It’s the same thing on warm evenings: you turn on your porch light and within minutes, you’re dealing with a swarm of bugs zooming around the lights—and bouncing off of your head or crash landing into your drink. What is it about light that brings out the bugs? Science tells us that this odd phenomenon is caused by phototaxis.
How Phototaxis Affects Insects
A phototactic animal or insect is one that instinctively moves to or away from light. The bugs that are swarming around your porch lights are positively phototactic, which means that they’re attracted to it. Bugs like cockroaches are negatively phototactic, which is why they scuttle away when you turn on a light.
What scientists aren’t quite sure of, though, is why insects are phototactic. Many believe that positively phototactic insects (like moths) use moonlight as a navigational tool. As they fly by moonlight, the Moon stays in a fixed position overhead. But when you turn on artificial light, the moths see it as another Moon and become confused. As they try to guide themselves by your porch lights, they end up zooming back and forth in an effort to keep the “Moon” in sight.
Another theory suggests that bugs use light as an escape route. In other words, if a predator were to shake a bush that insects were resting in, they’d fly towards the Moon where the predator can’t reach them. When artificial lights come on, because they’re generally brighter than the Moon, fleeing insects congregate around them instead.
Why Do Insects Continue Flying Around Lights?
You’d think that at a certain point, a bug confused by an artificial light source would realize its mistake and move on. However, bugs – especially moths – suffer from a temporary night blindness just like people do. Think about what happens when you’re standing in a brightly lit room and someone suddenly turns of all the lights – you experience a moment of confusion and almost total blindness as your eyes adjust to the darkness.
A moth’s eyes have light sensors similar to a human’s eyes (in insects, the sensor is called an “ommatidium”), but unlike humans, it takes a lot longer for the moth’s eyes to adjust to darkness after being exposed to a bright light – up to 30 minutes, in fact. So, as a moth swoops back and forth over your campfire or around a lightbulb, the bright light confuses it, but at the same time, the moth doesn’t want to fly into the darkness because it’ll be blind to predators or other dangers for up to half an hour.
What Kinds of Lights Attract Bugs?
While it isn’t the same for all insects, those that are attracted to lights are normally attracted to UV light and white light. That’s why hundreds of bugs will swarm around your bright white fluorescent lights but only a few will come to a campfire. If you want to keep the bugs away at night, try yellow or red bug lights. A few insects will still be attracted to these lights, but not nearly so many as are attracted to white lights.
In fact, this is one reason why bug zappers are supposed to work – the white and UV light that the device produces is supposed to attract and then zap mosquitoes. Bug zappers do attract a lot of phototactic insects, but only a few mosquitoes. While mosquitoes are somewhat attracted to light, they’re even more attracted to food sources, namely you!
If you want an evening outdoors without the bugs, try using yellow bug lights outdoors. Better yet, simply enjoy your evening by the light of the Moon. You can always use one of our natural repellents to keep the biting ones away from you.
Amber Kanuckel is a freelance writer from rural Ohio who loves all things outdoors. She specializes in home, garden, environmental, and green living topics.