Why are bugs attracted to light? - Smarter Every Day 103

Hey, it's me Destin. Welcome back to Smarter Every Day. So you've noticed that moths and other bugs sometimes get trapped around a light. Have you ever thought about why? [music] We are in the middle of the Amazon rainforest and we have a huge mercury vapor light that Gerson set up for us. And we are looking at all the different species that have shown up.

This is Phil Torres. He is a, what do you call it? - Entomologist. - Entomologist? Is that with a d or a t? - With a t. - Entomologist. So tonight we are studying why these insects get caught around the light. The term for this is called Phototaxis. So an organism is phototactic if it responds to light with motion. A positively phototactic animal will move toward a light and a negatively phototactic animal, it doesn't necessarily have to be an animal. I guess.. think about it; leaves are phototactic.

So here's what we're doing. They are going through, are you taking photos of all the different species? - Mmm hmm. A good amount of them, yeah. (Destin) You've got one on your ear, hold on. Got him. Man, I have been moving around and I haven't seen the same species twice. Oh, that's a pretty one. That is a crazy look.. Is that a moth? - Yeah. (Destin) Is this how entomologists find new species? - Yeah, this is a really good way because they just come right to you. (Destin) You just sit around? - Just kind of sit around and different things come at different times of night, so what we see now may be completely different in like three hours. - That's awesome.

What do you call these big ones, Phil? - A sphinx moth. - That's a sphinx moth? And then what about this guy with the little.. Princess Leia ears? - Ahh. I think that's a kind of silk moth. - OK.

We've demonstrated that the mercury vapor lamp is effective in bringing the species out of the forest and onto the bedsheet, but what we haven't answered is our question. Why are they attracted to the light in the first place? Well, I was surprised to find out that most of the literature on the subject was written in the 50s, 60s, and 70s. It's really old.

So here's what we're gonna do. I pulled out two theories from the literature and we're gonna demonstrate them both. This is an old Sphinx moth that I got two years ago. Check it out. If we take this and we use it and combine it with some lunar footage that I got from my buddy Jack, we can demonstrate theory number one. Moths use heavenly bodies like the sun and the moon to orient themselves in free flight. The idea is that they see the moon up in the sky and they try to keep themselves aligned at a certain angle with the moon.

Now, because the moon is so far away, that angle pretty much stays the same as they fly along. Now imagine we brought the moon down to the earth. As he goes underneath, his viewing angle would change more rapidly and he has to pitch up in order to compensate for it. This ends up pointing him just beyond the far edge of the light source. This would induce a logarithmic spiral, which is pretty easy to define with math. We should be able to look at the footage of the moth flying around that mercury vapor lamp and figure out if there's a logarithmic spiral, right?

I used the Phantom to record some high-speed footage of those moths flying around the lamp in hopes of plotting that logarithmic curve, but because the generator was running on AC, that light was blinking, and so I decided not to even analyze the footage. Instead, I found some photos that a potter named Steven took and submitted to Nat Geo. His open shutter method proved to be more effective. Look at those curves. Do you see any logarithmic spirals? Kinda maybe, but the problem is there's several moths that you can see flying directly in towards the light. That kind of debunks our theory, doesn't it?

This leads us to theory number two. If you're a moth in the woods and you see a little bright spot over to your side, you can assume that's a wide open area. Therefore, you can free fly without getting hit by trees and sticks and stuff like that. So the theory goes that these species of moths will align themselves with a light and fly directly towards it.

So, after looking at all these theories, and making my own observations, I took high-speed video of cicadas flying around lanterns. Something became painfully clear to me. Science does not know why insects that are phototactic are attracted to light. We think we know, but we don't. I mean at a base level, we don't know what's happening. This is such a simple thing for humans not to know.

We've been observing this for hundreds of years. So as long as humans have had fire, we've known that bugs are attracted to artificial light. So, it was only a matter of time before we invented things like the bug zappers. Hold on. [Hitting bug zapper] You buy a bug zapper, you expect it to work. Go. There you go. Sorry. Obviously, these things don't work as advertised. Let me explain why.

So if I read the literature correctly, more bugs will go to the UV light because more bugs can see that wavelength. Now, this is interesting, but the problem is, mosquitoes aren't the only type of bugs that are dying on these things. You might be doing more harm to the moth population than you are actually doing to the mosquito population. Mosquitoes are actually attracted to carbon dioxide, which is why they have these little inserts of octanol or something like that that they put in the bottom of these things.

One last thing. You see these black baffles on the side of the bug zapper? Now, that's there for a very specific reason. Remember how we talked about how a moth or another bug will try to go towards the side of a light source instead of directly at it? Now, what these baffles do is they create the illusion of multiple small light sources. So when a bug comes in and he sees those small light sources, he tries to go for the side of one, and unfortunately he goes directly in towards the electrified grid, [snaps fingers] getting zapped.

Anyway, that's pretty interesting. So, moths and other bugs do not fly solely on light sources alone. They also use other cues, like pheromones. How do I know that moths fly with pheromones, especially while mating? I read it in a book, while I was flying. [aircraft noise] You might be aware that Audible actually sponsors Smarter Every Day, so I recommend an audio book; you can download it for free and it helps Smarter Every Day.

Right now, I'm actually reading a book. It's by a guy named E.O. Wilson, Edward Wilson. He went to the University of Alabama just like I did. Pulitzer Prize winner, he's a really good guy. Anyway, what's interesting is, he is a bug expert. From my state. Pretty cool. The book I'm reading right now is called Letters to a Young Scientist; right now I'm reading about moth pheromones. Pretty neat.

It's actually pretty good, I'll let you look out the window real quick, just so you know where we're at. Anyway, the Letters to a Young Scientist is about Edward Wilson's long career and what he learned and didn't learn while he was studying insects. You can go check it out. The cool thing is you can learn from his mistakes and the things that he did that worked. Go check it out. Audible.com/smarter. You can download a free audio book. Top link in the video description, that'd help me out. Thanks, have a good one. [aircraft noise]

So the theory is that as you're flying, the closing angle between.. thank you.. the bug, and the moon, the artificial moon will change, and he tries to keep it overhead. And as he's flying... [loud insect sound] [laugh] [insect sound fades] Thank you. As he's flying, he always keeps the "moon" on top of him, and he just ends up in this rotational pattern. [Captions by Andrew Jackson] captionsbyandrew.wordpress.com Captioning in different languages welcome. Please contact Destin if you can help.