Leave the Leaves or Lose the Insects

Leave the leaves has become something of a rallying cry and discussion topic in recent years. When the weather starts to cool and leaves begin to fall, we’re presented with a valuable wildlife and gardening resource — but also something of a headache. Do I keep them all? Can I mulch them? Won’t they kill my lawn? What about pests? How do I keep my HOA happy?

Joining us to take a deeper dive into a pile of leaves, how they help insects, and how to manage them is Max Ferlauto, state entomologist with the Maryland Natural Heritage Program, where he is in charge of rare insect conservation. Before that, Max was undertaking research for his PhD at the University of Maryland, where he studied the impact of leaves (and leaf removal) on insects in gardens as well as on soil health.

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Photo: Steven Severinghaus/Flickr/Creative Commons 2.0

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Transcript

Rachel: 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/give.

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

Rachel: And I'm Rachel Dunham in Missoula, Montana.

Matthew: So “Leave the Leaves” has become something of a rallying cry and a discussion topic in recent years. When the weather starts to cool and leaves begin to fall, we're presented with a valuable wildlife and gardening resource, but also something of a headache. “Do I keep them all? Can I mulch them? Won't they kill my lawn? What about pests?”

Matthew: Joining us to take a deeper dive into a pile of leaves, how they help insects, and how to manage them is Max Ferlauto, state entomologist with the Maryland Natural Heritage Program, where he's in charge of rare insect conservation. Before that, Max was undertaking research for his PhD at the University of Maryland, where he studied the impact of leaves—and leaf removal—on insects and gardens, as well as on soil health.

Matthew: Welcome, Max.

Max: So great to be here.

Rachel: Yeah, we're very excited about this topic and to have you joining us. So we're gonna dive right in—can you talk about the benefits of leaving the leaves? And what I love about this question is that you've actually done research on this, so it's not anecdotal evidence that we're talking about today.

Max: Yeah. And I think that was partly how this research began. There was a lot of anecdotal stories and thoughts about the benefits of leaving leaves, but we really didn't know the details, which are very important whenever you're doing any sort of management or conservation work. And what's great is these are all conservation actions you can do in your backyard, which is why I was really excited by it. The idea of leaving your leaves, right, in the winter is that insects need a place to overwinter, and by removing leaves, the idea was you were taking away that place. So anyway, there were so many questions that arose. And, you know, even in my broader insect conservation work, really understanding the life history of species of insects is vital to doing any sort of conservation. And we really don't have a great grasp over where different species overwinter. So figuring that out is a huge part of doing any sort of management or conservation. And the hypothesis of many people is that they overwinter within the leaf litter or below ground.

Max: And in the past a lot of research—I wouldn't say a lot—but the research that had been done was mostly surrounding pest control. So if you had a pest outbreak and you wanted to make sure that didn't happen again for the next year, maybe you would remove or rake up and bag the decaying detritus from that plant that had the pest outbreak. Because the insect would presumably be overwintering within the leaves or below ground. So kind of the twist here was that well this time we actually want these insects. This time we're concerned about the lack of insects in our backyards, in our neighborhoods, because of all the benefits that they provide—being food sources for birds, their role in soil health—all of these values that we're now starting to realize that we're seeing the decline.

Max: So a lot of work has been done on how do we preserve these insects during their active lifecycle? Where they're flying to nectar resources, or they're eating their host plants. So we know to plant pollinator gardens, and we know to plant their host plant—their native host plant in your garden. There was this piece of the puzzle that was missing, which was: what happens over the winter? So that's kind of where the research began.

Max: And it went off in a lot of different directions. But kind of the core piece of it was I went to suburban yards in Maryland and we either removed leaves or we kept them. Haha. Kind of plain and simple. And then we placed an emergence trap over each of those areas. And the emergence trap is just this trap that collects everything emerging in the spring so we can quantify exactly what's in the yards and what is being affected. One question is: are our backyards even producing insects? Like, is this a habitat for them? And the answer is yes. We collected over 18,000 insects from a square meter of yard where leaves were retained.

