Why insects are attracted to light

Now most of us will have noticed that artificial lights at night seem to attract insects like bees around a honeypot, if that’s the right analogy! We didn’t know why though, other than perhaps the insects might be mistaking street lights for the sun and trying to navigate by them. Now though, with the help of some extremely fast photography, Sam Fabian, from Imperial College London, thinks he’s got to the bottom of it...

Samuel - The question that was really interesting to us is why, when you put a light out at night, do you get insects showing up? It's something that I think pretty much every human being on Earth has seen. The trouble was that a lot of the explanations, and a lot of the discussion around it just didn't seem to match our understanding of how insects operate. They just didn't seem quite satisfying. So we thought, well, what's the data? It actually turned out there wasn't that much data about the way in which insects were flying around lights at night.

Chris - And you're right about the ubiquity of this because I think I've certainly read references back in, I think Greek mythology, where people are referring to insects being drawn like a moth to a flame. What have people speculated might be going on.

Samuel - There are quite a few theories. A few of them were things like, insects are inherently attracted to heat. We actually now know that's not really true, or at least that's not the effect that's causing this. You can stick an LED out that chucks out far less thermal radiation and yet you still get lots and lots of insects showing up. There were other theories that it was something to do with trying to escape to where they saw bright spots because they thought it was like looking through the trees at a patch of light and they were heading for that. But actually, when we look at the trajectories of insects, they don't tend to fly around in direct lines straight towards the light. In fact, they assume these weird circuitous, roundabout paths that lead them in towards the light.

Chris - How did you test it, then? Did you set up a bonfire or bright lights? What was the experimental setup, and how did you actually see, in an objective way, what the insects are really doing?

Samuel - The way that we approached that first question, is it this compass cue, which we eventually ruled out, one way in which we approached this was that if you're using something as a sort of landmark reference and you are keeping it at a certain region, you should want it to either be on the right or the left of your body, most likely. Well, if we turn that light off and we turn a new light on at exactly the same time you should, after a little bit of confusion in the middle, start circling this other light in the same direction because you should always want the light to stay on your left. But actually, when I sat there and toggle in these lights, we found that they would actually change which direction they were circling around the light, they would change having the light on their left or having the light on their right. And they didn't seem to care which side it was on, which really argues against this compass cue. And the lights in this instance were UV LED bulbs. We would hang these around and we could see the way in which the insects would fly around it and then would record these with high speed cameras. So these are things that are able to take pictures at 500 to 1000 frames per second.

Chris - And when you did that, did anything emerge from these pictures that enabled you to explain why the insects appear to have a bad sense of direction that they flip when you flick the lights on and off? What are they actually doing? They're clearly not worried about right and left or they would've done exactly as you suggested and circled in one direction. So what are they doing?

Samuel - I think it really clicked when I had this beautiful underwing moth, and it sat on my hand and I got it to take off and we had this UV light that was shining upwards. What happens is the moth would cruise over the UV light and as soon as it was over and above that UV bolt, it flipped itself upside down and dropped out of the air. That is not matched by any explanation or any theory that we currently have. Actually, we know about a behavioural response where animals tilt their backs towards light because they think the light is the direction of up because they think that's the sky. The sky has been, for 370 million years while insects have been flying around, the brightest region that they can see. So if you just assume the bright region is the sky and therefore up, you can very quickly work out which way is up. Using where is bright to work out which way is up is a really robust, simple, beautiful way to solve this problem. It's a really great way to do it until somebody invents street lights, at which point it's suddenly not such a good idea.

Chris - So putting all this together, then, you would argue that what is happening when that moth or that insect goes to the streetlight at night is that they see this as a light source that they then think, that is the sky, and they then continuously orientate themselves so that their back is towards the light. But why does that make them go round in a circle?

Samuel - If they're constantly tilting their back towards the light, well, on average they should think that gravity or gravitational acceleration should be pulling them in the other direction. If they're tilting their backs over, it's a bit like if you had a helicopter or a plane and you just tilted it over. All of a sudden, those forces are asymmetrical, and that leads the animal drifting around. That drift is dependent on where it is relative to light. But what happens is they get locked in these very circular orbits, especially if there's very little wind and you have a very stable flight. Something like a dragonfly, you can get really, tight beautiful circles of about 60 centimetres in diameter if they fly around, that's because the body is rolled over to one side of their flying. It's creating asymmetry in which way the flight forces are going, causing them to constantly turn. As they turn, the light moves relative to them and they keep adjusting for that and they just get stuck in this pile.

Chris - So is there anything you can do to help me with the blue bottle in my bedroom that won't stop when I want to go to bed at night? Is there anything I can do to lure it away better?

Samuel - Certainly we can think about ways in which we can lure this away better. What seems to be important is that wavelength is super key. Wavelength is really just the colour of the lights and that's important because we find that things like UV are very important and they work on lots of different insects. But we also think about this in the other way: how do we have lights at night that don't influence insects? For those of us that love bugs and those of us that don't, we should care about them and we should want to not influence their behaviour. This tells us that actually the direction in which we're putting out lights is really important and that we shouldn't have lights that are just shining up into the atmosphere or shining out sideways if we want to try and restrict our effects on insects at night.