Bitter Nectar, Toxic Pollen: Pollinators and Plant Chemicals

We’ve spent some time on this podcast discussing pollinators and their life histories but today we are taking a deeper dive into plant chemistry to better understand the relationship between pollinators and their floral resources.

To help us explore this fascinating topic, is Leif Richardson, Xerces Endangered Species Conservation Biologist. Leif coordinates the California Bumble Bee Atlas project. His research focuses on the ecology, distribution, and declines of North American bumble bees. Leif is also the co-author of a range of scientific publications on bees, including Bumble Bees of North America: An Identification Guide. He holds a Master’s degree from the University of Arizona and a PhD from Dartmouth College.

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Photo credit: Barbara Driscoll (c)

Thank you for listening! For more information go to xerces.org/bugbanter.

Transcript

Matthew: Welcome to Bug Banter with the Xerces Society, where we explore the world of invertebrates and discover how to help these extraordinary animals. If you want to support our work, go to xerces.org/donate.

Rachel: Hi, I am Rachel Dunham in Missoula, Montana.

Matthew: And I'm Matthew Shepherd in Portland, Oregon.

Rachel: We’ve spent some time on this podcast discussing pollinators and their life histories, but today we are taking a deeper dive into plant chemistry to better understand the relationship between pollinators and their floral resources.

Rachel: To help us explore this fascinating topic, is Leif Richardson, Xerces’ endangered species conservation biologist. Leif coordinates the California Bumble Bee Atlas project. His research focuses on the ecology, distribution, and declines of North American bumble bees. Leif is also the co-author of a range of scientific publications on bees, including Bumble Bees of North America: An Identification Guide. He holds a Master’s degree from the University of Arizona and a PhD from Dartmouth College.

Rachel: Welcome back to Bug Banter, Leif! We’re happy to have you here.

Leif: Thanks. I'm glad to be here.

Matthew: Yeah, no, appreciate you coming in. I'm sure that for many people—and I have to include myself in that group—plant chemistry is something that we haven't thought much about. Can you give us a simple background of what it is, and maybe why it's helpful to plants?

Leif: Sure. Well, organisms in general produce chemicals that are necessary to growth and reproduction. That would include us, right? We—our bodies produce all sorts of different proteins and molecules that are for signaling and that sort of thing. Well, plants do this also. And everybody already knows this, even if they're not aware that they know it. So we get lots of things from plants that come from these chemicals, right? So the tastes in our food that we enjoy, the colors of our foods, the various medicinal and drug properties, and toxic properties of plants—all of these are mediated by chemicals. And of course, plants—their growth and reproduction is also part of this.

Matthew: In the introduction, Rachel said, we're gonna talk about pollinators and floral resources, so we may as well move on and do that. But maybe more accurately we should say insects and the flowers they visit, since not every flower visitor is a pollinator. What role does plant chemistry play in pollination, or plant insect interactions?

Leif: Yeah, that's a key distinction to make. Not all flower visitors are pollinators. Some of them are—you could think of them more as herbivores that are taking from the plant without delivering the service of pollination. And so plants make literally thousands of different types of chemicals, and for many different reasons. I guess in the early 20th century it was thought that there was one class of chemicals that were necessary to the growth and reproduction of plants, which were termed primary metabolites. And a second class of optional things we didn't really understand, which we called secondary metabolites. And it's the so-called secondary metabolites that are responsible for color in fruits and flowers, odors in flowers, toxic effects of alkaloids in leaves, and that sort of thing. So we now know these chemicals are not just waste products or some random production of the plants that's secondary to their life, these are also very important things that plants produce at cost to structure interactions with other organisms.

Leif: So the first example that I will give about how to think about plants interacting with animals using chemicals is herbivory, or the chewing on plants by animals, right? So if we think about a caterpillar chewing on a leaf, that's the food that's necessary to grow the caterpillar to pupation, to the cocoon stage, and then to becoming a butterfly, I'll say. But plants will mediate that interaction by putting chemicals into those leaves. And so we all know that lots of plants are toxic to bugs, right? And so we could think about tobacco plants—they produce nicotine in the leaves. And this is why we grow and harvest tobacco for human use of nicotine. But nicotine in leaves has been interpreted as an anti-herbivore defense that the plants deploy. And it turns out that many of the same chemicals are also found in flowers. They're found in the nectar, which is the sugary substance that plants make as an attractant for bees and other pollinators. They're found in the pollen itself, which is the male sex gamete plus other stuff, but it's also an attractant for pollinators—it's very important to bees and other pollinators. So there are just a range of other chemicals that are being expressed in flowers that turn out to mediate the interaction between animals and flowers.

