“Like a moth to a flame” may lose its meaning soon. Urban moths living in areas with heavy light pollution have restrained their fatal attraction and learned to avoid artificial light sources. The findings are published in Biology Letters this week. 

Nocturnal insects like moths are severely impacted by street lamps and city lights around the world. Not only does light pollution interfere with their natural day-night cycles, moths also die by being burned or becoming exposed to predators. Mortality at artificial light sources can be 40- to 100-fold higher than in areas where the night sky is actually dark. However, the evolutionary consequences remain unknown. 

To see if moths are adapting to changing light conditions, Florian Altermatt from the University of Zurich and Dieter Ebert from the University of Basel collected the larvae of the small ermine moth Yponomeuta cagnagella from 10 populations living in areas with low light pollution and in places exposed to long-term heavy light pollution in eastern France and northwestern Switzerland. They were raised in plastic boxes under typical garden conditions and were fed European spindle. After they emerged from their cocoons, the 1,048 moths were released into indoor flight cages with a fluorescent tube at one end.

Compared with moths from small villages and the countryside, city moths from parks and along streets showed a major reduction in their flight-to-light behavior. Additionally, city females were significantly less attracted to light than males. In many different species, males are the more mobile sex, and in this case, more often attracted to light. Overall, the average reduction in flight-to-light behavior was 30 percent. 

This reduced attraction to light sources likely benefits their survival and reproduction. While it seems natural selection has favored moths with a lower propensity to fly towards the light, this evolutionary change may have negative impacts for the community: If moths are becoming less mobile overall, this means reduced pollination of flowering plants and less food for nocturnal insectivores, like bats and spiders.