Max: We had on average 20 butterflies and moths, about 300 parasitic wasps, over 100 spiders, over 300 beetles, and, you know, thousands of flies. So these are productive areas of our yards, but you have to manage them correctly. So kind of the takeaway is: when you remove your leaves, you reduce your butterfly and moth emergence by about 45%. And you reduce your spider population by about 56%, and your beetle population by about 25%. So those are kind of just the—and we can go into more of the details, but I think, that was fairly surprising. First that we have all of these insects in our yards. And second that if you remove your leaves, you really can do a number on their populations.

Rachel: Wow, those numbers were quite shocking. So just for clarification, how many yards did you survey? And that 18,000 insects, was that collectively across those yards, or was that just like per yard on average?

Max: So it was 18,000 individuals—and that's on average—that would come out of a square meter. And that's not even including some of these other, you know, tiny—you know, we didn't count springtails or things like that. But this was a study across 20 yards in the suburban Maryland area. We also did a study at the Smithsonian Environmental Research Center, which was in more of a forested ecosystem, where we were able to better control for different variables like tree canopy diversity, and tree species. And we were able to add a lot of other manipulations and do more measurements. But it was cool to really be in the yards doing the—. You know, I'd be out there doing the litter manipulations when everybody else was blowing their leaves and raking. So yeah, I did yard work for my PhD. Haha.

Rachel: Haha. Oh, that's great. But it just gives you a sense of how important leaves are, and that leaving them really does provide a lot of really great—obviously necessary—habitat. So are all trees created equal in terms of creating leaf litter? I hate to call it leaf litter—like “leaf life.” Are all trees good, or are there certain trees that are not as beneficial? Do the insects care?

Max: So this adds a great question, and one that's fairly hard to answer. At the Smithsonian Environmental Research Center in that forested study, we had different species of trees that we manipulated the leaflet or under. And some were in monocultures and some were in polycultures. Unfortunately, we didn't have the power—like the statistical power—to be able to say things about the tree species individually. We were more looking at like mixtures of tree species versus a single-tree species. So I'd love to be able to answer that question through the research that we did, but that's a tough question. However, we do know that oaks support so many species of insects, especially butterflies and moths—especially moths, really lepidoptera. So my answer to that would be oak trees are the best. Haha. And, you know, for a couple reasons, right? Their leaves aren't gonna break down as quickly, so you're gonna kind of preserve that ground floor layer. And then of course, all the different species that they do host.

Max: What we were finding was certain groups of butterflies and moths were more affected than others, depending on kind of their life history traits. So if you were a leaf-miner, if you overwinter as a larva, and if your life strategy is to overwinter within the leaf layer itself, you are more affected by leaf removal than other groups that maybe overwinter below ground and things like that. But that information's really tough to find—you know, life history information about whether a caterpillar is on top of the soil, or underneath the soil—and yet those are vital questions when we're asking these questions.

Matthew: I just wanted to say that one example that I found pretty impactful is the luna moth, because that's one that does—its cocoon is in a leaf of the sweetgum, which then falls on to the ground and it overwinters in that leaf layer. And luna moths are something that people can identify and connect to. And so there's this idea that maybe the luna moth, we've been—. Have we been raking and blowing the luna moth out of our yards without realizing?

Max: It's also this question of: how is this—. You know, we're seeing these reductions—why are we seeing them? So one question was: is this direct mortality? Are we taking these leaves and putting them in a shredder, and obliterating your luna moth? Or are we, you know, putting them in a compost heap, and putting them in an area where they can't complete their lifecycle? Or is it something to do with the microclimate under the ground? Like are these species being exposed to drastic temperature changes because the leaf—their kind of blanket is gone? So those were kind of other questions we were interested in. And largely, I would say, we were finding that, for the most part, it's these species in the leaf litter proper. They're a leaf miner that is in the leaf when it falls, or they're a species that's overwintering as a caterpillar within the leaf layer. But definitely more work is needed to understand the temperature dynamics of that layer of our backyards and the forest.