Matthew: Kind of a follow up thought to that is, are all these chemicals produced by plants? Are they good for pollinators? Or, you know, do they harm them?

Leif: The correct way to think about how chemicals structure pollination interactions is to go back to the leaf herbivory example. So these chemicals produced by plants, some of them are beneficial to consumers, some of them are toxic to consumers. So we can all think of very toxic chemicals, again, like nicotine, that can kill insects—it's a potent insecticide. So some of these chemicals benefit insects, as we'll talk about—flower visitors, as we'll talk about more—but some of them also have harmful outcomes for the consumers.

Leif: So a great example here in California where I work, there's a plant called California buckeye, which produces big candelabras of beautiful white flowers—lots of odor, lots of nectar in them—and they are absolutely toxic to some of the consumers, which would include honey bees, who will feed on the nectar at the flower, and then just drop right off the plant dead. Other consumers can actually handle those toxic alkaloids in that particular plant. So a good way of summarizing that is that these chemicals can be both beneficial and detrimental to consumers, who may be pollinators, or just flower visitors who are not pollinating. And in many cases, insects are gonna experience a trade off in benefits and detriments as they consume plant materials. And the chemicals are kind of what underlies the pluses and minuses when you do the math there and look at whether the bug benefited from visiting the flower.

Matthew: Talking about the buckeye and the honey bees reminded me of when I was living in Britain. In Britain it's known as a lime tree, but here it's called the linden. It's an absolute magnet for bumble bees, and yet you can walk underneath the big one and the ground will be carpeted with dead, poisoned bumble bees.

Leif: That's a famous example. The genus is Tilia, or linden trees. And we still don't understand fully what's going on with that plant's nectar, and how it kills flower visitors. But it might have to do with some sugars that are produced that are not the ones that the bees are looking for, and that are toxic in certain contexts. Yeah.

Matthew: Yeah. It's obviously complicated, but, does understanding the chemistry of plants help us better understand pollinators?

Leif: I think it does. I think it does, yeah. So, the old way of thinking about pollination biology is that plants offer rewards for pollinators. Those would be nectar, the sugary substance that fuels flight and adult activity in many, many pollinators, and pollen, which supplies the protein necessary to reproduce, and make more bees and more butterflies, and so on. And that's absolutely true—those are the two most important nutrients that are coming out of flowers for pollinators. But they're getting all sorts of other things from flowers. There are flowers that make oils that bees then collect, and certain specialist bees need those oils. Of course, flowers make volatile chemicals that smell good or smell bad, and those can be attractants for flower visitors. And of course, we're talking about all of these other chemicals that do occur in nectar and pollen. And so what I found in some of my research, and other people have also found, is that some of these chemicals can actually benefit the consumers—they can actually help them. And in some contexts, those consumers, let's say they know this, and they are foraging for those chemicals.

Rachel: I have a follow up question to what we were previously talking about. What is the advantage to the plant killing the pollinators? Like is that just not their pollinator? I'm just very confused—like, why would a plant kill its pollinator?

Leif: That's the central question—that was great. Yeah. We used to call this whole domain of thinking about ecology, we used to call it “toxic nectar.” Obviously, you know, nectar that is toxic, but it extends to pollen, and other things that bees may interact with at flowers, and other pollinators, as well, of course. The original hypotheses for what these chemicals are doing in the plants include: they may have some antibiotic function in the nectar, so plants may be investing the nectar with things that are toxic to insects, but they help keep the nectar from spoiling. There's a theory that these things could be attractants for pollinators, so they could benefit pollinators in some way. There's a theory that these things are deterrents to visitors.

Leif: And so in this case we might imagine a pollinator—so that is the flower visitor who can end up moving male gametes, or pollen, to female flower parts, right? The pollinator—as opposed to just a flower visitor who comes in and forages but doesn't end up doing that. So there's a theory that some of these chemicals could exclude the casual visitor who is not also a pollinator, while not excluding the pollinator. There's a credible hypothesis called the “Drunken Pollinator Hypothesis,” which just stipulates that these chemicals are intoxicants, and that a drunk or inebriated pollinator is more messy and less accurate, and its movements around the flowers—they tend to spread more pollen. This is a serious hypothesis that has some, you know, has at least some evidence behind it. So it's a great question. Why would you invest flowers with potentially toxic chemicals when flowers are—seem to be designed through natural selection to attract these pollinators that plants absolutely need for pollen transfer?

