It's a common literary trope, but moths really are attracted to the flame. And that usually goes poorly for the insect, with death a common outcome – either by burning or by predators that use the light source as a buffet table. As humans occupy an increasing share of the planet, we are lighting up ever more portions of the night sky, which may affect nocturnal species like moths that are attracted to light. Swiss researchers Florian Altermatt and Dieter Ebert, based in Zürich and Basel, thought that moths may be able to evolve changes in their behaviour to resist anthropological changes in light conditions. As artificial light can be so dangerous to moths, they hypothesized that moths from areas with bright lights at night might be less attracted to lethal light than moths from dark areas as a result of evolutionary changes in their behaviour.

To test this, the researchers set out collecting young caterpillars of the ermine moth Yponomeuta cagnagella from cities and towns in Switzerland and France that varied in their night-time light levels. They reared these caterpillars to adult moths in a laboratory where all the animals received the same light levels. They then set up an indoor flight cage with a light trap at one end, left the moths overnight to flutter to the light, and counted how many were collected by the trap in the morning to test how attracted to the light the moths were.

The duo found that female moths were much less likely to be trapped than male moths and that, in agreement with their hypothesis, moths from bright areas were much less likely to be trapped than moths from dark areas. This suggests that the moths from bright areas have undergone selection to avoid lethal bright city lights. As the researchers had collected the moths as young caterpillars, they think this response must be a genetic difference, rather than a difference in light exposure during development.

Altermatt and Ebert suggest this might be good news for moths and other nocturnal insects. If insects can evolve reduced attraction to light, perhaps light pollution won't be as dangerous for these species. But they caution that their results also show that as light pollution has caused systematic changes in animal behaviour to evolve, it may take several generations before moths see the benefits of efforts to reduce light pollution. In addition, it is unclear how the reduction in attraction of the moths to light might impact the insect's ecology. Our light pollution has taught the moth to avoid the flame, but perhaps at a cost.

Reduced flight-to-light behaviour of moth populations exposed to long-term urban light pollution
Biol. Lett.