Rachel: I feel like this is another really hard question, but what if there were no leaves? You know, a part of my heart dies when I drive around our community and I see giant plastic bags which I know are full of leaves in the fall that we're throwing away. But I wonder like if a yard, for example, you take all the leaves out—where do these insects go? Are they just sort of, well, there's nowhere for them to overwinter, or do we even have that answer?

Max: Yeah, I think a lot of them are in those bags, I think. Haha. And I get asked a lot about composting leaves, as well. And I think, you know, anytime you're taking these leaves and you're putting them in an area where that insect can't complete its lifecycle—. And a compost bin that's properly running gets fairly hot, you know? And that will kill the insects. Or it's so deep that they can't find their way out, or they're being sent to a landfill, or shredded, or sucked up in a vacuum truck, you know? So those insects, they're not completing their lifecycle. They have kind of—their path has ended.

Max: And I say that because if we think about their life as a path, there are certain things we do along that path to improve their population. Planting a native plant garden, planting an oak tree—all of these kind of eco-friendly landscaping practices. But if somewhere along that path we're cutting them off, and we're saying, “Well, I'm glad you enjoyed your native plant garden, but now you're going into the landfill.” Not only are you stopping the rest of their lifecycle, but you could potentially be attracting insects to an area where they can't survive, you know? So I think if you are going to plant a native plant garden, or plant oak trees, and pollinator flowers, and all of these resources, you need to make sure that you follow all the way through their lifecycle.

Rachel: Yeah, I think that makes a lot of sense. It all just is quite devastating to think, “What have I been doing all these years raking up my leaves?” Haha. My last question before we move on a little bit—kind of flipping it to the more positive side—you've alluded to this a little bit, but aside from providing overwintering habitat, what are the other benefits of leaving the leaves? Do they help with soil health, for example?

Max: Yes. So we also looked at soil carbon and decomposition rates. And what was interesting was we looked at homes that had areas of their yard where they had kept the leaves—they had retained the leaves for years on end—and areas that had pretty much always been a lawn, or an area where these were removed. And we found that there was dramatically more soil carbon in the areas where the leaves were left. Carbon is so important in soil ecosystems, but especially in our residential areas—where soil quality generally is not that great—that soil carbon can increase the ability of the soil to hold water to, you know, help avoid runoff. It provides nutrients to plants, and can stop weeds and other things from growing. So there's so many benefits just to soil carbon in and of itself, kind of irrespective of thinking about carbon sequestration and things like that.

Max: What was interesting was even if you started leaving your leaves again, if it was an area where you had removed your leaves for many years, there was kind of a legacy of that management, and your soil carbon and your decomposition rates, they don't rebound. So these actions that we've done have staying power. They—those legacies are in our soils and it takes a while to kind of build that back up.

Matthew: It's almost like we're not thinking enough, in a way, about the impacts of what we're doing. We're guided by experts that we see on social media, or we read in magazines, and so on, and there's this like long tradition of neat and tidy. And we have to rake, we have to look after our neighborhood, we don't want to devalue our property or our neighbor’s. Leave the Leaves has been a great name for a campaign to encourage people to get beyond neat and tidy. It's been a hashtag that's spread all over social media, but I know that it's a very simple concept, and yet it kind of hides a whole world of nuance, and you've already been talking about all of that. At Xerces we often get asked if leaving all the leaves is necessary to have a positive impact. We get complaints from people who worry about leaves smothering grass, or they still desire that neat and tidy leafless yard. Is there a happy medium, do you feel, where we can get benefits from leaving leaves and avoiding the downsides?

