The Fungi Factor: The Surprising Discovery of Mold’s Impact on Leafcutter Bees (Ep. 52)

We’re exploring the surprising relationship between leafcutter bees and Aspergillus mold. Our guest, Dr. Victoria Luizzi, sheds light on how this mold might actually protect these solitary pollinators. Despite mold usually having a bad rap, this particular type, commonly found on rose leaves (a favorite nesting material for leafcutter bees), might offer some unexpected benefits. Dr. Luizzi discusses their field studies and experiments aimed at uncovering why these bees are drawn to Aspergillus.

Dr. Victoria Luizzi, a researcher in the Pringle Lab at the University of Nevada in the US, focuses on the interactions between species, especially how microbes influence their hosts’ interactions with other organisms.

Good to know

Leafcutter bees cut leaves to build their nests, but don’t worry—they don’t harm the plants. The little round cutouts you see are totally harmless and a sign of these bees doing their thing. Leafcutters are crucial for pollinating wildflowers and crops like blueberries, onions, carrots, and alfalfa. They use the leaves from just about any broadleaf plant to make their nests.

Transcript

[00:00:00] Welcome to the Bee’s Knees, a podcast wild about native bees. Wild and native bees are under threat worldwide. In each episode, we look at actionable things we can do to support these adorable little guys whose pollination work is crucial for maintaining biodiversity. I’m Jacy Meyer, and I thank you for being here.

The leafcutter bee, the bane of many proud gardeners due to its quirky habit of nibbling away at the leaves of flowers. But that foliage is crucial to these bees, who use the leaf bits to build their nests. As vital pollinators of wildflowers, fruits, and vegetables, supporting them is essential. But what if a mold, something that doesn’t typically sound beneficial, could actually have a positive effect on these bees?

Today, we’re delving into this intriguing topic with Dr. Victoria Luizzi, who will introduce us to the world of leafcutter bees and the mold they studied. Can you please introduce us to the leafcutter [00:01:00] bee?

Yes. So leafcutter bees are solitary bees, like actually the vast majority of bees. So that means they don’t live in a big colony with a queen and instead each individual female leafcutter bee sort of builds its own nest, stocks it with food, and lays eggs in it.

And leaf cutter bees are named for their unique behavior of cutting pieces out of plant leaves, sometimes flower petals as well when they use these leaf pieces to actually build the lining of their nest cells. So that’s, you know, pretty unique among bees. Most bees are not bringing all of this plant material into their nests, but leaf cutter bees have these really cool leafy nests and you can really easily tell that a leaf cutter bee has visited a leaf because they leave behind really smooth.

Sort of semi circle or semi elliptical cuts. They look really different from, you know, a caterpillar munching or something like that.

So please tell us about the mold that you identified.

Yeah, I’ll tell you what I can about it. So the mold was actually [00:02:00] several strains of mold in a genus called Aspergillus.

And this mold was really common on rose leaves, which is a preferred nesting material source for leafcutter bees. Aspergillus is a really big genus. It contains a lot of molds, but also some plant and animal pathogens. And, you know, identifying fungi to species is pretty challenging, so I can’t tell you a firm species name.

But I do know that all of these molds that I will sort of collectively refer to as aspergillus are all pretty closely related to each other. And for anyone who happens to be in the know, they are not related to fowl brood, which is caused, uh, that’s a disease, honeybees, that is caused by a completely different Aspergillus group.

So what sparked your interest in studying the leafcutter bees and their preferences for certain leaves?

Yeah, so I definitely had an existing interest in species interactions, plant insect interactions, plant microbe interactions, coming into graduate school. [00:03:00] And I was, you know, trying to figure out what to work on for my dissertation.

And leafcutter bees were actually pretty common on the University of Arizona campus, which is where I did my PhD. And a couple of people that were associated with One or both of my labs, I’ve had two labs, a couple of people associated with those labs had worked on leafcutter bees in the previous couple of years.

And this was kind of a new study system that was coming up for them. And so they were kind of on people’s radar, and there was some really interesting preliminary data about them, but a lot more work needed to be done. And I started just, you know, walking around campus as I was. You know, going to my classes and also thinking about research, I started noticing their damage, really distinctive damage on leaves.

And in particular, I started noticing this really striking pattern about how it was sort of distributed across leaves. It’s really clustered, usually. No matter what plant you’re looking at, you will often see, you know, some leaves that have tons and tons of cuts [00:04:00] on them. Right next to leaves, even on the same plant that are totally untouched.

