Convo with Susannah Lerman: Lawn mowing frequency affects bee populations in suburban yards


Here’s a recent conversation I had with Susannah Lerman, a wildlife ecologist with the U.S. Forest Service. She and her colleagues Alexandra Contosta, Joan Milam and Christofer Bang recently published a paper showing how lawn mowing frequency can affect bee abundance and diversity in suburban yards. The transcript below has been lightly edited for length and readability.

Matt Kelly: So how does a wildlife ecologist with the Forest Service get into bees?

Susannah Lerman: That’s interesting. So, I’m trained as an ornithologist. I mainly research backyard habitat, and I’ve been doing a lot of work on how people landscape their yards and how birds respond to these different changes that people make. But then also how people respond to interacting with the birds in their neighborhoods.

But this is really kind of looking at the long-term component — like, “I’m going to plant five oak trees and three maple trees and a bunch of different types of shrubs, and then wait 15 to 20 years for them to grow and provide some of these habitat features” — and I wanted to figure out other ways to look at the short-term management behaviors that people could address to improve wildlife habitat for their yards.

At the same time, I read a paper that talked about the expanse of lawns — there’s something like 65,000 square miles of lawns in the United States, that’s roughly the size of Florida — and many of my colleagues have really dismissed lawns as non-habitat because you look at a lawn and it’s just grass, they’re these sterile environments. And I used to think that too but realized, well, we’re not going to get rid of the 65,000 square miles of lawns. So how do we make them less bad?

One of the things that I noticed in these lawns was that there were all these flowers that were growing, like dandelion and clover and violets. I started to think this actually could be some resources for bees. It was also around this time when our news feeds were filled with the plight of bees: bees are declining, they’re losing their habitat, what’s going on with the bees? So it really seemed like an obvious choice to study bees to really look at this question of how do we do these short-term decisions for managing our yards.

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Photo: Susannah Lerman

Matt: You said one of the things you were studying with birds is how people interact with the birds in their yards. Tell me a little bit more about that.

Susannah: This is work I’ve been a part of since 2005. I work with the Central Arizona Phoenix Long Term Ecological Research Project that’s based at Arizona State University. So in addition to working with ecologists I also work with social scientists. Part of our research looking at these landscape designs is also integrating a social survey. What we were able to do was to ask people in roughly 40 different neighborhoods scattered throughout the city of Phoenix about their level of satisfaction with the birds in their backyards. And what we found is that neighborhoods that had more of the desert specialists — like greater roadrunners or cactus wrens or verdins, these birds that you can’t see in New England or most other parts of the country — people were a lot more satisfied with their birds. And what we really wanted to understand is, is this just important for those of us who keep our bird lists? Or is it more of a general phenomena that people really like seeing birds outside their windows?

Matt: The reason I’m curious about this is have you given any thought to whether that same question could be asked about bees in the backyard?

Susannah: Yes. We have some current research that is assessing that, looking at pollinators in general, again in Phoenix. We’re hoping to learn a little bit more about people’s interactions with wildlife in their neighborhoods. We’re really trying to assess whether people see them as a nuisance or as a benefit, as a service.

Matt: One of the things I’ve found really interesting in talking with a wide range of people, particularly outside of the bee specialty, is that people will be really enthusiastic about bees but then not understand all of these species differences within bees.

Susannah: Absolutely.

Matt: Is that something you think you could tease out with a survey like this? Or does that seem outside the range of what you could get from the average American?

Susannah: So, I think most people when they think of bees, they think of honey bees. And honey bees have really gotten a lot of the press with their link to agriculture. But all of our native bees are also suffering as well. And the one issue is that most of the bees that are in people’s backyards are minuscule. They’re like the size of a grain of rice. So I think it might be difficult for people to really get excited about the 40 different species of bees that are buzzing around in their yard.

Matt: Right.