Max: Yeah, absolutely. It's not all or nothing. We were interested in that question. Is there—like, if you're gonna leave your leaves, but you can't leave them in your entire yard, is there an area where it's more beneficial to do that? So we looked at areas of high maintenance and low maintenance. So like a front yard, as opposed to maybe an area behind a shed in your backyard or something like that. We actually found it was more beneficial to leave leaves in a high maintenance area—in the front of your house where you mow your lawn, where you usually had raked your leaves—than in an area that's more unmaintained, and that was kind of interesting to us. It was counterintuitive.

Max: But if you think about it, in these more maintained areas—these barrens, so to speak—if suddenly you have a resource, you have an overwintering resource—just like a, you know, a nectaring resource, now it's an overwintering resource—you are creating habitat in an area that it otherwise wouldn't be. Whereas in these unmaintained areas, generally there's lots of leaves, lots of brush, you know, kind of those areas that nobody sees—they're behind your house, and so the benefit to you leaving your leaves there isn't as great. But that was something we were noticing. Also in your front yard, you do have resources. You have—generally—gardens, you have flowers, and as I mentioned, a square meter—that can increase your insects overwintering population because they have a place to go. So I would say absolutely do not have to leave all the leaves on your property. You can set aside a small area in an area where generally you had raked—so this time do something different, just a small area—and that is actually incredibly beneficial.

Matthew: Yeah, no, I'm still shocked by the number of individuals that you are finding from a square meter. To think that there could be such life in such a small patch is quite remarkable. It's also encouraging when you think about if every person down a street were to leave an extra square meter of leaf litter—or 10 square meters of leaf litter, if you've got the space—they really could have a serious impact. I was going to ask about areas of like most helpful, and it seems like you've already covered that. Do you think there are leaves—and maybe this is going back to something that Rachel was asking about—I mean, are there leaves that are better to keep or maybe better to dispose of? And again, I'm thinking about some of the questions we get where people are concerned about disease, or whether that's a fungal disease, or maybe a pest species. And certainly black walnut is one particular tree species that I've heard mentioned a number of times in conversations.

Max: Yeah, I think—. Right, each part of your yard can be used for different purposes. I'm a big gardener—a vegetable gardener. Haha. I wanna make sure that I get my vegetables, and pests are a huge concern for me. So in areas like that I am definitely going to remove diseased material. And if I have a pest outbreak, I'm gonna remove those leaves. I might not even plant the same vegetable in the same area next year.

Max: But something else to consider is that your whole yard is an ecosystem, and it's going to have different uses. In some areas maybe you wanna leave that for nature—you want it to be a native plant garden—but in other areas you want it to be a lawn to play with your dog, okay? And in other areas you want it to be a vegetable garden. Just because they're separate areas, they do all—they're all interconnected. So if you ignore the nature kind of part of your yard, that can actually have impacts to other parts. If you lose that balance, you can get pest outbreaks. So one of the groups we were looking at were parasitic wasps. If you remove all of your leaves and you basically ignore all of the ecosystem that's naturally occurring in your yard, you can reduce those natural enemies, those beneficial insects that have value in other parts of your yard. So for me, in my vegetable garden, I have—you know, it's kind of surrounded by wildflowers, by native plants, by a diverse ecosystem. And I use that to host those parasitic insects, those beneficial insects, those predators to eat my aphids, to, you know, deal with those pest problems I have in my vegetable garden.

Matthew: Yeah, seems like everybody can achieve the right level of balance.

Rachel: So in terms of location, I do have a follow up question, because I've heard—I think it was Heather Holmes that talks about this like soft landing under trees. So we have like a pretty big yard and we push them to the outskirts of our yard and underneath all of our trees. Are those spots like more helpful than having them out? If people are trying to figure out where to put them in their yard—is under trees like a good option?

Max: Yeah, it is. And one of the other things we were looking at were alternative leaf management strategies. So we looked at what happens when you shred your leaves, we looked at what happens when you remove your leaves, and we looked at what happens when you move them from one area to another. And we were finding that that was—I mean, it wasn't perfect—but it was pretty close to our kind of baseline control of just leaving your leaves where they fall. So definitely, if you have to move them, retain them, just move them to an area in your yard where they can be—under trees, in your garden.