Rachel: Yeah. That's so interesting. I'm just imagining these like semi-drunk bumble bees, like bumbling around, like all confused and being them bumbling selves. Haha.

Leif: Yeah. I should add, just sort of a corollary to this whole conversation, is the fact that when flowers open, nectar is more or less sterile—it doesn't contain microbes. But as soon as the flower opens, microbes start raining into the flowers. And as soon as bees and other insects start visiting those flowers, they are bringing microbes. There are some microbes that specialize on living in floral nectar. It's the only place they can really live. And we think that bees and other floral visitors might be the way that they get around. One of those converts the sugars and nectar to alcohol—you know, just basic fermentation. Essentially, they make beer out of the nectar. And there's one study, maybe more than this now, that show that these alcoholic nectars are more attractive to bee pollinators than the sterile sugary nectars that do not contain alcohol. So it's—it goes even beyond just these chemicals that may deter or attract flower visitors. But some of them are actually changing the plant product of nectar into something more palatable for some visitors.

Rachel: Wow. That's so interesting. So you had hinted this in the previous question that Matthew had asked—so for your PhD, you studied bees and how they used the nectar and pollen of certain plants for medicine. And before we dive into your research, I am just wondering what led you to study this interaction?

Leif: Yeah. I think when I was trying to figure out what to do with my life work-wise, I was very interested in plants and plant reproduction for whatever reason. But also when I had days where I didn't wanna be a plant biologist, I thought I would be an ethnobotanist. Really interested in the way human beings use plants, and the way we've domesticated plants for food, the way we use plants medicinally, as drugs, et cetera. And so I think these two things came together to some extent for me, with the realization that plants produce all sorts of chemicals that have context-dependent effects on people, but also on these bugs that are so important to plant reproduction.

Leif: And so the revelation that bees—my favorite pollinator—were flying through this sort of, this environment that included benefits like sugary stuff, and protein-rich pollen, but also all of these chemicals that could have actual impacts on their lives got me really excited about thinking about what the plants are doing, what the bees are doing, how those chemicals may or may not be structuring their interactions.

Rachel: So how did you conduct this study?

Leif: So the first thing I did—and this was with my mentor, Becky Irwin at Dartmouth College. She's now at North Carolina State University—and our colleague, Lynn Adler, among others. Becky and Lynn had this idea that these chemicals—. One thing they might be there is—one reason they might be in the nectar is to attract the bees, and that the bees may benefit in some way from them. So we know that bumble bees typically have an array of parasites and pathogens that they struggle with that affect their fitness, their reproductive output, and even their lifespan, and things like this. So we had this idea that that some of these chemicals might kill the parasites that live in the guts of the bees without actually killing the bees. That would be, quote, unquote, “good for the bees,” right?

Leif: And so the first experiment was to take bumble bees that were recently adults [and] inoculate them with a parasite, unfortunately. They all got a measured dose of this gut parasite. Then, I fed them nectar solutions that either contained one of these potentially toxic chemicals, or did not. And so we had, I think, six or eight different chemicals. And then the two treatments—either the control, or you got the chemical. And what we found is that for some, but not all of those chemicals, the bees had a statistically significantly lower parasite load after seven days, I think it was, than those bees that got the control diet—they just got the straight sugar.

Leif: And so these are things like the terpenoid thymol, which is found in many different plants like oregano, and thyme, and other things in the mint family, but other plants, as well. In fact, I think—Matthew brought up the linden trees—and I believe they also make thymol in their nectar for whatever reason—they're not related to these guys. We also studied nicotine. Anabasine, which is another alkaloid closely related to nicotine. It's the molecular precursor—as the plant's making nicotine, it makes this thing called anabasine. We studied gallic acid found in many plants’ nectars. We studied caffeine, and then two iridoid glycosides, which are these potentially very toxic and extremely bitter chemicals that come from a plant that I was also working on called turtlehead, which is in the figwort family, or the—we now call it the Plantaginaceae—the family that includes plantains.