Susannah: Because they are so small. And you need a microscope to see the really interesting features. Other than things like bumble bees and honey bees and carpenter bees, people probably have no idea that the majority of bees that are flying around are there. So I think it might be challenging from that perspective.

But one of the nice things about bees and pollinators in general is that people really understand the direct ecosystem service that these pollinators provide. And so that might be a different way of really assessing these questions of, “How much do you value the bees and pollinators in your yard?” Recognizing: Do you like strawberries? Do you like tomatoes?

Matt: One of the things I really appreciated in your paper and in your findings, and one of the things I’m really appreciating in this conversation right now, is this idea of cultural norms. Which is really interesting in terms of how it applies to lawns, but is also interesting in terms of talking about how the average person might perceive bees in the backyard.

But before we get into that, can you give me the short summary of the lawn mowing research and the findings you came up with?

Susannah: Sure thing. So continuing on with the “why I got into bees”, it’s really trying to figure out are there these specific behaviors that people can change to create wildlife habitat. And the main hypothesis, the research question we were trying to get at was: If you mow your lawn less, then you have more flower like dandelions, clover and some of these other weedy species (we call them spontaneous plants). And if you have more flowers, do you have more bees?

To assess this, we conducted a study in Springfield, Mass., in 16 yards that were lawn dominated, yards that were mostly lawn. They had a few foundational plantings, maybe one or two trees. These were yards that did not have pollinator gardens because we were really interested in the plants that were growing in the lawn. The yards were divided into three different groups: lawns were either mowed every week, every two weeks, or every three weeks. And one of the fun parts of this study that I think a lot of people liked about it is that we actually did all the lawn mowing, me and my team. So this was a way for us to control what was happening in these lawns, recognizing that some people would feel they had to mow it. But, no, we have to mow on these specific dates so that we can address our research question. For a lot of people in the study, they were delighted. They got their lawn mowed for free for two years.

Roughly every three weeks or so we went out and did our bee assessment. What we found was that the yards that were mowed less frequently had more flowers. That was pretty cool but was something that was kind of obvious. The yards that were mowed every three weeks had two and half times more lawn flowers. So in addition to looking at what bees were present we also counted every lawn flower that was present in these yards right before we mowed.

But in terms of bees, we found that the yards that were mowed every two weeks had the most bees, but it was not the most diverse community of bees.

So when we first started looking at the results of our study, it was just a little puzzling. Why is it that when you have more flowers, you don’t have more bees? One of the reasons we discuss in our paper is that in these yards that have more flowers, they also have taller grass.

Matt: So just to be clear: Logically you would expect a lawn that gets mowed every three weeks to have more flowers and therefore more bees. That didn’t happen. And the reason could be that even though they have more flowers, they also have the taller grass.

Susannah: Exactly. And you would think that more flowers means there’s more habitat for bees. So these yards that were mowed every three weeks have taller grass, and one thought is that because most of these bees are about the size of a grain of rice — most are sweat bees — that it was really hard for them to gain access to these flowers. That even though there were more flowers, the taller grass might have prevented access.

We had another thought as well, that even though you have more flowers it doesn’t necessarily equate to quality. So although our study did not look at pollen and nectar availability in these lawn flowers, there are other studies that have looked at the density of flowers and didn’t find a direct relationship between those two.

But then when you think about the human component, the people part of this study, it was actually kind of a good story. Because, first and fore most, these lawns are for people. They’re an extension of somebody’s home, they’re going to be managed so they look nice and are easy to maintain. Most people are not really thinking about wildlife habitat when they’re making these decisions. So when you go to the yards that were mowed every week, it felt like, “Do we really need to mow it again?” The grass was still pretty short. But knowing that this is what lot of people do, mowing their lawns every week, we have to stick with our protocol and mow the lawns every week.

The lawns that were mowed every two weeks, they definitely looked like they were ready to have a trim. And it was somewhat satisfying. You go and you mow and you can see it was little bit shaggy and then all of a sudden it looks really good.

