Moths at Crossroads?
Polyphemus Moth. Photo by Marguerite Gregory © California Academy of Sciences.
JB: This is Earth and Sky. Ken Geohegan of Colorado Springs writes, “We get a lot of moths in town. The birds fly stunning acrobatics at intersections to catch them. Why at intersections? Are the moths attracted to traffic lights?”
DB: Ken, you didn’t say if you saw these flying creatures at night or in the day. Most moths are noctural – they fly only at night. If you’re seeing these insects in daytime, chances are they’re caddis flies – close relatives to moths and butterflies. Caddis flies swarm together in order to find mates – typically near rivers or streams where females lay their eggs in the water. To a caddis fly, wet asphalt looks like a stream. Especially after a rain, you might see caddis flies swarming around puddles at an intersection.
JB: On the other hand, night-flying insects are probably moths – and, yes, it’s probably the light that’s attracting them. Moths often use moonlight to navigate – but artificial lights can interfere.
When a moth confuses an artificial light for the moon, it ends up spiraling closer to it. Moths are solitary – they don’t actually swarm. But, it’s true, Ken, hundreds of moths might be drawn to a light at an intersection.
DB: More on why moths are drawn to streetlights – at earthsky.org. Thanks to the U.S. Forest Service and to the National Fish and Wildlife Foundation – supporting the conservation of native fish, wildlife, plants, and their habitats. We’re Block and Byrd for Earth and Sky.
Author’s Notes:
Moths aren’t so much attracted to artificial lights as they are disoriented by then. A nearby light source is like a powerful drug to a moth. That makes sense because – aside from the occasional forest fire – earthly light sources have been in existence for only a very short time in contrast to the history of the moth.
Like many flying insects, moths are able to find their way partly by using light as a compass. When the source of light is the sun or moon, that light source is very distant – and the incoming light rays striking the insect arrive just about parallel to each other. So moths – and many other flying insects – have evolved to expect to receive light at a fixed part of the eye. As long as the moth flies more or less in a straight line, this visual pattern remains unchanged.
Now consider what happens when the light source is a nearby streetlight. Then the angle at which the light strikes the moth’s eye quickly changes while the moth holds to a straight-line course. The moth tries to maintain a constant angle to the source – and as it does so, it spirals in toward the light. So the moth seems “attracted” to the light – so much so that it may end up colliding with it!
- Moths don’t typically swarm. If they are congregated at a light, it is mostl likely because they are disoriented and confused.
- Moths navigate by keeping the moon at a fixed angle to their line of travel. Artificial lights confuse moths-they use the light, not the moon, to institute their fixed angle of travel. The light is much closer than the moon, so their “fixed” angle of travel is always changing. This brings them on a spiral course towards the light.
- If you see a swarm of insects that look like moths during the day, they are more likely caddis flies or mayflies.
- These insects use polarized light to find water.
- Some female mayflies actually lay their eggs on wet pavement, so sure are they that the road is a stream.
- In ancient fossil tar pits, along with saber toothed cats and mastodons, people have found many beetles. Scientists believe the beetles, like caddis flies and mayflies, were confused by the polarized light reflected by the slick tar. They swarmed over it, and many were trapped and eventually fossilized.
The following individual was interviewed for today’s show. Our thanks to:
Christopher C. Caudill, Ph.D.
Postdoctoral Fellow Program in Aquatic Chemical Signaling
School of Biology
Georgia Institute of Technology
Atlanta GA
Web sites:
Bugs To Light (from All Things Considered, August 18, 2002)
