Some Butterflies, Moths Benefit From Warm City Temperatures

City butterflies and moths have a longer flight season than their rural counterparts, a new study finds.

Cities are typically significantly warmer than surrounding areas. City centers are usually 1-7 degrees hotter during the day and about 2-5 degrees warmer at night than their outlying neighbors, according to the U.S. Environmental Protection Agency (EPA).

Most cities experience what is known as this urban heat island effect to some degree. Cities also have light pollution at night, which artificially extends daylength.

Having warmer temperatures creates a longer growing season for insects as they’ve adapted to start their overwintering later in the year. Many insects benefit from this longer season and may even produce an extra generation with that extra time, says lead researcher Thomas Merckx, a biologist at Vrije Universiteit Brussel.

Recent research has found that climate warming is lengthening the flight periods of butterflies and other insects.

“Also, a few studies demonstrated that rapid evolution in such insects is correcting for the mismatches between photoperiodic [light and dark cycle] cues and how they respond to seasonal change,” Merckx tells Treehugger. 

“Indeed, while many organisms use daylength as a cue to know how far advanced the season is, the warming climate messes up the information within this cue. Evolution, however, allows to re-align this daylength cue with the appropriate developmental response, so that developing organisms are able to make the right choice near the end of summer as to whether to risk direct development into the adult stage or opt for developing into the overwintering phase.”

For this new study, Merckx and his colleagues wanted to test if climate warming was similarly having an impact on butterflies and moths in urban settings.

“Our idea proved right, which is remarkable given that urban populations are typically connected to the rural populations, and that this evolutionary effect is present at small spatial scales (the scale of individual cities),” he says.

The results were published in the journal Proceedings of the National Academy of Sciences.

Interesting and Important Adaptation

For the study, researchers analyzed the green-veined white butterfly (Pieris napi) and the latticed heath moth (Chiasmia clathrata). They performed laboratory experiments, raising offspring from wild-caught insects with different controlled photoperiods, in order to see if shorter daylengths had an impact.

They also analyzed citizen science data, comparing population data on the insects from six urban areas in Sweden and Finland.

They found that urban populations have adapted to have longer growing seasons, starting their overwintering later in the year.

“In general, warming temperatures are a bad thing for species as most species are fine-tuned to a relatively small range of temperatures, with climatic warming pushing the ambient temperature over their optimal range. However, some warm-adapted organisms do benefit from rising temperatures, as it allows them to colonise new sites,” Merckx says.

“Moreover, as we show here, some organisms will evolutionary adapt to rising temperatures. However, it is likely that this evolutionary response will be more prevalent in already common, generalist species, with many species not able to respond in time to rising temperatures. How general our findings are is definitely something that now needs more attention.”

Researchers found that the warmer urban setting allows the insects to develop into adults during the same season, which allows them to mate, and the offspring to develop sufficiently before winter comes. Rural insects instead will overwinter at that time.

“As such the urban populations can get an extra (partial) generation within the same year, and this is highly beneficial for the local urban population,” Merckx explains.

This adaptation is both interesting and important, researchers say.

“It is interesting because it shows that urbanization can lead to rapid evolutionary change. It is important because it shows that humans are having evolutionary effects on other species. It also shows that the urban heat island effect has a very strong selection pressure, impacting urban communities,” Merckx says.

“As such, this also shows that reducing the extent of the UHI in cities by various measures (having more trees, water, less impervious surfaces...) is an important aspect of making our cities more hospitable for more species, leading to more biodiverse cities in the end.”