It's common knowledge that insects are attracted to lights, especially at night. As PMPs, you may be even more familiar with this phenomena, using it against pest insects with technology like insect light traps (ILTs). Whether its a customer complaining of all the bugs around a porch light or a pile up of dead insects within a ceiling light, this is a common occurrence. In a recent study, researchers took this behavior and went to figure out exactly what was going on and why this occurs.

Popular theories regarding light attraction included ideas about how the insects were attempting navigation, but this recent study shows otherwise. Researchers took moths and observed their behavior when exposed to light with 3D flight trackers. This technology was able to show how the insect moved through the air. They found that the insects didn't move directly towards the light, but are angled towards it in such a way that they end up orbiting the light, stalling, and generally getting mixed up. Their movement pattern is described as orthogonal which means right angles. 

What I found most interesting was that the moths oriented themselves so that the light was on their back. The researchers propose that this orientation response to light may be a way to orient themselves vertically based on the fact that for millions of years, the sun and moon were the only light sources the moths had to worry about. The artificial light sources therefore, make it difficult for these insects to orient themselves vertically. They even observed that when artificial light was below the insect, it would crash.

To take things a step further, this suggests that the navigation hypothesis would not be entirely correct and instead the moth's confusion is a matter of figuring out which way is up. If moths only flew towards a light, they would all just fly straight up into the sky towards the sun or moon. The moths were not flying towards the light as we may think of them doing, showing that they are not trying to escape by going into the light. Orienting themselves to fly orthogonally, would keep them flying normally.

The study looked at a few species of moth as well as fruit flies and observed the described behaviors among most of them but not all. It's not clear yet where those differences may lie, but its given us a start in understanding this behavior. So next time you get a curious customer asking why so many bugs end up by their porch lights, you'll have an explanation.

Article by Ellie Lane

References

Fabian, S.T., Sondhi, Y., Allen, P.E. et al. Why flying insects gather at artificial light. Nat Commun 15, 689 (2024). https://doi.org/10.1038/s41467-024-44785-3