Why are Moths Attracted to Flame?

In this week's Science out of the Box, we consider why nocturnal insects are drawn to flame or porch lights.

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DEBBIE ELLIOTT, host:

This is ALL THINGS CONSIDERED from NPR News. I'm Debbie Elliott.

Imagine you're sitting on your porch this evening, sipping a mint tulip to help you get through the sultry summer heat when suddenly, duck, an enormous moth comes swishing by, a little too close for comfort as it succumbs to its fatal attraction to your porch light. Everyone knows moths will fly to a flame, but we wondered why?

(Soundbite of music)

ELLIOTT: As we pursued the moth-flame connection for this week's Science Out of the Box segment, we found a few theories out there. For the first, we turn to Dr. May Berenbaum. She's the head of the department of entomology at the University of Illinois at Urbana-Champaign.

Hello there.

Dr. MAY BERENBAUM (Department of Entomology, University of Illinois): Hi there.

ELLIOTT: So why do moths have this suicidal attraction to flames and light bulbs?

Dr. BERENBAUM: Well, the sort of standard line of explanation is that it's something of an evolutionary short circuit that moths and other nocturnal insects use celestial navigation for orienting about in the dark, the same way that explorers could find their way by charting a course relative to the North Star or some celestial far-distant point source of light. And what has happened since that time is that humans have come along and developed terrestrial point sources of light. So very intense light that is not millions of miles away.

So in a behavior called transverse orientation, many animals, including insects, can move or fly to maintain a constant angle relative to a distant point source of light. So that - yeah.

ELLIOTT: So is this similar to what - you know, we've always heard about sea turtles and how they are looking for the moon to go back into the sea and when you have your lights on all along the beach, it confuses them and they don't know which way to go?

Dr. BERENBAUM: Yeah. It's, I think, another manifestation of more or less the same phenomenon. The reason that moths end up at porch lights so often, along with other nocturnal flying insects, is to maintain a constant angle relative to a nearby point source of light. They had to keep kind of moving closer and closer and adjusting their relative position. And ultimately they end up kind of at the light itself.

ELLIOTT: I can understand why they might bump into light bulbs and keep flying around that, but what about flames? When you have a candle and they tend to fly into the candle, and it's hot and often times it leads to their demise. It seems like the heat would push them away.

Dr. BERENBAUM: Well, that's kind of a short circuit part of it. If there's no evolutionary history of a close-by point source of light that also puts out enormous amounts of heat then they have no history of dealing with those conflicting kinds of signals.

ELLIOTT: Now, we've heard some other theories on this so we're going to let you hear one of those. We talked earlier to Henry Hsiao, a professor of biomedical engineering at the University of North Carolina. And back in 1972, he wrote a whole book on moths.

Here is what he had to say.

Dr. HENRY HSIAO (Professor, Biomedical Engineering, University of North Carolina): My best hypothesis is that it's net(ph) response of a night-flying insect to the morning light and let me explain that. During the day, almost all moths try to hide from predators. That's why their wing patterns have camouflaged patterns in drab colors. And at night, the darkness of light do not allow a moth to select the best hiding place so when the first morning light appears they have to fly up and towards the light and seek a place to hide. So you'd expect the behavior where they first seek light and then fly down and hide.

ELLIOTT: So Dr. Hsiao has studied the moth's flight patterns and says if moths really use the moon to navigate, they would end up flying in spirals around the light. His research seems to show that they actually make a beeline for it, if you pardon my mixed insect metaphor there. Which theory seems more plausible to you, Dr. Berenbaum?

Dr. BERENBAUM: He has a valid point, you know, and why they appear to fly straight at the light, everything is relative. The transverse orientation may begin at some considerable distance - 30 feet, 50 feet from the light source.

ELLIOTT: Well, thank you very much for talking with us.

Dr. BERENBAUM: Thank you for the question.

ELLIOTT: Dr. May Berenbaum is the head of the entomology department at the University Illinois at Urbana-Champaign.

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