So I really just started wondering, like, what do these bees know? What are they, like, paying attention to? What are they cuing into? What is affecting their choices and why? So, you know, just very basic curiosity. If there’s a pattern and it looks like something is happening and I want to know what, was kind of the basic impetus for that.

And then adding that to the fact that these leaves are their nest material, like that’s the environment that their offspring are developing in. So, which leaves they choose is probably consequential in some way, and I wanted to understand what goes into those choices.

So how did you conduct the experiments to determine the presence of this Aspergillus mold on the leaves that you discovered the leafcutter bees prefer?

Yes, good question. So this is going to be sort of a multi part, um, answer. So, kind of one of the first things that I did in the system, kind of as I was still honestly deciding if I wanted to work on leafcutter bees, was I just did a [00:05:00] really broad survey of All of the bacteria and fungi living on the surfaces of rose leaves and roses were the plants that I was working on for this study.

And I did this by just sort of hanging out, watching bees forage for leaves during their nesting season. And as soon as I saw a bee cut a leaf, I sort of snatched it away and Pressed it on a plate of sort of a general nutrient medium that’s good for sampling lots of different kinds of bacteria and fungi.

And so I had those cut leaf samples, and then I did the same thing for sort of nearby similarly aged leaves that the bees had ignored and not cut. And then just compared the microbes that grew on the plates that had made contact with the cut leaf, and those that had made contact with the non cut leaf.

And I found that bees were choosing leaves with distinct microbial communities. They were different from the microbial communities on leaves that the bees didn’t choose. And in particular, you know, there was obviously a lot of variation, a lot going on in these patterns, but [00:06:00] there were a couple of groups of microbes that really stood out as driving a lot of that pattern.

In particular, there was this Aspergillus mold that I found a lot more commonly on cut leaves. And then there were, you know, several strains of some other fungus called Alternaria, um, and then a bacterium called Bacillus that were more common on leaves that they did not cut. So that kind of suggested to me that bees could have preferences for or against these microbes, but at that point it was really just correlation and not causation.

So I did a follow up experiment to test that by adding those microbes back onto the leaves in sort of a controlled way and then seeing how the bees responded. So I basically made, you know, three different microbe soups, if you will. Kind of a liquid suspension of those microbe cells. So Aspergillus, Alternaria, and Bacillus.

And then I just went back to that same field site where I had collected the original data on the microbes [00:07:00] and spritzed those microbe soups onto different rose leaves. So each leaf got one of the three microbes or they got just kind of a sterile spray that had all of the soup components but no microbes.

Just to control for bees maybe being attracted. Or deterred by the leaf being sprayed, right? And so then, you know, I went back and saw how often each of those leaves got cut. And when I compared the leaves in the different treatments, I saw that the leaves that had been sprayed with aspergillus got cut by bees about twice as often, on average, as the leaves that were sprayed with just the sterile, no microbe soup.

The other two microbes didn’t have any effect. Sometimes that’s how it goes. Um, but this was still really exciting, the aspergillus results, um, because, you know, now there’s experimental evidence that leafcutter bees are more inclined to cut a leaf if it has aspergillus on it, you know, suggesting a preference for that fungus.

At this point, though, I still had no idea [00:08:00] why bees might like aspergillus. You know, it might just be sort of a bias towards the smell of it for no reason. It might be something You know, adaptive something beneficial to them. And so I did one more experiment to try to understand what might be behind that potential preference.

And this is based on, you know, something that’s known about aspergillus molds in general, which is that a lot of species of aspergillus produce toxins, which can decrease the growth of other microbes. And so I wondered if You know, the aspergillus strains that I was seeing on the rose leaves might be able to decrease the growth of microbes that would be harmful to bees in their nests, such as pathogens.

So to test that idea, I grew aspergillus in the lab on plates of nutrient media and essentially competed it against some bee pathogens. And in particular, I was looking at several strains of the fungus that causes a disease [00:09:00] called chalk brood. Um, and this is best known for affecting honeybee colonies, but can also affect solitary bees, including leafcutter bees.

And so, I did these sort of direct fungus fight assays where I pitted them against each other and tested whether aspergillus depressed the growth of the pathogen. And I found that aspergillus was actually able to substantially slow the growth of chalkbrood fungus on a, on a plate of nutrient media. And so that also suggests that aspergillus might have a protective effect in nests.

