A Beetle’s Chemical (And Plastic) Romance
For many species of beetle, the key to finding a mate is scent: Both females and males give off pheromones that signal their species, their sex, and even their maturity level. How do researchers know? In experiments with dead beetles that have been sprayed with female pheromones, live males reliably attempt to mate with the dead insects.
But when one team of researchers based at the Chinese Academy of Sciences in Beijing and Syracuse University in New York tried to investigate whether this was true for the flea beetle Altica flagariae, they got a strange result. Males seemed confused when presented with scented dead beetles, leaving the team wondering if the dead beetles were still exuding their original chemicals. What is a research team to do? They attempted the same experiment, but with 3D-printed replicas. This time, the male beetles seemed clearly attracted to the female scent, the researchers wrote in the journal Chemoecology last month.
Producer Christie Taylor talks to Syracuse University biologist Kari Segraves about the intricacies of studying beetle intimacy, and the implications for evolutionary biology.
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Kari Segraves is a professor of Biology at Syracuse University in Syracuse, New York.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. Later in the hour, we’re going to talk about the clues scientists look for to predict the wildfire season in California, plus how common wood chips are helping to clean the water all over the world. But first, you know the song, right? Birds do it, bees do it, even beetles do it.
Yeah, I’m talking about– what else? Mating. And in the insect world, smell is a big part of the process of finding a suitable mate. SciFri producer Christie Taylor is here with a story about the odorous quest for love and an unusual new method for studying it. Hey, Christie.
CHRISTIE TAYLOR: Hey, Ira.
IRA FLATOW: So I’m guessing when we’re talking about seductive scents for beetles, we’re not talking about Chanel No. 5, right?
CHRISTIE TAYLOR: No, I think that’s a bit out of their price range. Today we’re actually talking about pheromones. You and I wouldn’t be able to smell them, but for the beetles in this story, they are a highly personalized cocktail of chemicals that can tell a would-be suitor everything from what species his date is– that’s important– to whether she’s actually even fertile yet.
IRA FLATOW: Yeah, I can see how this is useful for a beetle, for sure. And I think I can see why biologists would be interested in studying those chemicals.
CHRISTIE TAYLOR: Yeah, there’s a lot to learn there, for sure– not just how they work to convey information, but even how they might be helping new species form.
IRA FLATOW: OK, tell us the rest of this story.
CHRISTIE TAYLOR: Well, for that, we turned to Dr. Kari Segraves, a professor of biology at Syracuse University. I talked to her about an unusual innovation her research team just published that might make researching beetle mating and the chemistry involved easier to study. For starters, here she is introducing her main character– a beetle called the flea beetle.
KARI SEGRAVES: They’re called flea beetles because they sort of look like fleas do when they jump. They have really strong hind legs, and they can hop and basically fling themselves through the air to get away from predators. And they are actually gorgeous little beetles. They can be many different kinds of iridescent colors or have little stripes and things on them.
This particular one that we’re working on is mostly black with an iridescent thorax and head. And the iridescent color comes out really kind of a blue. It’s a really beautiful color.
These beetles are herbivores. They feed on lots of different plants. Particularly in the United States, they can be bad pests of many crops, including things like cabbage. Flea beetles can really cause a lot of destruction.
CHRISTIE TAYLOR: I’m a beetle. I’m looking to reproduce. What is the standard process for locating and wooing a mate?
KARI SEGRAVES: A very common theme is that they use chemicals in order to seek out and make choices. But beetles also do other crazy things. Like, you may have heard of the Hercules beetle. So they have these huge horns on their heads and either heads or thorax as well. And they engage in battles with other males so that the males will fight using these horns, and then one will throw another away, basically, out of the picture.
But we have other things like fireflies. Those are actually beetles. And they use these really complex light shows to be able to communicate between males and females. It’s all over the board. It’s hard to pinpoint one general system.
CHRISTIE TAYLOR: So for the story that we’re going to talk about today, we are going to focus on those that use those chemical signals. So when you’re studying that process, the pheromones themselves, how does that research process usually go?
KARI SEGRAVES: The projects that we’ve been working on generally involved understanding how insects make choices about their mates. That is, how do they find other individuals that are the same species and then make decisions about which of those individuals they’re going to actually mate with? Usually, what we’ll do is we’ll set up arenas that are usually trays or some kind of container. And we’ll place a male in with two different models of things that they need to choose between. So it might be a male versus a female beetle, or it might be an immature female versus a mature female beetle. And then we just let the male go and say, all right, in this assay, which of the models do they choose?
In these studies that I’ve been doing with Huai-Jun Xue at the Chinese Academy of Sciences, we’ve been using dead beetles. We’ll kill a beetle, then we’ll let the male go and say, which one of these options does it prefer to mate with? They’ll mount and attempt to copulate with the dead beetles, as odd as that sounds. But essentially, if you do enough males, then you’ll be able to figure out overall what the preference is for that species.
CHRISTIE TAYLOR: OK, that’s what usually happens. But you recently got some results that suggested a new approach might be kind of important. What went wrong?
KARI SEGRAVES: We were looking at three different beetle species. And essentially, we were asking the question, do they recognize the differences between males and females based on their chemistry? So can they find a recognized chemical signal, and do they know which ones are male versus female?
So they prefer the females, but we don’t actually know why they prefer the females. It could be because of the chemicals, or it could be the way the beetles look or the way they feel when they start to mount them. And so in order to figure out if it was the chemicals versus something else, what we had to do was essentially remove the chemicals from these dead beetles and then swap it, so put the male scent onto the female bodies and put the female scent onto the male bodies.
Two of the species, the males preferred the female scent. So they were mounting the males that were coated with the female scent. That third species, it was kind of a mix. It was like they were completely confused. They didn’t know which to go to.