Leif: So for about four or more of these chemicals we had a significant reduction in the parasite load of the bee. And we did not see increased mortality of those same bees. Some of these chemicals we saw non-significant declines in the parasite load. Also, none of them caused an increase in the parasite load. And so this was pretty exciting. It just suggested that—. We tried a small handful of chemicals and we found a few that were beneficial to bees. We know that there are thousands—tens of thousands—of different chemicals that naturally occur in nectar and pollen. And so if we can, you know, find this with a couple of educated guesses maybe they're—maybe this is a general, a more generalized part of pollination ecology, that these chemicals are there that may benefit the insects that turn out to be the pollinators.

Rachel: Do you think that the bees are able to distinguish like which plants have this special pollen?

Leif: Some of what I'll say in response is a little bit speculative, because I can't get inside the brains of these bees. But I'll tell you what we've found, and what we know. And so first of all, yes, bees and other—I use the term bees frequently to refer to flower pollinators or even visitors, but I study bees, so I'll just keep saying that. So bees we know can taste some of these chemicals in nectar, and experiments will show that they're either attracted to foraging on nectar with the chemical, or deterred from doing so. This is also true for some pesticide residues. There are some studies that show that bees are—will drink more nectar if it contains a neonicotinoid pesticide metabolite, than if it does not. Not a good, this is not a good thing for bees, but just to make the case—they can tell the difference, and they can act on that.

Leif: And so what we did with the knowledge we got from that first experiment was to ask, “Okay, so bees can benefit from some of these chemicals. For one of the plants that produces those chemicals in its nectar and pollen, how might that affect bee behavior and pollination?” I did an experiment where I made an array of flowers—. Well, let me back up. First, [for] about five to 10 populations of plants, I sampled nectar from individual plants—hundreds of different plants—and we looked at the chemical concentrations in that nectar. We found that the populations differ a bit in their average chemical concentrations—and that this is catalpol and aucubin, which are two iridoid glycoside chemicals. So one population might, on average, have more of these bitter chemicals than another one. And then within the population we see a lot of variants, from flowers with not very much to flowers with quite a bit.

Leif: So taking that information, I designed this experiment where I used a micropipettor to move tiny little concentrations of artificial nectar solutions into flowers to either lower or raise the concentration of those two chemicals in those flowers. I then followed wild foraging bees through that array of flowers, and I mapped all of the different transitions from one flower to the next. And then I timed the duration of the flight between the flowers, and then the time that they spent on the flowers, and did this with many hundreds of bees. At the end of their movement through my array, I actually caught the bee. And the bee had to go to the lab and be sacrificed and dissected. And so now I knew if the bee was parasitized or not. And I—we actually looked at one gut parasite, and one other parasitoid fly larva, which lives in the abdomen but not inside the gut where it would be directly exposed to these chemicals.

Leif: So what we found is that bees that had a high count of these parasites in the gut would move through the array until it got to the flowers that were the most bitter. They would drink longer at those flowers than at other flowers, and they were much more likely to fly up and come right back to the same flower than bees that did not have a lot of parasites. So parasite infection was driving this behavior. And so bees without the parasite would forage through the array without respect to how bitter the flowers were. And we assume those bees could also taste these chemicals but they just move through the array, drinking the very bitter one, the less bitter one, without spending more or less time on this. So to us, this tells us that there's a context-dependent change in how bees are interacting with flowers that has to do with their parasite load, and the relative strength of the chemical in a flower that's embedded within a population of flowers. And in short, this is called self-medication. It is the same thing you do if you're sick and you go to the pharmacy and get cough syrup, or whatever it might be.

Leif: And so this opens a lot of questions about these insects. We know that lots of other animals do self-medicate—from other primates like ourselves, to a variety of other birds, and other vertebrates. There are some other insect examples. But you can see the consequences for pollination here. If sick bees will spend more time on flowers that are more bitter, the plant might be pushed to produce greater bitterness, right? This is a very simplistic way of looking at the evolutionary outcomes of this interaction. But plants are probably being affected by the bees’ behavior, and obviously vice versa.

Matthew: Wow. I'm—. Yeah, I'm watching Rachel's face, as well, whilst you're explaining all this. And I mean, for me, I'm just like, I—the dedication it takes, and the hours to be out there watching individual bees and, you know, recording all that.

Leif: It was a really fun experiment to do. And it was challenging because the plant is a wetland plant, and it grows in marshes that can be up to maybe two or three feet of water, and the plants are coming right out of the water. So I mostly did this work solo, and I would just wear high rubber boots and slosh around in the wetland. Haha. But I had an iPad, essentially, that I was using to record all the data on this little database. And I had to make sure not to drop that in the swamp, right? So it was like, it was a little bit more complicated than your average field work, in my experience.

