SACHA PFEIFFER, HOST:
It's time now for our science news roundup from Short Wave, NPR's science podcast. And I'm joined by Regina Barber and Hannah Chinn. Hi to both of you.
REGINA BARBER, BYLINE: Hey.
HANNAH CHINN, BYLINE: Hey.
PFEIFFER: You have brought us three science stories that caught your attention this week. What are the three?
BARBER: So bean plants' surprising chemical counterattack against pests.
CHINN: We've got new ways to speed up learning in mice.
BARBER: And how our strongest mosquito repellent may have a weakness we didn't know about before.
PFEIFFER: Ah. So since fiber is such a wellness trend right now...
BARBER: Yes.
PFEIFFER: I keep hearing about fibermaxxing.
BARBER: Yes.
PFEIFFER: Why don't we start with the bean? Tell us about that one.
BARBER: Yeah. So this story is an amazing example of plant versus animal warfare, and it might make you think a little differently about how crafty plants can be in defending themselves. And it all centers around the common bean plant. So this is the one I feel like a lot of grade schoolers grow in classrooms. It's a garden staple, the bush bean or pole bean. And it turns out it has an elaborate chemical defense mechanism.
PFEIFFER: Ah, and what's it defending itself against? I assume some pest or some blight or something?
CHINN: You're right. In this case, it's caterpillars. So, Sacha, in a study out this week in the journal Science Advances, researchers described how when caterpillars munch on bean plants, a compound in the caterpillar spit causes the bean plants to release a chemical signal into the air.
PFEIFFER: (Laughter) OK.
CHINN: Think about it as a kind of distress flare that calls in a fleet of wasp bodyguards.
ADAM STEINBRENNER: The types of chemicals we were able to measure were known to be attractants to predatory wasps, parasitoid wasps that lay eggs in caterpillars.
CHINN: So this is Adam Steinbrenner. He's a plant biologist at the University of Washington, and he's one of the authors on the study. He says some of the wasps eat the caterpillars. Others lay eggs inside the caterpillars' bodies so their babies can eat them, really effectively removing the threat to the bean plants.
BARBER: And, Sacha, this kind of chemical signal isn't rare. Plants release them all the time. Like, cut grass smells a specific way. That's the plant's chemical response to being wounded.
PFEIFFER: Interesting. The cut grass smelling a specific way reminds me of flowers smelling sweet because they're trying to attract bees.
BARBER: Yeah.
CHINN: Yeah. And these are both chemical responses, right? But the specific compound that attracts wasps is really only triggered via caterpillar spit. So the bean plant doesn't call wasps in if, say, you cut a leaf or if you damage the plant in a different way. And, Sacha, this is just one of multiple ways that plants have really evolved to defend themselves, if you think about the thorns of a rose or the spines of a cactus or bitter, poisonous leaves. Or I guess, in this case, this strategy - right? - calling in some aerial reinforcements from the insect world.
PFEIFFER: It's like calling in the drones.
CHINN: Yes. Yes.
BARBER: (Laughter) Exactly.
PFEIFFER: All right. You also mentioned, for your second topic, new ways to speed up learning in mice. Tell us about that one.
BARBER: Yeah, so I brought you a study just published in the journal Science. In lab experiments, researchers typically give little rewards to help them learn tasks. But in this study, they found that giving them bigger rewards less frequently actually helped the mice learn a lot faster. Here's Josh Dudman, one of the authors of the study.
JOSH DUDMAN: We discovered that we had been substantially underestimating how efficiently animals can learn. We could essentially take some of the slowest learners and move them all the way up to the fastest learners.
PFEIFFER: I really want to know what the rewards were. I'm thinking about cheese or peanut butter we put in a mouse trap. What did they use as rewards for these mice?
CHINN: So in this case, the reward was artificially sweetened water, which, for what it's worth, is also something that humans appreciate.
PFEIFFER: And what sort of things were the mice trying to learn?
CHINN: Well, one was the classic Pavlovian cue. They were learning that certain sounds meant a reward was coming. And they also had to learn harder things, like pulling a joystick or turning a tiny steering wheel right or left to get a reward.
PFEIFFER: So I'm interested in this idea that a bigger amount of reward given less frequently resulted in faster learning rather than smaller, more frequent rewards. Tell us more about why that worked better.
