Abhijit Shylanath. 'Moths to light,' via Flickr. CC BY 2.0.

So you’re sat outside on a gorgeous summer’s eve, soaking in the atmosphere, only to be pelted with bugs left, right and center that seem to have a one way ticket towards your mood lighting. Why do certain insects carry out this irritating, yet slightly entertaining, behavior that often results in their rapid demise? There are a few theories surrounding this topic which we will look into, but so far it seems that no one is in agreement on the subject.

An organism’s response to light with motion is known as phototaxis. Positively phototactic organisms, such as moths, move towards light sources. Negatively phototactic organisms, on the other hand, move away from light, such as cockroaches that scuttle into a dark corner when you switch the light on.

A popular theory proposed to account for positive phototaxis in insects is that unnatural sources of light interfere with their internal navigation systems. Before the introduction of artificial lights, nocturnal insects such as most moths evolved to use natural light sources such as the moon or stars in order to navigate. These insects navigate by keeping themselves aligned at a certain angle relative to a light source.

Since the moon is so far away, the angle stays the same as the insect flies along, but this isn’t the case with smaller light sources such as a candle flame or light bulb. This is because the angle to the light source changes as the insect passes the source, so in an attempt to keep themselves aligned the insect ends up flying round in circles. The entire situation is, no doubt, very confusing for the poor little things.

This theory has a couple of problems, however, since unnatural light sources such as man-made fires have been around for thousands of years.  We therefore might expect that natural selection would have plucked out the insects that engaged in this suicidal behavior. Furthermore, many moth species are not in fact migratory, therefore it doesn’t really make sense that they would all use moonlight for navigation.

Another idea is that seeing an unobstructed light source indicates that the pathway is clear, therefore the insects fly directly towards it in an attempt to avoid obstacles. This could explain why some insects seem to kamikaze right into light bulbs.

Some people have postulated that since many flowers reflect UV light, bugs may be attracted to artificial light sources that also emit small amounts of UV because they mistake them for a flower, aka a food source. Indeed, bugs tend to be more attracted to UV light rather than longer wavelength light such as yellow and red.

A final, intriguing idea suggested back in the 70s by an entomologist proposes that moths actually mistake certain light sources for female moths. While this may sound quite bizarre, it was discovered that the infrared light spectrum given off by candle flames actually has a few common frequencies with the light given off by the pheromones of female moths. The same researcher that made this discovery previously found that pheromones are actually weakly luminescent.

Male moths may therefore dive into a candle flame because they mistook it for a female looking for a mate. Unfortunately for them, instead of getting lucky they end up burnt to death. Sad.

However, once again this idea doesn’t really make sense given the fact that, as discussed, insects are far more attracted to UV than light with longer wavelengths, such as infrared.

In sum, while it seems scientists haven’t quite made up their mind about why insects perform this behavior, it seems that the most likely explanation is that they are attempting to use the lights as a form of navigation, but it’s difficult to definitively prove this.

[Header image, "Moths to light," by Abhijit Shylanath, via Flickr, used in accordance with CC BY 2.0]


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