Matthew: Yeah, no, that's amazing. You've probably touched on some of what I wanted to ask next, and that you found that with some flowers for specific chemistry, more pollen was moved from those flowers because, you know, the bees would feed on the flowers for longer. Which suggests that these flowers may have higher reproductive success. Is my simplistic statement—is that vaguely accurate? And if so, and kind of, how did you discover this?

Leif: Yeah. And that evidence is strong, but limited to—. I—. There may be other studies out there now that show that plants have higher reproductive success when they have higher concentrations of these chemicals. But my study, you know, it is just one study, so take it for what it is. But what we did is once we found out that the bees when parasitized were moving—were spending more time at the most bitter flowers is we designed an experiment where the flowers were again altered in terms of their chemical constituency. And then I added some fluorescent dye powder to the male parts of the flowers. And then we let bees forage. And after they foraged, we counted the number of dye particles that they, I think it was, that they took away. And so dye particles are widely used as a pollen analog in pollination biology.

Leif: So the idea is we think that the longer the bumble bee is in the flower—these turtlehead flowers—the more pollen just passively rains down on the body. They're also collecting pollen, so they're causing pollen to come off of the anthers. So the longer they're in the flower, the more pollen they leave with is essentially the story.
Now remember, bees are there to collect pollen for their own food reasons, and for rearing their offspring, but pollen is also the male gamete. So from the plant's perspective, you want that pollen to end up on a stigma, on a female part of a flower on another plant, right? And so both things are happening at once. But we found that plants are exporting more pollen when bees are spending more time in the flowers as a consequence of lapping up that super bitter nectar that they might be after.

Matthew: Does this mean that bees could be driving evolution of the flowers? And did they kind of—are they acting in a way that shape plant populations and communities?

Leif: Yeah, I think so. When we remove all the context and just look at these two interacting organisms, the bee—the bumble bee—and turtlehead—this plant. But remember that there's a parasite in this story. So it's entirely possible that the parasite is driving. If it's true that the bees’ foraging behavior, that is context-dependent, is driving natural selection on this plant trait, on this floral trait—nectar chemistry—it's also possible that the average prevalence of the parasite in the population is affecting, is upstream of that. And so it's a what we call a tritrophic interactions—it's a food web, if you will. And so this is a general rule in ecological systems. There are usually more than two organisms that are interacting by themselves. They all have different mutualists and antagonists, right? So pollinators, but also leaf herbivores, or the bees may have animals that they interact with that might benefit them in some way. But they also have these parasites living in their guts that definitely do not benefit them. So there's a lot going on at once. But the discovery that nectar with these toxic chemicals in it can benefit bees, I think is—shows that yes, that’s one place that floral evolution might be under some selection, depending on the context, the environmental context.

Matthew: Yeah, and also since the plant, the chemistry of the nectar kills the parasites.

Leif: Yeah.

Matthew: It's almost like, ah, which of that trifecta is driving evolution?

Leif: Yeah, yeah. One thing that's really interesting about this system to me is that we think—and there's some evidence for this—that these chemicals are, in fact, some of them are in fact, toxic to the bees. But just as with our medical system, and our physiology, the dose makes the poison. So it is a dose-dependent situation. So, I believe the nicotine in those studies was at about five parts per million, which is an average dose that we found in the scientific literature. Other people had just measured the concentration of nicotine in tobacco flowers. So at that dose, the bee is not killed and the parasite is. But if you were to increase that by an order of magnitude or something, a hundred parts per million of nicotine in those flowers, those bees would probably be dying, right? And so this is dose-dependent, just like chemotherapy is. You know, might save your life but may also cause some negative outcomes. So what I want people to understand is that there are trade-offs for the bees and other consumers of these chemicals. They may benefit in some way, like this anti-parasitic effect, but they may also experience some detriment.

Leif: And in this study, we didn't find higher—there's some other research that I haven't described yet—but we didn't find higher mortality among the bees that consume these chemicals, as we might have hypothesized. In one case, we found that the chemical had an apparent negative impact on the bees, which was that it slowed their reproduction. It gave them a few—it caused them to take another—I'm working from memory here—but maybe five to seven days before they could reproduce, before they could produce adult offspring themselves. So these chemicals may be having negative impacts in the body, even as they're having this one positive impact on gut parasites.

