If you’ve been keeping up with social media the past week, you may have noticed a fluttering of memes depicting, in various ways, moths’ strong attraction to artificial lights. The memes all stem from a photo posted to Reddit by user No_Reason27 of a large moth peering in through a window, with bright light reflecting off its eyes. A small selection of moth memes can be found here, but a substantially larger sampling can be found on the Facebook groups Entomology Memes and Entomemeology, which have covered a spectacular sampling of memes people have created.
With a common theme of moths loving lamps, you may be tempted to wonder – just what is the attraction of moths to lamps? I briefly mentioned it in my post about Moth Week 2018, but I will elaborate now. Why are they drawn so inexorably toward artificial lights? There are plenty of hypotheses, and the “true” answer may vary from species to species or multiple answers may apply simultaneously. However, the conclusion most commonly cited, and explained in this post by AskAnEntomologist, is that many night-flying insects (including many moths) use light to navigate at night.
Imagine you are a moth in a pre-human world. When night falls, the only light sources available are the stars and the moon. Relative to the ground, these do move, but their movement is slow, constant, and predictable, so a good internal clock (otherwise known as circadian rhythm) can account for their movements. The visual acuity of insects is pretty poor, so the stars are most likely invisible, with the moon being the only visible landmark for an insect to fly by.
With some exceptions, insect brains often work by following simple rules rather than complex abstract thought, and the best evidence suggests that a night-flying insect’s simple rule to maintain a constant trajectory is to identify the brightest landmark in the sky and keep it in the field of vision of the same facets of the compound eye. When the moon is that landmark, it is far enough away that keeping it fixed in the same portion of the field of view works fairly well for maintaining a straight trajectory. The problem for insects arises when the landmark is a lamp, which is not so far away. As the insect flies, the relative position of the insect and the lamp change. For the insect to keep that lamp in the same spot in its field of view, it must turn toward the light. As the insect tries to follow its simple rule of maintaining straight-line flight, it actually spirals in toward the light. Because insects are typically poor at critical thinking, they become disoriented rather than realizing that the lamp is a poor landmark to follow, and remain trapped at the lamp, spiraling back to it any time they attempt to leave.
Many insects follow this basic rule or some variant of it, and as a result are attracted to lights. Other insects that are attracted to lights likely are attracted for a different reason, as some of them typically charge light sources head-on rather than spiraling in like a disoriented moth. Still other insects flee from light or cease flying when exposed to bright enough light. The responses of insects to light can be useful for pest management in a variety of ways, which are reviewed here. Guardian primarily uses light in the form of ultraviolet-emitting insect light traps, which monitor and in some cases reduce populations of insects that are attracted to light, such as moths and flies.