City Moths Have Learned to Avoid Light, According to Study

Scientists have discovered that city-dwelling moths may be more savvy than their country counterparts when it comes to avoiding artificial light sources. According to a recent study published in Biology Letters, urban moths are less attracted to light, in general, than rural moths. While that means urban moths have a leg up when it comes to staying away from death traps like street lamps and building lights, scientists think the adaptation could have an unexpected—and possibly negative—impact on their ecosystem.

To study the light responses of urban and rural moths, Swiss zoologists from the universities of Basel and Zurich collected larvae of the spindle ermine moth (Yponomeuta cagnagella) from both low-light regions like the village of Kleinlützel and regions exposed to light pollution, like the city of Basel. Once the 1050 moths reached adulthood, researchers studied their flight-to-light behavior in the lab. They found that moths from regions with heavy light pollution were significantly less likely to fly towards light sources than those from low light areas; they also found that female moths were, in general, less attracted to light than male moths.

This isn’t the first study to illustrate the impact that human civilization and modern technology have had on moth evolution. Famously, the peppered moth took on a much darker hue in response to the smog and soot-filled atmosphere created by the Industrial Revolution. Over several generations, the once light-colored moths grew darker, an adaptive feat that allowed them to better camouflage themselves in dark, polluted regions. Likewise, when pollution decreased in the UK in the 1970s, the peppered moths began to grow lighter again. 

Urban moths in the Basel region, meanwhile, changed their behavior, rather than their appearance, over generations. Now, through natural selection, those moths are born with a lower flight-to-light propensity—an adaptive trait that allows them to stay safe in cities full of potentially disorienting artificial lights.

While that adaptation is undoubtedly beneficial to urban moth survival, researchers are concerned that it may have a negative impact on the moths’ broader ecosystem. For instance, urban moths may travel shorter distances, and end up pollinating fewer plants. Their light adaptation is both a fascinating example of natural selection in action, and a reminder of the unexpected impact human technology can have on other forms of life.