But then when we got to the yards that were mowed every three weeks, we really had to convince ourselves that it didn’t really look that bad. But they did. They looked messy. And even though we were mowing these lawns for free, the homeowners of these yards would run out and say, “I’m so glad you’re here. My neighbors were getting really upset and kept offering to come over and mow the lawn and dig up the dandelions.” And I kept saying, “No, you can’t! It’s for science.”

So from that norms perspective, if we want to start slowly changing the way our lawns look, we have to do it in accordance with what people want.

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Photo: Susannah Lerman

Matt: You mentioned this in the paper and hearing you say it again now, I think it’s really interesting to consider what the purpose of a lawn is. It’s a place of activity. There are humans moving through it, they’re playing, there are dogs running around. And what that made me think about is, when you’re talking about lawns as contributing to bee habitat, it’s really about food resources. You’re not looking at nesting places, are you?

Susannah: Exactly. We did observe a couple of nests throughout the course of the study, and this was particularly in the yards that had a lot more bare soil. A majority of the bees here in western Mass, in Springfield yards, are ground nesters. So having these opportunities for them to nest is another key component. But I think a lot of the bee research, especially in urban-suburban areas, is really focused on the food resources and not so much on the nesting opportunities. So that’s definitely a key area that we would like to start delving into at some point.

Matt: The other thing you explain in the paper is that how we mow our lawns is part of the bigger solution. Lawns as habitat is not a solution that lives in isolation.

Susannah: Exactly. So when I give presentations, I make the point that I don’t want to be known as the wildlife ecologist who says you should have lawns for wildlife. That’s not how I want to be known. It’s more about recognizing that we can’t get rid of all these lawns. I have a lot of colleagues that say, “We have to significantly reduce or replace lawns.” And I think that’s great but we’re never going to get there, or we’re not going to get these anytime soon. It’s really about trying to figure out how do we make these lawns less bad. And that’s in terms of how do we change our management behaviors.

So in addition to mowing less, it’s also not applying any chemicals. And that was another big component of our study too, that all the yards we worked with were not applying anything. You know, it’s kind of obvious when you look out and you see dandelions and clovers and the 53 other species of flowers that we identified growing in these lawns. It’s pretty obvious when you walk around in these suburban neighborhoods and you can really pinpoint that yard is spraying, that yard is not.

Matt: And between the two, if you had to pick, which is more important? Not spraying or mowing every two weeks?

Susannah: I think spraying because that has a whole bunch of other implications as well.

Matt: And in your particular study you were only counting bee numbers and counting flower numbers. You were not documenting relationships of which bees went to which flowers, for how long, and how often?

Susannah: Correct. So that’s, again, another avenue of research. I know there’s been some really interesting research in England that looked at these visitation rates in these urban meadows with bees and hover flies and other pollinators. But, yeah, that’s a different question that hopefully we’ll be able to address some day.

Matt: So what’s the ideal outcome from all the research you’re doing? How are you hoping to see behaviors change? And what’s the ultimate goal: Are we looking for greater abundance of bees, greater diversity of bees? You talked about “bee evenness” in your paper. Can you explain that a bit?

Susannah: I think the main message that is really important to take home here is that everybody has an opportunity to contribute to bee conservation. There are some people that are going to go gung-ho and they’re going to plant pollinator gardens. But what this study is demonstrating is that there are additional things that people can not do that’s actually going to be beneficial. We call it the “lazy lawn mower” approach. This is for those of us who don’t have the time, the money or the green thumb — or maybe don’t really like what these pollinator garden look like, they’re just a bit too messy — so here’s another way that we can contribute to bee conservation without doing anything. From that perspective, it’s really a nice way of letting people know that you don’t have to do anything and you’re still doing good. I’ve had a lot of people who’ve told me that they feel vindicated from this study. That they’re able to tell their neighbors, “That’s why I’m not mowing. It’s for the bees.”