Matthew: It's never simple, is it?

Leif: It's never simple. Haha.

Matthew: It seems that the chemistry within the plant probably makes some plants more successful. Could there possibly be that some plants are less successful? I don't know whether that makes sense, but it seems a diverse plant community needs a diverse bee or pollinator community. And so it's like, do the changes in bee populations, or the pollinator population, do they change plant communities? Or is it kind of the plant community driving the pollinator community?

Leif: I think that this is just one of many studies that shows that there's this dynamism in the system where plants can drive bee behavior, and are probably responsible for natural selection operating on some bee traits, like tongue length, right? Flowers with long corolla tubes where the nectar is way down at the bottom of the flower probably lead to selection on the pollinators to have longer tongues, so as to be able to reach the nectar, right? So plants are, we think, are affecting their consumers in all kinds of ways. But of course, as we've just been talking, bees can affect plant reproduction, too. They can differentially impact the reproductive success of a given flower based on the quality of the flower and how they behave on it.

Leif: And so yes, I think your question is really about diversity—plant diversity and pollinator diversity—and how those things are maintained. And how chemicals might lower diversity, or something. It would be really hard to speculate on exactly how that works. But there's certainly natural selection taking place as a result of these interactions at the flower. And I would also just remind people to keep in mind that plants are these dynamic organisms that are producing—they're alive just like you, and doing all of this stuff—they're producing all of these chemicals. And an old way of looking at plants is that they weren't able to pro—they weren't able to regulate the distribution of these chemicals in their bodies. And so an easy way to explain toxic nectar back in the day was just that the plants need these chemicals to defend their leaves from herbivores, and it's just passively leaking into nectar and pollen. Which is bad, but there's nothing they can do about it. Not so, right? Look at any given plant, they are sequestering chemicals that result in color, or flavor, or odor in one part of the plant, like a fruit, that they are not putting in another part of the plant like a leaf, right?

Leif: So it's true that my focal plant, this turtlehead thing, had these iridoid glycosides in the leaves, in the stems, in the roots, in the nectar, in the pollen—all over the place. But actually, the concentrations are really different across those different floral parts. As just one example, there's two chemicals—one is called catalpol, and the other is called aucubin. They're actually very closely related. In leaves, there's much more catalpol than aucubin. In nectar, it's reversed—there's much more aucubin than catalpol. Both of them seem to benefit bees in this way, but one may be more toxic to bees. We could just—you know, it's pure speculation—but maybe bees are more attracted to the aucubin than the catalpol, and that has led to selection against the latter in nectar, but not in the leaves. So there's all this complicated stuff going on within the plants themselves that we have to account for when we think about, you know, pollinators or flower visitors exerting natural selection on those traits.

Rachel: This has been really interesting. I feel like you've uncovered this like whole world that I didn't—I knew existed, but didn't know anything about. And the studies that you've done are very interesting, and I think that, as we do on this podcast, we always sort of lead into like, “Well, what does this mean for bee conservation?” If plant chemicals benefit bees, can we use this knowledge to help bees?

Leif: My first answer is yes. I think we could do something with this knowledge to help pollinators, help flower visitors that we wanna keep on the landscape. Because we know that there are some benefits, right? Not only are the bees attracted to certain plants more than others, and we wanna plant those in restoration, and things like this. But there is some trait of the plant that might benefit a bee that's sick, you know, and we know that parasites are one of—or pathogens and parasites are one of the drivers of decline for bees anyway. Some other flower visitors, as well. So yes, I think it's possible that these—we could use these plants in a structured way so as to support the pollinators or flower visitors in any given place.

Leif: However, I don't think we know enough about these systems to really design a program to do that yet. I would encourage people to look into this stuff, and then go get those plants and grow them. It can't hurt. Well, it might hurt some insects, but maybe not others. But, yeah, it seems like it's a potential place where we should look for benefits—differential benefits among the plants that these bees will use, and other pollinators will use. But I would say we have a lot to learn before we could do it in a way that would be no question beneficial that would select—you know, it would help the right group of insects without hurting some of them, and that sort of thing. Yeah.

Matthew: With bees, some of them are very specialized on the plants that they can—they will collect pollen from, or forage on. And if some of these specialist bees are lost, is it possible that another species will—a bee will fill the, you know, the pollination gap? Or is it possible that the chemistry of the plant may deter another pollinator coming in? I mean, it's like, is there possible incompatibility between pollinators and plants here?

