City butterflies keep flying for longer

A new study led by VUB biologist Thomas Merckx shows that because of the warmer urban environment in which they live, butterflies and moths display a longer flight season than those in the surrounding countryside. The experiments he and colleagues conducted demonstrate for the first time that urban populations are evolutionarily adapted to start their overwintering state later in the year.

“This result shows that there are urban populations of butterflies and moths that have managed to adapt evolutionarily to the warmer, urban climate,” says Merckx, who started the Global Change Biology group within VUB’s Department of Biology on the 1^st of December.

Cities are often warmer than surrounding areas. This creates a longer growing season for insects, when it is warm enough for them to reproduce. Many insects, including several butterflies and moths, benefit from a longer growing season and some even manage to produce an extra generation. Genetic adaptations can help, says Merckx:

“Because cities provide extreme environments for many organisms, such evolution can be very rapid: genetic adaptations can take place in as little as five to ten years. It is a misconception that evolution would always take hundreds of years.”

In insects, daylength is used as a cue to decide on the time they enter diapause and start overwintering. These results now show for the first time that caterpillars from urban populations of the species studied develop into adults even given shorter days and thus later in the season than caterpillars from rural populations. The higher temperatures in the city make it less important that the summer season is further advanced, as an extra generation of adult butterflies can often still reproduce successfully before the onset of winter. The study therefore concluded that urban butterflies have successfully adapted to their environment by having evolved a different response to daylength, resulting in later diapause induction, consistent with adaptation to urban warming.

Merckx: “Because the study focused on two species, and both recorded a similar pattern, it is likely that such evolutionary changes in the seasonal responses of urban populations could occur in other species that manage to survive in cities. Urban environments, thus, as drivers of evolutionary change.”

“It is interesting that earlier studies of evolutionary consequences of current climate change have identified very similar changes. Hence, urban environments seem to provide similar selection pressures on insect populations,” adds co-author Karl Gotthard from the Department of Zoology at Stockholm University.

The study carried out laboratory experiments on the green-veined white butterfly and the latticed heath moth from Stockholm, Helsinki and the surrounding countryside. The experiments were topped-up by an analysis of a long series of citizen science data from six urban regions in Sweden and Finland. The study was published in the renowned journal PNAS.

The original research article: Thomas Merckx, Matthew E. Nielsen, Janne Heliölä, Mikko Kuussaari, Lars B. Pettersson, Juha Pöyry, Juha Tiainen, Karl Gotthard & Sami M. Kivelä. Urbanization extends flight phenology and leads to local adaptation of seasonal plasticity in Lepidoptera.