In terms of evenness, this is a way of documenting diversity. Think of it this way: Say you have ten difference species in a yard, and of those ten species they’re each represented by ten individuals. Therefore, you have a pretty even bee community. Whereas, say you have another yard that also has those same ten species, but nine of the species are represented by one individual and one of the species is represented by twenty individuals. If you look at the number of species in both yards, they’re the same. But one of the yards is dominated by one particular species whereas the other is more even.

Matt: So what does that tell us? Anything in particular, or just interesting to notice?

Susannah: If we know which species are present and we know what their requirements are, that can give us some indication of what types of features are in these habitats. By looking at evenness, it’s recognizing that a more even bee community is going to have different implications in terms of the what the quality of that habitat is compared to a less even bee community.

Matt: Because knowing that, we can then reverse engineer the connection. If we want certain species or to support certain species in an area, then these are the sorts of features we might need to have.

Susannah: Exactly.

Matt: In the paper, you actually documented a rare species in a suburban lawn that you wouldn’t have expected to see there.

Susannah: So it’s not “rare”. I guess this is kind of semantics, but it’s “not common”. It’s Lasioglossum illinoense. This is a bee that is incredibly common further south and the last time it was collected in Massachusetts was in the 1920s. And it was represented by one individual back then. It was our most abundant bee that we found in Springfield—

Matt: I’m sorry, stop for a second. I’m sorry I didn’t catch this while reading the paper. So the last time this bee was documented in Massachusetts was in 1920. It typically has a range that’s farther south of Massachusetts. But now you’re telling me that it was the most common bee you found in these yards?

Susannah: Yes.

Matt: Why do I think that’s interesting?

Susannah: (Laughing) We did too. So if you go across the border in Connecticut, there’s a handful of records just about twenty miles south of us. And Massachusetts is a pretty well-collected state, so it was really surprising that this was the most abundant bee.

So maybe this is representing a range shift. Our research can’t really go beyond the fact that, wow, this is pretty interesting. Again, we’re looking into other ways to further document why we see this. One of the things we want to do is try to find their nests to see what is it about the soils up here that might be beneficial to them, that make them want to be in these Springfield yards. But, you know, it was a really fun finding.

The other thing that was really neat was letting the homeowners participating in the study know they had this uncommon visitor that hadn’t been in Massachusetts since the 1920s. It’s the most blah looking bee that I’ve seen of the bees that we collected. It’s just drab, brown and small. But people were just so excited — “Wow! We have this uncommon bee that’s flying around in my yard.” It really gave people a sense of pride. Which I think is another outcome of the study: Pretty much all the people who participated, they never thought about bees before. But now they do.

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Lasioglossum illinoense / Photo: Barcode of Life Data Systems

Matt: Did you have a chance to talk to any of the neighbors who had a problem with the grass getting longer?

Susannah: We did. Because we were out in these yards every week doing something, we did have some interaction with neighbors. Sometimes people would come up to us when we were running around with our nets trying to catch bees, or mowing, and they’d start telling us about their gardens or about bees they’d seen, or some story. And then there’s also the cynics that would come up to us and be like, “You know that yard really needs to be mowed.” We got everything.

But, again, seeing scientists in these yards got people to start thinking, “Wow. People do science in the suburbs?” I think most people either think it happens in the lab or, as wildlife biologists, we’re conducting our research out in these wild places. But this showed there’s a lot going on in your yards that we’re really interested in, and that gets people to think about their behaviors and how their behaviors affect wildlife and conservation.

Matt: So this is my last question for you, and it’s interesting that we actually arrived at it. It’s interesting to hear you talking about how people responded to that “not common” bee, and your conversations with the neighbors. It strikes me that the answer to getting over this cultural hurdle with lawns and people’s perceptions of bees exists somewhere in those interactions that you had.