Max: What we did find though was mulching was just about as bad as removing the fallen leaves. So although that might be a good strategy for like composting and improving your soil quality, you're destroying the insects that are being run over by a lawnmower, put through a shredder. But soft landings, yeah. There's this idea that there's neat and tidy and then there's ecologically conscious gardening, and they're definitely not separate. You can have a beautiful, kind of woodland garden where you retain your leaves. And it'll keep down weeds, too, so you'd have to do less weeding. And you're also providing that ecosystem for plants that aren't gonna grow without any of that nutrients.

Rachel: Yeah. I was gonna ask you about mulching. So obviously, mulching during the season as the leaves are falling—not helpful. But is there a time of year, like in the spring, when you could potentially use leaves as compost? Or is it best just to leave them all the time?

Max: So we had our emergence traps out from about April to July. And you know, I understand everybody's coming from different places, but that’s pretty much the spring season here in Maryland—and even into the summer, really, I mean, getting into July. And we were finding that insects just kept on emerging. Moths kept on emerging, parasitic wasps kept on emerging. We didn't see—and we were looking for it. We were looking for kind of that peak and then, “Okay, now it's okay to rake your leaves.” We didn't see that. And there are multiple generations, too, of insects. So, you know, that first cycle, there's some that they're gonna have another one later on. They're gonna need that resource. So, as much as I'd like to say, “Oh, when the ground hits this temperature,” or, “This many, you know, days in,” unfortunately, I think, you pretty much wanna think about where you wanna leave your leaves instead of like when to remove them. So, I understand you can't leave them everywhere, but in the places that you're gonna leave them, they should be there year-round.

Rachel: Yeah, that makes sense. Well, Max, I'm gonna be changing my practices this fall. Haha. I appreciate all of the great information.

Matthew: Yeah, I'm glad you mentioned where because that was kind of the direction I was going in next. And I realize that you just mentioned that you were gathering—the emergence traps was still out ‘til July, and we're—it's July when we're recording this episode, which may seem like a weird time of year to be thinking about leaves, but, you know, you've just been saying how the benefit from them is not just limited to one time of year, it's a full-year thing. And that does bring us to that some gardeners—. Obviously, the leaves will not have rotted away, and so I'm kind of thinking of if gardeners are not mulching, but moving leaves to gather them in one place, do you have a sense of how deep a layer? I mean, because it's like, do you have a foot of leaves? Or two feet of leaves? Or six inches of leaves? It's almost like, could you have too many leaves?

Max: Yeah, no, that's a great question. And I would say we're thinking more about six inches than feet. Because insects, they have a lot to pay attention to—there's a lot of cues that they need. They need to know when to emerge, they need to know when to mate, when their host plant is gonna be ready, and all of that can get disrupted if you're burying them under a massive pile of leaves. So really this is, yeah, as you mentioned, a soft landing. This is like a doubling of leaves, not quadrupling, or tripling. So I would definitely say, a shallow layer of leaves around six inches is kind of more what you wanna lean towards.

Matthew: Yeah, no, that makes sense. And then it does mean that in some gardens you'll keep the leaves where you can, but you will also be disposing of some.

Max: Yes, and that's fine. You have to do what you have to do. And, you know, a lot of times I get asked, “Won't this kill my lawn?” And it really depends on the type of tree that you have above. Some leaves actually decompose fairly quickly, and your lawn will be okay. But yeah, and there's other tree species, like oaks, that that will probably smother your lawn. And so you have to be okay with that in certain areas. And if you're not, then take those leaves and move them to an area where they're okay.

