The great outdoors can produce baffling mysteries. MVTimes Wild Side columnist Matt Pelikan tries his best to solve them. Got a question for the Wild Side? Send it to firstname.lastname@example.org
Here’s a good one – a picture of a luna who battled his way inside my house and thrashed around until he finally settled on a jar of capers in my pantry. He was attracted to our porch light. Why is that? At one point I used to leave the porch light on overnight so I could go out in the morning and see what had gathered. But then I felt guilty watching the chickadees pick them off one by one from the screen in the morning.
The Pelikan brief:
You think a porch light is attractive to moths? Try putting out an ultraviolet “black light” sometime! Many kinds of moths just can’t resist black light; in fact, biologists studying moths routinely use ultraviolet light sources to attract moths for collection or observation. But while the tendency of moths to fly toward light, and toward ultraviolet light in particular, is well known, nobody has come up with a fully satisfactory explanation.
The rest of the answer:
Most likely several factors play a role, and it’s also probably that different moth species respond to light for different reasons. And it’s worth keeping in mind that, for most of the evolutionary history of moths, all or most of the light at night came from the moon, stars, or the sky glow before sunrise and after sunset. So today’s situation, with artificial light sources popping up everywhere in moth habitat, is not what moths evolved to experience.
Moths are simple animals, and their needs in life boil to the usual basics. They need food, they need a mate, and it’s helpful to have some way of finding their way around. Explanations have been offered that relate to all three of these needs. For example, many adult moths feed on nectar from flowers. And many flowers glow under the ultraviolet lightt that makes up part of incoming sunlight (it’s the flower’s way of advertising for pollinators to come visit). So there may be some primitive tendency in moths to fly toward ultraviolet light in expectation of a meal. This tendency, then, would bring moths in to human light sources, at least ones that emit part of their energy in the ultraviolet range.
Many kinds of moths may also patrol large areas to find mates, and in order to be efficient about it, would want to avoid going over the same area repeatedly. One way to do this is simply to fly in a straight line, and some moths may use the moon or bright starts to help this. If you head toward the moon, or keep it on, say, your starboard beam, you’ll keep moving in a roughly straight direction. But if you try this with a light source that is much closer than the moon, you either arrive there and then don’t know what to do next, or you end up circling the light in an effort to keep it at the same angle. Both mechanisms could bring moths to light.
Finally — and I get in over my head on this one — some biologists noted a similarity between the frequency of ultraviolet light and the resonant frequency of some of the chemicals, called pheromones, that moths produce to lure potential mates. In other words, ultraviolet light somehow remind moths of the irresistible smell of a mate, even though one stimulus is a visual one and the other is a scent.
So take your pick: any, or all, or none of the above. Many kinds of moths are attracted to light, and that’s really all anybody knows for sure. But there is also agreement that in today’s environment, moths are paying a price for their fondness for light. The moths around your porch light are essentially wasting their time, hanging around in a sterile habitat that is unsuitable for them to live or reproduce in. Disruption of moth life cycles by artificial lighting is a real problem, and it’s one you can help with by turning outside lights off unless you are actively using them.