Sunday, May 17, 2009

Apple #384: Moths to the Light

So I know I said I was going to post a bunch of entries about my recent trip to California. But most of those entries have to do with plants. So instead of giving you a bunch of plant entries in a row and making you thoroughly sick and tired of plants, I thought I'd space them out a little bit.

So I'm going to tackle a question recently posed by faithful Daily Apple reader Tim, who asked:

Hey, I've got a Daily Apple question: Why do moths go toward the light? Had a beautiful moth bouncing around in here for a couple nights. What draws them to the light? If they like light so much, why don't they just get up in the daytime like the butterflies? Riddle me that!

I'm going to spare everyone the atrocity that would result if I tried to put the answer in a form of a riddle, or a rhyme even, and I'll just give you the facts. Or I should say, theories.

  • Most moths are nocturnal, so they wake up and go looking for food at night. They're not going to get up in the daytime like the butterflies because they're wired to do otherwise. This would be like the Apple Lady going to bed before midnight. Unthinkable!
  • Of the moths that are nocturnal, not all of them fly toward a light. But as for the ones that do, they tend to zoom toward the light bulb and ping around it for a while. Or, in the case of an open flame, they fly toward that and flutter around it and sometimes even fly into it and get incinerated.

  • So once again, we turn to the scientists for the answer to a "why" question. As I've learned in the years I've been doing this here Daily Apple, scientists are terrible when it comes to answering the "why." This case is no exception. They don't really know why moths fly toward the light. But they have some theories.
  • One theory that's probably the oldest, or anyway gets tossed around the most, is that because moths are nocturnal, they use the moon as their navigational reference point and they think that artificial lights are the moon.
  • Scientists have discovered that moths have a kind of internal compass (similar to that which Monarch butterflies have) which they use to keep themselves oriented at a certain angle relative to the moon. When the moths see an artificial light that they think is the moon, they fly toward it thinking that they will never actually get there but that it will remain far up in the sky and they will have to stay at that certain angle relative to the moon. But, surprise surprise, they do get to the moon (which is really the light bulb). This confuses the heck out of them, they back off, and then their navigational compass kicks in again, and they try to maintain that angle relative to the moon/light bulb, only to fly into it once more.

If you're a moth with a miniscule brain, would you know which of these lights is the moon?
(Photo by Olga Levina)

  • A few moth scientists don't like this theory so much. They say the patterns of moths' flight as they circle the moon/light bulb don't match up with the patterns that would result if the magical navigation-angle theory were true. I'm also wondering, if moths are so easily confused by artificial lights, how the heck do they manage to get where they're supposed to go, especially if they're migrating moths?
  • One moth scientist in particular, Henry Hsiao, has proposed another theory, which has two parts. The first of these is that moths are trying to protect themselves from predators. If a moth is sitting on a bush, let's say, and something is sneaking up from the ground to eat it, the moth is going to fly away. To a moth active at night, dark equals ground while light equals sky and moon and therefore safety. So according to this theory, when a moth zooms toward a light bulb, some predator has just tried to eat it and the moth has just flown like a bat out of hell to get away from it and, surprise, finds itself at the moon.
  • Once the moth is at the moon/light bulb, the moth realizes it's too close and tries to get away, or back to the darkness. However, surrounding every light source is a thing called the Mach band, which is a darker band of light. The moth thinks this darker band of light is the actual darkness and it circles around the light, trying to keep itself in the Mach band which is about a foot away from the light.

Look at the white circle of light in the middle of the purple. After a second or two, you'll notice a darker ring of purple about halfway out from the middle of the circle. This darker circle is not actually there; it's only an illusion produced by the eye. This darker circle is what is called the Mach band, and it's what Dr. Hsiao thinks that the moths try to keep themselves in as they're circling a light bulb.
(Image from Perceptual Stuff)

  • I have some problems with this theory, though, too. If this were true, the moth would fly in concentric circles and it would not keep going back to the light bulb, pinging off it, and back again. Also, why does light equal safety when the moth is in the bushes, but then when the moth is at the light, the moth decides that dark equals safety? And that Mach band is pretty small, especially compared to all the dark outside the circle. It's tough for me to believe that the moths really prefer the Mach band to all the dark that's farther outside of the circle.
  • It's also worth noting that Dr. Hsiao's theory includes the assumption that the moths are confusing the artificial light with the moon. In part one of his theory, they're assuming the light equals the moon and that they should fly toward it. So I don't see him necessarily contradicting the moon theory after all.
  • But what about when the light source is a candle? One scientist named Philip S. Callanan did a bunch of work with optics and infrared rays, and he said that it is male moths who are flying toward the candles, and they're following their sense of smell as well as sight. Callanan says that moths use a combination of smell and sensitivity to infrared light patterns to detect the presence of female pheromones. A candle emits an infrared spectra of light that's really similar to that of a female moth, so the male moth flies toward the candle expecting it to be a female moth. And then the male tries to mate with the supposed female, but instead gets scorched.