Leif: I'd say there's potential incompatibility. You're right, just talking about bees, there are bees that specialize on a limited set of pollens. They—at the extreme end, there are bees that can only take pollen from a single plant species. So that plant species occurrence on the landscape determines presence or absence for those bees—they cannot live without that pollen. At the other end of the spectrum, there are bees that can eat hundreds, thousands of different things. Bumble bees and honey bees are the classic examples at that end of things.

Leif: From the plant's perspective—a plant that hosts a highly specialized bee pollinator that can only get its pollen from that plant—they are generally not in a one-to-one symbiotic relationship with that bee. Generally, there are other flower visitors besides the one hyper specialist who are coming in. So even if you lose the hyper specialist, there are probably other bees or other pollinators in the system that are gonna visit those flowers and may be able to pick up the slack. I think this is generalizing, but many of these specialized pollen foragers—so they have adaptations to gather the pollen from that particular plant—in some ways, they're more efficient at taking the pollen home to feed to offspring as opposed to moving it to other flowers. But as a generalization, I think that there are many systems where the specialist bee is actually quite good as a pollinator because of these adaptations.

Leif: One example to give you is squash bees. Squash bees are found in two or three different genera of bees. And they only eat pollens from the genus Cucurbita, which includes squash and pumpkins, also coyote gourd, which is found in the southwest, and there's one species in the southeast. There aren't any in the north on this continent. And so this bee cannot exist in places that don't have Cucurbita. And they do exist in the north because we grow squashes and pumpkins there. This is all they can eat. But nectar speaking, they could go to other flowers, but pollen wise, it has to be this. But there are other pollinators for squash and pumpkins. You can remove the squash bees, and bumble bees, and honey bees, and a bunch of other stuff will get in there and use that nectar and pollen and pollinate those plants.

Leif: So it's not to downplay the role of the specialists, but these systems have a lot of redundancy built into them in terms of lots of different flower visitors, some of whom are pollinators, some of whom are not. And the loss of a single a species that's an important pollinator, it could have a serious detrimental impact on that plant, on its populations, but it could also be relatively minor given that there are other pollinators in those systems.

Matthew: Wow, I have learned so much from you today, in this last half hour or so, Leif. I opened saying I really didn't know much about plant chemistry. And now I know a lot more.

Matthew: So thank you so much. This has been really, really, really interesting and enjoyable and informative. And we always wrap up our episodes with a standard question or two. So the first one I have for you is: if you could see any bug in the wild, what would it be?

Leif: Hmm. Any bug in the wild? I did know this question was coming and I don't have a suitable answer. There are so many. I really like insects. There's a bunch that I'd like to see. I'm really interested in carpenter bees in East Africa and the Mediterranean. I don't know why, but there's good diversity, they're important pollinators—they're important crop pollinators in East Africa—and they're relatively understudied. That's one.

Leif: But there's a bumble bee that is a life lister for me, and that would be—it's called Bombus dahlbomii. It occurs only in Chile, and Argentina, and pretty far south, so in the colder parts of those two countries. And it is the largest bumble bee on earth. It's bright orange all over its body—very large, beautiful bee that's co-evolved with things like fuchsia that you might buy at the gardening store, [and] a bunch of other native plants from that area. And unfortunately, this bee is declining quickly because we brought commercial bumble bees to Chile and Argentina to pollinate berries. These are the same berries that you might eat in wintertime in California, or somewhere else in North America. We needed a bumble bee to do this pollination. It came from Western Europe, from other parts of the so-called Old World, and those are competing with this bee and causing it to decline. So, it's just an animal that I’d love to see before it becomes too rare to find. Yeah.

Rachel: I'm gonna go look up that bumble bee after this. So our last question—'cause you've been on the podcast before we actually added this new question this last episode—I'm excited to ask you. What is your most memorable experience with wildlife?