CHINN: Yeah. Well, we don't know the whole picture, but Josh and his colleagues think it has something to do with dopamine and engagement, right? Dopamine is a chemical that makes you feel good, and then your brain is like, oh, I want to do that again. And when the mice got these big rewards, they got a bigger burst of dopamine. And the scientists say that lasted longer and kept them more engaged than these little rewards. They also pointed out there might be other factors at play, not just dopamine.
PFEIFFER: Since this is just a mouse study, what does it tell us potentially about human learning?
BARBER: Yeah, so these experiments give insight into what keeps the brain engaged, which might be helpful for future animal experiments. We aren't quite at the point of comparing these experiments to human learning, but it's a step in the right direction.
PFEIFFER: All right, so topic three, we move on to another type of pest in some people's eyes, and that is how our strongest mosquito repellent may have a weakness we didn't know about before. What repellent, and what weakness?
BARBER: Yeah, so many people know DEET is an important chemical in many mosquito repellents, but it may have an Achilles heel. Mosquitoes can learn to associate its smell with dinner.
CHINN: Which is a pretty big deal - right? - because having an effective mosquito repellent isn't just good for avoiding itchy bites. It also saves lives. Mosquitoes can spread really deadly diseases, like malaria.
PFEIFFER: Yeah, so tell us more about this because, of course, the reason we wear DEET is to keep mosquitoes away.
BARBER: Yep.
PFEIFFER: So remind us, what is it that DEET does that repels mosquitoes?
BARBER: Yeah, so that's the part that's wild to me. We still don't fully know how exactly DEET works because...
CLEMENT VINAUGER: It is not as easy as one might think to ask mosquitoes, you know, how does it feel? How do you smell, taste that?
CHINN: So this is Clement Vinauger. He's a neuroethologist at Virginia Tech who worked on the study, which was published recently in the Journal of Experimental Biology. He said, the reason DEET generally works is because it repels mosquitoes. We just know they don't like it for some reason or another.
Here's the thing. Previous research shows that repeatedly exposing mosquitoes to DEET made them less repelled by it. And Clement and his colleagues wanted to take it a step further and find out if mosquitoes could learn to actually be attracted to DEET.
PFEIFFER: This all seems so counterintuitive.
BARBER: Yeah.
PFEIFFER: So how do you teach a mosquito?
BARBER: Like one of the experiments in the mice study about rewards, these researchers went back to Pavlov - you know, that study where you ring a bell, you make the dog salivate because they think they're going to get fed.
CHINN: And the researchers used the same concept, but instead of a bell, they use the smell of DEET. And as a reward, they used a bag of blood or sugar that the mosquitoes could bite and feed on.
BARBER: And the training worked. They found that in a little over half of the trials, mosquitoes actually began to associate DEET with feeding time, and they would actively approach it, even if the reward wasn't there. And when they tested with actual humans, they found that trained mosquitoes would also gravitate towards a person's hand sprayed with DEET, more than their other hand without the repellent.
PFEIFFER: All right, so I feel like I'm confused by this. You put DEET on, and it's going to attract a mosquito, even though we want DEET to repel mosquitoes. So what does this tell us about DEET's use and effectiveness in the real world?
CHINN: This study shows it's possible for mosquitoes to learn to be attracted to DEET, and that surprised Ali Afify. He's an entomologist and a neuroscientist at Drexel University. He wasn't involved in this study, but he pointed out that it's unclear whether this sort of thing could happen in the wild because remember, this required a lot of training mosquitoes, and usually wild, untrained mosquitoes will fly away from DEET.
ALI AFIFY: Because mosquitoes are not attracted to DEET in real life, so it's very difficult for mosquitoes to experience DEET while blood feeding.
BARBER: So for now, the researchers say, please don't stop using DEET. They emphasize that it's still a very effective repellent in real-world situations.
PFEIFFER: All right. For those of us who like hiking in the woods and other places where mosquitoes congregate, we'll hope that DEET remains effective. That is Regina Barber and Hannah Chinn from NPR's science podcast, Short Wave. You can follow it on your favorite podcast app for new discoveries, everyday mysteries and the science behind the headlines. Thanks to both of you.
BARBER: Thank you.
CHINN: Thank you.
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