It’s common knowledge that insects are attracted to lights, especially at night. As PMPs, you may be even more familiar with this phenomenon, using it against pest insects with technology like insect light traps (ILTs). Whether it's 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 occurred.
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 toward the light, but were angled toward it in such a way that they ended 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.
Figure 4. The effect of reflected light was strongly dependent on whether it came from below or above the insect. An example trajectory of insects attempting to fly above a white sheet illuminated by a downward-facing UV light tube. b Example trajectories of insects flying under a white sheet illuminated by an upward-facing UV light tube. c A diagrammatic representation of the hypothesized behavioral effect of ‘light trapping’ (left) vs. flight under a diffuse canopy (right). The strong effect of light directionality was also present in Honeybees and Diptera, both being unable to sustain flight when UV light came from below.
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