Moth flying toward a candle.
(Photo by Tony)

  • This doesn't account for the moths' behavior around light bulbs, though, so I'm dissatisfied with it.
  • Another scientist says that when the light source is flame, the moth has no "evolutionary history" to tell it that the heat associated with the flame is too hot and that it should not fly straight into it. It's flying to the flame, thinking it's the moon, but because it doesn't know to avoid heat, it gets toasted.
  • At first this seemed pretty ridiculous to me because flames have been around for a lot longer than light bulbs. I mean, when there's a forest fire, are all the moths just diving into the fire? But it turns out, somebody researched exactly this, and yes, the researchers saw the moths heading straight into the fire.
  • Elizabeth Gerson and Rick Kelsey, who work for the US Forest Service, did an experiment using pandora moths that live in Oregon. They caught about 200 of the pandora moths in two light traps, and then they set a controlled fire near the traps. They put the traps on the ground near the fire, opened the traps, and watched what the moths did. Very few moths flew out of the light trap. Of those that flew, three of them "spiralled into the flames." Most of the moths crawled out instead of flying, and half of those that crawled out walked straight into the fire and burned. "Radiant heat did not seem to deter pandora moths from entering the flames."

Coloradia pandora, the moth that Gerson and Kelsey saw walking into the fire. I think that unit of measurement is centimeters.
(Photo from Moths of Southeastern Arizona)

  • Even though I have seen a moth fly into a candle, I find this utterly shocking, that moths would walk straight into a forest fire. I mean, where's the sense in that?
  • By the way, this study also tested the moths' responses to different colors of infrared light. They found that the moths didn't seem to care what color the light was, which suggests that Callanan's theory is a bit wobbly. Callanan and Gerson were each testing different species of moths, so it's possible they could both be true. But I find Gerson's study in which they saw the moths walking into the fire to be more compelling.

Apparently if you're a moth, when you see a fire like this, you say, "Hey, wow, I want to get right in the middle of that."
(Photo of a forest fire near Big Bear Lake, CA by Fotoguy77, I think)

Man, I just can't get over that. I ask again, how does that make any kind of sense?

Well, after all this, I haven't answered Tim's question -- at least, not to my satisfaction. But it looks like the light bulb = moon theory seems to be the one that's winning. That is, this is the best theory scientists have right now, until somebody does some more research and maybe finds out that moths are doing something other than confusing the moon and light bulbs.

For now, I think the short answer is, "Moths fly to the light because they're stupid." Along those lines, these people are demonstrating for us the logic of moths. Except they have sense enough to quit after a while.

P.S. After calling all moths stupid, I have to amend my statement. Because I remembered the moths we learned about in the Joshua Trees entry, the ones who are solely responsible for pollinating those wacky and friendly plants. So I'm going to say that the ones who do not head straight for the flames -- in Gerson and Kelsey's study, that was slightly less than half -- those moths are not as stupid.

The Straight Dope, Why are moths attracted to bright lights? January 27, 1989
Howstuffworks, Why are moths attracted to light?
NPR All Things Considered, Why are Moths Attracted to Flame? August 18, 2007
Happy News, Why Are Moths Attracted to Light? September 26, 2007
James K. Adams, Why are moths attracted to lights . . .
Wikipedia, Moths
Philip S. Callanan, Moth and candle: the candle flame as sexual mimic of the coded infrared wavelengths from a moth sex scent (pheromone),
Applied Optics, December 1, 1977
James L. Oschman and Nora H. Oschman, Electromagnetic communication and olfaction in insects,
Frontier Perspectives, September 22, 2004
Elizabeth A. Gerson and Rick G. Kelsey, "Attraction and direct mortality of pandora moths, Coloradia pandora (Lepidoptera: Saturniidae), by nocturnal fire," Forest Ecology and Management, October 22, 1997


  1. Yup. It sounds like they're stupid.

  2. I always thought it was the teenage moth version of dragracing.

  3. A lot of interesting information but: they still appear suicidal to me and drive me nuts flapping around the light bulb!

  4. is it true that they don't fly straight

  5. This was a lot of help!! Thanks!!


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