, which the butterflies can let themselves be carried by.
Similar to migratory birds, various types of insects also undertake long hikes, some of them long, to switch between their winter and summer quarters. Often the journeys take place over several generations, whereby the conditions along the way determine how well the insects can reproduce. Insect migration can have massive effects on agriculture: some species act as pollinators, others attack the plants in large numbers and can cause baldness. Some insects can also transmit diseases. “It is therefore essential that we better understand the movement patterns of insects,” emphasizes a research team headed by Gao Hu from the Nanjing Agricultural University in China.
From Africa to Europe
For the current study, the researchers examined how painted ladies migrate from Africa to Europe and which factors determine their population size. “It is well known that the number of painted ladies in Europe fluctuates widely, sometimes a hundred times more from one year to the next,” explains co-author Tom Oliver from the University of Reading in the UK. “However, the conditions causing this were unknown, and the presumption that the butterflies could cross the Sahara desert and oceans to reach Europe was not proven.”
In order to get to the bottom of the prerequisites for large painted lady populations in Europe, the researchers evaluated data from butterfly counts from 1994 to 2015 and compared them with weather records. “This research shows that this unlikely journey across desert and ocean is possible, and that certain pre-migration season climates have a huge impact on the number of people who make it,” says Oliver. In the 21 years studied, there were five in which particularly large butterfly populations arrived in Europe. In 1996, 2003, 2006, 2009 and 2015, the climatic conditions on the route must have been particularly favorable for the painted lady. Using statistical models, the researchers calculated which climatic factors best explain the population fluctuations.
With a tailwind across the desert
To their surprise, they found that an important influencing factor appears to be the precipitation during the winter months in the savannah south of the Sahara. “This is unexpected for two reasons,” the authors write. “First, the savannah is typically very dry in winter, so it may come as a surprise that this region appears to produce large numbers of painted ladies in certain years. Second, our results suggest that the butterflies must be able to cross the Sahara desert against the prevailing seasonal winds – an enormous challenge for insects, which are comparatively short-lived and weak in flight compared to migratory birds. “
However, further analyzes and modeling showed that there are also winds in winter and early spring that enable the painted lady to cross the Sahara with a tail wind. At an altitude of one to three kilometers, the butterflies could reach their next breeding area in North Africa within four days. If there is also sufficient rainfall there in spring and thus plenty of fodder plants for the caterpillars, the painted lady can multiply again explosively. The next generation will then be able to cross the Mediterranean and reach Europe.
The results can also contribute to the understanding of other insect migrations and help predict which insect species will be found in different regions in the future due to climate change and how many they could arrive there. “We enjoy seeing the beautiful painted ladies in our gardens in Europe, but climate change will also shift invasive species that pest crops or spread disease,” says Oliver.
Source: Gao Hu (Nanjing Agricultural University, China) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2102762118