Some of the males went to the females. Some of the males went to the males. And they didn’t exhibit any preference. That made us question our setup, basically, and our experimental design.
CHRISTIE TAYLOR: So you had taken all the scent off these dead beetles and put male scent on dead females and female scent on dead males. And the alive male beetles didn’t seem to know which was which, still, somehow. So maybe there’s some scent that didn’t get washed out.
KARI SEGRAVES: Right, right.
CHRISTIE TAYLOR: So this is where 3D printing beetles comes in. How did you come up with the idea to try this?
KARI SEGRAVES: Actually, I took inspiration from one of my colleagues and friends, Rob Raguso at Cornell University. He had been using 3D printing to make scent-free flower models. And so I really took Rob’s idea to heart and suggested to Huai-Jun that maybe if we made a 3D-printed beetle, that means we could have something that didn’t already have these chemicals. We could then paint the chemicals on, and then we wouldn’t have to worry about maybe having a mixture of signals, because those dead beetles might have been leaking out more pheromones. And if they were doing that, then we would have had a mixture of chemical signals, and that would explain why we didn’t see a preference.
CHRISTIE TAYLOR: So you 3D printed some beetles. How did that go?
KARI SEGRAVES: Well, it was a little tricky, because these are tiny beetles. They have very fine legs and very fine antennae. 3D printers aren’t quite good enough to be able to print those super fine structures. In the end, our models didn’t– they didn’t have legs or antennae, but they had the overall shape, so they looked pretty much like the beetles.
CHRISTIE TAYLOR: And did the male beetles go for these imitation beetles?
KARI SEGRAVES: Yeah, so you would be surprised that they were very willing to attempt to mate with these little 3D-printed plastic models. Most of them chose the female-scented 3D models.
CHRISTIE TAYLOR: So then you took your blank slate of plastic, and you sprayed it with chemicals, and you got this pretty strong confirmation, then, that these male beetles are really drawn to scent?
KARI SEGRAVES: It was really cool, because we had that result before that had said they don’t actually have a preference when we swap the chemistry. And so this actually allowed us to be able to show that the beetles do actually cue in on the pheromones and that they are using them to decide whether or not a specific individual is a male or a female.
CHRISTIE TAYLOR: What does this get us to understanding on a deeper level? Is there an evolution component, for example?
KARI SEGRAVES: You know, there are so many beetles in the world. One in four animals is a beetle.
CHRISTIE TAYLOR: Wow.
KARI SEGRAVES: Yeah, there’s maybe about 400,000 species of beetles described. People have talked about how important beetles are in the sense that they are really species rich. And so I think we can use this as a model to kind of learn more about how new species form.
It’s one of the key questions in evolutionary biology, is how do new species form? And the interaction between plants and plant-feeding insects might be one of the key ways that we can get new species to form, because plants contain a lot of chemistry themselves. I’m talking about chemistry today.
And so essentially, what this does is it sets up an evolutionary arms race where, literally, the plants are gaining defenses, and the insects are evolving counter-defenses. And they’re sort of in this race where they’re battling it out to be the one on top. That escalation can drive what we call specialization, or where insects tend to have a really narrow diet. And so in this way, we might see, actually, both groups of plants and plant-feeding insects becoming more species rich over time.
CHRISTIE TAYLOR: Well, and you mentioned one in four animals on Earth is a beetle, which is– that statistic alone is kind of blowing my mind. But I know, personally, when I see a beetle, I tend not to find it very easy to tell what kind of beetle it is. There’s so many little black and brown beetles. If I can’t tell a beetle apart, how does a beetle tell a beetle apart?
KARI SEGRAVES: Right. Especially in this group that we’ve been working with, these beetles are extremely similar to one another. They’re notoriously difficult to identify by humans, although I think the beetles do a just fine job. We’ve been working with these three closely related species, and as far as we can tell, despite the opportunities for them to interbreed, that is a very rare thing that happens in nature. So they’re really honing in on the cues on the outside of the beetles, these chemical signals that are distinct between the species. And also, then, within the species, they have these distinctive characteristics that allow them to tell male from female and immature from mature individuals.
CHRISTIE TAYLOR: I’m thinking about beetles like emerald ash borer, which are these terrible, terrible pests that have created a great deal of environmental destruction. Could understanding beetle chemistry help us control them better or manage them?
KARI SEGRAVES: So it’s possible that we might be able to co-opt– if we understand more about these cues, they’re also using these types of chemicals to make choices about what to feed on. In addition to looking at mating in these beetles, we’ve also looked at their attraction to their host plants. We were doing this one particular study where we were interested in two species. They’re each other’s closest relatives, and one species feeds on mature elm trees, and another species feeds on young elm trees only. It’s really strange.
And so we were asking questions about, well, what is it about old versus young trees that is allowing them to figure out what they should be feeding on? So we did a similar study to the one we’ve been talking about, where we took extracts of the chemistry off of the surface of young and old tree leaves and then painted it on to filter paper disks, so literally a piece of paper. And the beetles that were attracted to young trees would go to the filter paper disks that had the young signals painted onto them, and they would chew on those filter papers. They would lay eggs onto them. It was just like they were sitting on their host plants. And so if you can trick them into doing something that they shouldn’t be doing, then you might be able to use that as pest management.
CHRISTIE TAYLOR: Well, I think that’s all the time we have, and I want to thank you so much for joining me.
KARI SEGRAVES: Thanks for having me. It’s been great.
CHRISTIE TAYLOR: Yeah, it’s been really fun. Kari Segraves is a professor of biology at Syracuse University in Syracuse, New York. I’m Christie Taylor.