Of course, this should be verified in a bee nest, but if that holds up, that would be one potential reason why Aspergillus is beneficial to bees is that, you know, maybe having it in your nest means that you have less pathogen attack and your offspring are able to survive better.

Oh, I loved that breakdown of the steps you went through.

In addition to the possible protective element of Aspergillus to leafcutter bees specifically in their [00:10:00] nests, can you think of any other protective elements this mold might offer a leafcutter or actually any native bee?

Yeah, that’s a good question. I mean, it could potentially, if we’re talking about, you know, benefits of Aspergillus, It could also slow the growth of other microbes that maybe aren’t specifically pathogenic, but, you know, are just sort of molds that you don’t want in your nest, or, you know, fungi that might compete with bees for their pollen provisions.

Um, so each larval bee in a nest has sort of a little, Cake or puddle, depending on species of sort of nectar and pollen, mushed together as food. Um, and there’s lots of microbes in those provisions that are beneficial to bees actually. Um, but it’s also possible that there are, you know, fungi that might just take the nutrients from them, sort of make them moldy and overgrown and not good for bees.

So aspergillus might interact with, you know, microbes in the pollen provisions in ways that are either beneficial or harmful to the [00:11:00] bees. That’s something that we don’t really know anything about yet.

So it’s probably hard to know why the bees are choosing the leaves that have the aspergillus mold on them.

However, do you know why they are not choosing them?

Yes. So I, in terms of how they sort of know that the aspergillus is there, how they sense it. I wasn’t able to nail down for sure how they do it, but some of the data that I’ve collected along the way can rule out sensory pathways that they’re not using.

So I can tell you that they’re not using visual cues to sense it. So bees, as you may know, can sense ultraviolet light, unlike humans. And aspergillus does reflect in the UV range, but the reflectant spectra meaning. Sort of the color profile of a leaf as it would appear to a bee is the same between leaves that have aspergillus on them and leaves that don’t.

So [00:12:00] bees are probably not tasting the presence of aspergillus because out of all the chemicals in a leaf that a bee would probably be able to taste in that way. So sense the chemicals on contact, those are mostly similar as well. Between leaves that do and don’t have aspergillus and also between leaves that get cut and leaves that don’t get cut.

So chemistry is not playing a big role. The one element of chemistry that I don’t have data on is what we call volatile compounds and those are Really small molecules that go into the air and that we would describe as smells or scents And so that would be my guess as to how bees probably do tell the Aspergillus is there, but I don’t necessarily.

I can’t, I don’t have sort of positive evidence of that. I just have evidence against other modalities.

If you could encourage people to do just one thing to help support native bees, what would it be?

Yeah, I thought about this a [00:13:00] lot and it was very hard to narrow it down to one thing. I mean, if I had to choose one, and this is not necessarily easy, but I would say, you know, do what you can to help protect native plant and bee habitat.

You know, there’s a lot of things that you can do in urban spaces that are helpful and better than nothing. And I don’t want to discourage that, but sort of the bottom line is that some bees do thrive in urban areas, including. One of the species that I worked on, but some just don’t and may never thrive in urban areas.

And so they need places to live and, you know, we need to do what we can to make sure that those places remain intact and protected. If people are looking for a sort of more in your own backyard solution though, um, I will, I’ll sort of plug one that I think is less In the public consciousness than like planting flowers, which is, if you have any sort of outdoor space [00:14:00] that is under your control, I would encourage you to just kind of leave parts of it alone and let them be not pretty.

And specifically what I mean by that is leave patches of bare soil or sparsely vegetated soil, you know, maybe with some little pebbles, maybe not whatever you have. That bare ground is really important as a nesting site for ground nesting bees, which again is like the majority of bees. So, leaving those areas, not trying to plant a bunch of plants on them or groom them all the time and just kind of leave them alone, that can provide a really important resource for bees that even if they have all the pollen in the world, they don’t have a place to build a nest, they’re still not going to do well.

Plant pollinator interactions are a crucial aspect of biodiversity. If plants struggle, bees can’t get the pollen they need. If we lose the bees, there won’t be any pollination for the plants. We need to care for and nurture both. Victoria’s actionable advice is to protect wild natural spaces. Urban environments aren’t suitable for all bee [00:15:00] species.

And we need all bee species alive and well. Thank you, Victoria, for sharing such a fascinating study, and thank you for your time and tuning in. Until next time, nurture wild nature.