Leif: Okay, I'll give you two. One is—and just a very memorable experience with javelinas, which are these pig relatives that are wild animals in the Southwest that run around in packs. You can see them in Southern Arizona, maybe also adjacent states, but that's where I got to know them. They have very poor eyesight, but very good sense of smell. And they're nocturnal, or spend—they're active at—they may be corpuscular to some extent—they're active at dawn and dusk kind of thing. And I used to live in Arizona and I was—I had a friend who worked at the Biosphere, which was that experiment to have human beings live in a completely closed system where they would be growing their own food and the gas exchange would allow them to live and all that stuff. At this point we had discovered it didn't work and it was turning into something else—an experimental place. But I went to go see it one time after hours with a friend, because I think we weren't allowed to go in and we had to go in late at night. Anyway, we got there and we got out of the vehicle, and this whole pack of javelinas was in the parking area and started—we had a dog with us—and they got excited by the dog and started just running in circles around our van. And I don't know, 15 or 20 of these hogs running wild at night around the van, with a dog who wasn't trying to attack them, but was very excited by the whole thing. Yeah, it really stuck with me.

Leif: I wanted to add one bug experience from this year that really will probably be something I talk about when I'm in a nursing home. I was at this place a few months ago called Point Sal in California. It's just a coastal spot. It's public land. There's no way to drive to it, you have to hike about five miles over a small mountain and then down this steep, rocky slope to get to the beach. I got down there and found bumble bees that were foraging on some plants right in the beach sand there. And I like to take photos of bees, so that's what I was doing. And it was very, very windy. And what I found was that it was so windy that I could not open my eyes to take photos at the same elevation off the sand as they were flying, because there was so much sand coming at me. I had to close my eyes and just take pictures.

Leif: And I got interested in this, you know, if it was hard for me to exist at that elevation off the sand, it must be hard for them to live that way all the time. And I got some great photos of them just being absolutely pelted by sand grains. Of them being knocked off plants by the wind and then instead of flying back, crawling back and kinda struggling up the plant to get to the flowers. And the reason I remember this, and I'm bringing it to your attention, is that it's just a little vignette into the life history of bumble bees that you will not find in a textbook, or in any of my publications, or anyone else's. They are dealing with regular day-to-day stuff that we tend to overlook, and this is just one of these things. Living on a beach means that you have to fly in high winds, which is energetically expensive, is more difficult for navigation, and so on. But it also comes with these other risks like sandstorms. So I don't have any grand conclusions to give you about outcomes for the bees or anything like that, but it was just a cool little observation of these bumble bees on the beach at Point Sal in California this summer.

Rachel: Yeah, it's interesting. I mean, they deal with the same things that we do in a lot of ways.

Leif: Yes.

Matthew: But also, like you say, these observations give such insight into the life of these insects. And it's not the kind of thing that gets recorded, or in a textbook, or academic paper necessarily. But also the kind of way in which, you know, anybody, community scientists, anybody out and about can discover things about these insects that contribute to broadening our knowledge, which is just great.

Leif: Yeah. I love that. We've been talking about, you know, research I did as a graduate student and, you know, in a lab—science in a lab. This sounds removed from the average insect enthusiast. And I will cosign what you said, Matthew. Anybody can go outside and experience some of these organisms and make observations that they're not going to find validated by a textbook or something. Because, you know, they’re—these animals are complex. They're living their lives—individuals are doing things differently from other individuals. And so there is so much to observe and understand about insects, including pollinators. And I think that is a domain that's open to everybody.

Rachel: Definitely. And I know how much you love bees. So being able to observe, and having observed so many bees for so many hours that we now know you did with your studies, and to be able to see this new behavior must have been really cool, and kind of a new connection that you were able to make with these amazing animals.

Leif: Yeah, it was really exciting. And I'll say that this research—lots of other individuals and labs have been involved in this work during my time doing it, and then since. And so we're starting to know more and more about the plants that are important to these bees. And we’re finding that in some cases there are benefits from consuming plant pollen that don't come from the chemicals, it's actually the physical structure of the pollen grains that help deal with parasites in one case with sunflower pollen. So I just wanna make the case that—or point out that there are lots of people working in this area, and we're learning more all the time about how plant chemistry mediates pollination interactions and bee health, I'll say, yeah.

Rachel: I'm gonna make a note to get in touch with you in a few months and then do another podcast on this 'cause I have more questions, and I think there's so much more to explore here. So thank you so much, Leif, for your time today and for sharing all this great information. I've certainly learned a lot. I know Matthew has, and I'm sure our listeners did, as well. So thank you for your time. It was great to have you back.

Leif: You're welcome. I was glad to do it. Thanks.

Matthew: Thank you, Leif.

Rachel: Bug Banter is brought to you by the Xerces Society, a donor-based nonprofit that is working to protect insects and other invertebrates—the life that sustains us.

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