Bug Off: Why Mosquitoes Have An Annoyingly Amazing Sense Of Smell
Mosquitoes use their sense of smell to find their next meal: us. So what would happen if you tweaked their smell so that humans smell really gross to them?
That’s what Dr. Chris Potter and his lab recently tried to do—they changed the neurons responsible for the insect’s smell detection, so that in the presence of animal odors, their olfactory systems would be overwhelmed. Instead of smelling like a nice meal, mosquitoes would be repelled by the scent of humans, like if you were stuck in a small room with someone wearing too much cologne.
This method worked in Drosophila, the common fruit fly, so Potter and his team were hopeful that would also be the case for mosquitoes. Instead, the experiment didn’t go as planned. Because finding a blood meal is so important for mosquitoes, those little buggers evolved backups for their backup receptors. When Potter turned one pathway off, another one kicked in.
Ira talks with Dr. Chris Potter, an associate professor of neuroscience in the Johns Hopkins University School of Medicine, based in Baltimore, Maryland, about his findings, and why we can never quite get mosquitoes to bug off.
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Chris Potter is an associate professor of Neuroscience at the Johns Hopkins University School of Medicine in Baltimore, Maryland.
IRA FLATOW: Summer means camping, beaches, barbecues, and pesky mosquitoes to bug us for another season, right? They use their amazing sense of smell to sniff out their next meal– us. Hoping to figure out a way to get mosquitoes to leave us alone, one team of scientists asked what would happen if we tricked their olfactory neurons, those cells responsible for smell, into being repulsed by the smell of humans. The answer turned out to be a lesson in biology.
Here to tell us more about that experiment and its surprising results is Dr. Chris Potter, associate professor of neuroscience in the Johns Hopkins University School of Medicine, based in Baltimore, Maryland. Welcome to Science Friday.
CHRIS POTTER: Thank you for having me.
IRA FLATOW: So your original goal was to trick mosquitoes into being repelled by human smell. How were you going to do that?
CHRIS POTTER: Well, what we were trying to do is we were trying to take an odorant receptor that normally responds to human odors in Anopheles mosquitoes. And instead of having that just in a very small subset of olfactory neurons in the antenna of the mosquito, we were going to put it in almost all their olfactory neurons. And in this way, when a mosquito would fly towards the human, for example, all those olfactory neurons would now get activated, when before, a lot of them were silent. And our hope was that that would be a repellent signal to the mosquito.
It’s sort of like if you’re in a room that has somebody who has very strong cologne or perfume. You just need to get away from that. And we were hoping the same thing would happen with mosquitoes, that if we were to trick them, humans would now smell so strong to them that they would try to get away from us.
IRA FLATOW: Well, do you have any past history with other insects where this has worked?
CHRIS POTTER: Oh, yes. We tried this originally in Drosophila melanogaster. It’s a vinegar fly. And it works beautifully well there. So we thought, perhaps naively, that we could just bring this over into the Anopheles system, into the mosquito system, and it would also work really well there.
IRA FLATOW: Because they would think we’re just annoying. They’d be so overloaded they’d want to get away from us. But did it work?
CHRIS POTTER: It did not work, no.
IRA FLATOW: It did not work.
CHRIS POTTER: Surprisingly, it did not work. And so what happened instead, instead of turning on these neurons, we actually shut them down. We were expecting that the olfactory system of the Anopheles mosquitoes would now be overwhelmed, but the exact opposite actually happened, that now the mosquitoes’ olfactory systems was shut down completely.
IRA FLATOW: Wow. Wow. That must have been surprising.
CHRIS POTTER: That was surprising, and a little bit confusing in the beginning. We weren’t sure what was going on. And so then we had to go and figure out why wasn’t it working like we expected it to.
IRA FLATOW: Did you ever figure that out?
CHRIS POTTER: We did. So it turns out that in mosquito olfactory neurons, there seems to be a mechanism in the neuron itself where if you overexpress an odorant receptor, it actually shuts down that neuron. And so what was happening is when we were expressing this odorant receptor in all the olfactory neurons, all those olfactory neurons were now being triggered to shut down their odorant receptor signaling.
IRA FLATOW: So looking at the big picture, what did this experiment tell you about mosquitoes or about doing research in general?
CHRIS POTTER: It’s a little harder to do these things in mosquitoes. What it did tell us is that the olfactory system in mosquitoes is a little trickier than we expected, that it has a lot of aces up its sleeve. And so we think that this mechanism that we’ve identified in mosquitoes might suggest that they might be a lot more adaptable to change. You know, we try to knock out one aspect of mosquito olfaction, there’s another aspect that will come in and take care of that. It’ll fix it for them.
IRA FLATOW: So they have backups, but maybe backups of backups.
CHRIS POTTER: That’s right, yeah. So we were looking at one class, one type of olfactory receptors called odorant receptors. And we essentially knocked all those down. They were no longer functioning. And so what they did instead was they used their backups, their backup olfactory receptors. Those can then take the place of the odorant receptors that we knocked out.
IRA FLATOW: Because this is very important, for those receptors to work. That’s how they live. They need to suck blood from people.
CHRIS POTTER: Exactly right. So this is such an important drive for them that if they don’t get a blood meal, that’s essentially the end of the line for them. The blood is required for the females to produce eggs. So no blood, no eggs. That’s the end of the line. And so there is such a strong evolutionary drive for them to get that blood meal that they have figured out all sorts of tricks, all sorts of ways to make sure that they’ll always find a human and always get our blood.
IRA FLATOW: Do we know what attracts them? I mean, to repel them, you’ve got to know what attracts them, right?
CHRIS POTTER: That’s true. Yes. There’s a number of odorants that we give off on our skin that seems to be attractive to mosquitoes. These are breakdown products of certain oily substances in our skin that give us the human odor to the mosquito. And they’re super attracted to those. Unfortunately, there’s nothing we’ve identified yet that is more attractive to a mosquito than a human. So there’s no good lure that you can use at this point that would work better than having a human.
IRA FLATOW: Did I hear you say that humans are their favorite food?
CHRIS POTTER: For these anthropophilic mosquitoes– so that is like the Aedes aegypti mosquitoes, the Anopheles gambiae mosquitoes– they are the favorite food for those mosquitoes, yes.
IRA FLATOW: Wow. So if you’re among animals, we’re the choice meat in this thing.
CHRIS POTTER: It’s frightening. It’s amazing.
IRA FLATOW: Yeah, it’s frightening and disheartening, because we know that we’re their number one target. Is there anything that will keep us from getting eaten alive, any kind of repellent we can suggest?
CHRIS POTTER: Well, yeah, there’s a number of repellents that are on the market that are fairly effective. So like DEET, for example, is the most commonly used insect repellent. We’ve been using it since the 1950s, and it does work quite well.
One thing that does seem to increase your attraction, something to keep in mind for the backyard barbecue, is alcohol consumption. Drinking a beer, for example, can make you a little bit more attractive to a mosquito. The thinking is that it helps– your skin gets flushed, a little bit warmer. And so when you’re a little bit warmer, you’re giving off a little bit more odors that the mosquito can pick up on.
IRA FLATOW: Wow. Thank you for that, because I’m not going to see that on a beer commercial this summer. Thanks a lot, Chris.
CHRIS POTTER: Yeah, thank you.
IRA FLATOW: Dr. Chris Potter is an associate professor of neuroscience in the Johns Hopkins University School of Medicine, based in Baltimore.