Susannah: I think that’s exactly it. Again, when people have these direct interactions either with nature or with scientists or with research, I think that’s when they really kind of have that light bulb moment. There was another one of the participating householders, and I remember talking to her about her lawn. She was complaining about this one patch in front of her yard that they had to replant. It was a different species of grass and she just thought it was a little bit of an eyesore. Then she pointed at the neighbor across the street and she said, “I wish my yard looked like theirs.” I looked across and it was just this rich, lush, green lawn, and it was so obvious that it was treated.

I pointed that out to her and she was shocked. She didn’t realize that that was what was going on. She just thought that’s what a lawn should look like. And I told her her, well, if you want to have a lawn like that then you’re going to have to put all these pesticides, herbicides and fertilizers on it to make it look like that. And she said, “Oh, no no. I don’t want to do that because I love having the cottontails bouncing around here, and I love when my niece comes over that she can run around in the yard barefoot. But I just wish it didn’t look so ugly.”

Matt: It seems to me that this goes back to your satisfaction study, asking those questions to the average person about the types of birds in their backyard. There’s some sort of value system at play there, which people may or may not realize they have. And it sounds like you were starting to get a sense of it with these homeowners whose lawns you were mowing. There’s this value system that not everybody has. That seems to me to be the culture hurdle. And you’ve got me thinking about how interesting it is to get people over that.

Susannah: Yeah. And the other thing is people are so different, right? With the work in Phoenix, when I’d be out there are six o’clock in the morning with a pair of binoculars counting birds in people’s neighborhoods, a lot of people would come up to me and be like, “What are you doing?” And I’d explain I was looking at the birds in their neighborhood. And then they’d tell me this wonderful story about hummingbirds coming to their feeders or some of the plants that they have in their gardens. Then there were other people who would just look at me and say, “Can you do something about the pigeons?” So to me it was an indication that they were interacting with the birds in their neighborhood but they were having this negative interaction. The whole values component is also very complicated.

Matt: The crossover work you’re doing between ecology and sociology is really interesting. It’s a fascinating dynamic to think about.

Susannah: You know a lot of us that are doing urban research sometimes refer to ourselves as socio-ecologists. The field of urban ecology has kind of gone through a transformation over the last 10 years or so. In the early days when people were studying wildlife in cities, they kind of looked at it as ecology in the city, to see how bees or birds or other species were responding to the amount of pavement or buildings or the number of people in the neighborhood. And then about twenty years ago there was this shift to thinking, well, people are a big part of the system and kind of the main drivers. So we really need to understand their decisions and behaviors and values to really understand why they do what they do. Because that is a big feature of how wildlife is going to respond.

With a lot of these other studies, especially in national forests, some of the features that you look at are what kinds of plants are present or elevation or soil conditions or precipitation. All of that goes out the window when you go into cities because you can change the soil, you can change the amount of water, you can decide what species you want to have in your yard. So we really need to understand why people are doing what they’re doing, if we want to think about the conservation implications. And that’s really a big part of my work: How do we make these cities better for people and wildlife?

Matt: And it seems like it would be really easy to start saying, “People are the problem.” We’re causing all the problems. What you’re getting at is the idea that people are also the solution. We change the environment, yes. But we can change the environment in ways that are conducive to supporting biodiversity and all these different features. It doesn’t just have to be for the worse.

Susannah: Yes, that’s exactly it. When I first got into the field of urban ecology, back in early 2000, I was of that mindset: “We have to get rid of cities, they’re awful. There’s too many people.” And then I realized that’s not going to happen. Similar to my coming around with lawns, we’re not going to get rid of lawns. Instead, how do we come up with some solutions? How do we solve some of these problems? And it’s exciting for me. It’s really trying to figure out how we get people excited about the wildlife in their yards and their role in conservation.

Matt: This is really fascinating work. Thanks for taking the time to talk.

Susannah: Sure thing. Thanks so much for your interest.