Matthew: You've talked already about how gardens—you can kind of divide your garden up and decide what you want from it. And I know that I’ve also had similar conversations. And it's encouraging people to take the steps that they can, and they feel comfortable with, but also accepting that yeah, you're gonna have an area of your dog to run around, or because you wanna throw a ball with your kids, or have somewhere to sit your morning coffee. And it is not feeling guilty or—. Although maybe we should hope people do feel a bit of guilt about, “I could have done more.” But no, we don't want people to feel bad, because their garden provides multiple benefits. But yeah, always reaching that balance. There's a comfortable compromise to be had in the middle of it all.

Rachel: Well, Max, thank you for all of the wonderful information you have provided. I have a lot to think about, and like Matthew had kind of alluded to, it's pretty complex, but what's nice is there's options for everybody. But I think what's really cool is just that there's a whole living ecosystem in these leaves, and whatever we can do to maintain that is just as important as planting native plants and, you know, leaving logs and other nesting habitats. So I've really enjoyed this and have learned so much. So we're gonna go to the two questions that we ask everyone—and this one is my favorite, so I'm excited to ask you—but what inspired you to pursue a career in insect conservation?

Max: Well, I was interested in plant conservation before insects. I started getting interested in native plants, and backyard gardening, and providing habitat. And then I started focusing on the things that were utilizing those plants. And yeah, plants are cool. It's hard for me to say they're not cool. But I mean, to see something moving around, it's so much more fascinating for me. And I was inspired by the work of Doug Tallamy, actually. And, you know, it was kind of groundbreaking for me to think about our yards as a place of habitat, and not just suburban sprawl that has no value. Because it’s positive, it's hopeful. You know, that this is something that we all can do. We don't have to rely on the government, or tons of money, or things like that. If we just all band together, we can do a lot with the land that we have. So that was really inspiring to me.

Max: And actually, I did my PhD working with Karen Burkhart, who was a co-author with Doug Tallamy on a lot of papers, even got involved in this type of research in the first place. And now, I'm in Maryland where we don't have a ton of open wild land, so to speak. A lot of it is human-dominated. And when you're working on insect conservation, like I am, working with people, and their properties, and their yards is a huge component to creating corridors, and allowing these insect populations—which are fairly small— to grow—these rare insect populations, specifically—to flourish.

Matthew: I'm similar. I didn't start with insects, but discovered insects along the way and became fascinated with them. And for sure, plants and insects, they go together. You can't separate them out. I was intrigued to hear how people ended up working with insects, because it's not always the most—it's not something that like your career counselor at high school said, “Hey, insects!” You know, so it’s—.

Max: Yes. Haha.

Matthew: —it's great. Though, that—. Anyway, also, I want to say thank you because this has been a great conversation. Leaves have been something that I may have taught and thought about more than many people in recent years, and so to have this opportunity to sit down and talk with you more has been great. Last question for you is: if you could see any bug in the wild, what would it be?

Max: So, you know, part of my job is to survey for rare insects. So there's a lot of species that I could say that are rare and not a lot of people have seen them. But I have never seen a spicebush swallowtail caterpillar in the wild. And it is something that I—every time I go to a spicebush, I spend like five minutes looking it over, and I've just gotten so unlucky. But they look really cool. I mean, their whole kinda life history. First, they start out looking like bird droppings, and then they get eye spots, and everything. And I just have not been lucky enough to see one of them. And it's the type of thing that—. Like a big thing for me is plant the native plant in your garden, and then the insect will show up. It's just so fun for me to kind of cultivate that community in your backyard.

Matthew: Now do you have to go like, go around and like touch every bird dropping you come across, just in case? Haha.

Max: Haha. They're well hidden.

Rachel: I love that story. Well, thank you so much, Max. This has been really wonderful. I appreciate your knowledge, and answering all of our many, many questions. Thank you so much for your time today. It's been really nice.

Max: Absolutely.

Matthew: Yeah, thank you.

Matthew: 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.

Matthew: If you are already a donor, thank you so much. If you want to support our work, go to xerces.org/donate. For information about this podcast or for show notes, go to xerces.org/bugbanter.