Invasive Pests vs. Minnesota Cold

Minnesota's cold weather can have a big impact on invasive pest such as insects and noxious weeds. Learn how cold temperatures affect pest behaviors and the consequences of warmer winters. 

Smarty Plants is a podcast of the Minnesota Department of Agriculture. Visit www.mda.state.mn.us/plants-insects/smarty-plants for more episodes. Look for a new episode of Smarty Plants every month.

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Jennifer Burington: 0:20

Welcome to Smarty Plants, a Minnesota Department of Agriculture podcast, informing citizens about invasive species that affect our environment and agricultural resources. My name is Jennifer Brangton, and I will be your host. This episode, we will be covering information on the cold tolerance of invasive insects and plants in Minnesota. Our cold winters can influence what invasive pests are present and how much damage they can inflict. As our winters become warmer, we are also witnessing change in pest behavior. With us today to discuss how the cold affects invasive insects and plants is Rob Vernett with the USDA Forest Service and the Minnesota Invasive Terrestrial Plants and Pest Center, and Tony Cotterlett, weed scientist and noxious weed section manager from the Minnesota Department of Agriculture. for making time to speak with us.

Tony Cortilet: 1:06

Thanks for having me, Jennifer.

Rob Venette: 1:07

Yes. Thank you, Jennifer.

Jennifer Burington: 1:09

Thanks. And with us is Dr. Rob Venette. He is a research biologist with the Northern Research Station of the USDA Forest Service and is the director of the Minnesota Invasive Terrestrials, Plants, and Pests Center at the University of Minnesota. With the Forest Service, he conducts research on the distribution and impact of insects, pathogens, and weeds that are new to North America that may affect forests and grasslands. Studies on the cold tolerance of many invasive species have become a cornerstone of his research. At MITPPC, Vinette coordinates a portfolio of research to prevent or reduce the effects of some of the greatest invasive species threats to Minnesota. Tony has served as a weed scientist with the Minnesota Department of Agriculture since 1998, and as mentioned is currently the section manager for the seed, noxious weed, hemp, and biotechnology programs. He has over 30 years of professional experience working with plant species, habitat management, and ecological preservation. Tony holds a bachelor's degree in fisheries and wildlife biology, as well as a master's degree in agronomy with a concentration in weed science, both from Iowa State University. Thanks again for joining us here. So we're going to be talking a little bit about, uh, the cold. Now this fall has been really warm and I wish it were kind of cold so I could get out some skis, but, um, tell us a little bit, what is cold tolerance in respect to invasive insects and plants?

Rob Venette: 2:33

Yeah. So when we start talking about cold tolerance, what we're really talking about is the ability of really any organism to survive a period of cold and then resume its normal development and continue to go on through its life cycle, um, almost as though it never had gone through the cold in the first place.

Tony Cortilet: 2:53

Yeah. And the same for plants, uh, everything from seed to reproduction, to continuing that cycle again, how. Uh, the survivability of that individual species is going to continue on.

Jennifer Burington: 3:06

So, over the past couple of years, we've had some cold snaps, and it's gotten really cold over those cold snaps. And, how cold does it actually have to be to really affect these insect pests that are coming in?

Rob Venette: 3:21

That's a great question, and it really depends on the insect that we're talking about. So, if we're talking about an insect like the emerald ash borer, We know that it needs to get around minus 20 Fahrenheit to kill about half of the insects. If it gets to minus 30, we're looking at close to 90 percent of the insects being killed. So it really does depend on, um, where the insects are and what actual temperatures they're going to be exposed to. Now, there are other insects, say for example, uh, the Harmonia lady beetles that come into people's homes. For that insect, it only needs to get to be around minus 10 to begin to see pretty significant mortality. But of course, those insects are overwintering in our home, and so they don't necessarily experience the bitter cold that might come along with a polar vortex.

Jennifer Burington: 4:12

And then, tell us a little bit, how does plant pest cold tolerance factor into the work that each of you do?

Rob Venette: 4:21

So thinking about cold tolerance is a major part of my, my effort, and one of the things that we're always trying to think about is how might a pest make it through a Minnesota winter? And we think about that from a couple of different angles. We have a lot of native pests. We have tons of mosquitoes, as people are familiar with. Uh, people are familiar in agriculture with the European corn borer, um, lots of insects that are very well adapted to a Minnesota climate. And those insects are not going to go away just because we have a cold winter. Um, it's the insects that are new, particularly on the east coast or the west coast that might be moving in. And those are the ones that we're kind of curious about. Might they actually be able to make it through a Minnesota winter because Our winters are often colder than many of these insects in particular have ever experienced. And so there's a very good chance that they may not be able to make it here. And so we think about cold in a lot of different ways. The simplest way is simply to say, Will the cold kill this insect outright? And, um, if it does, and if our winters typically would be enough to, to get rid of it, then that's one that we would advise the Department of Ag, other land managers to say, hey, don't put a lot of effort, don't put a lot of energy into worrying about this particular pest. There are though, quite a few insects that are new to this country and they're perfectly well adapted as well. Where they come from, either in Asia or Europe. They've already experienced cold in their history. And so the winters that we have here are really nothing new to them. And so those are the ones that we've become more concerned about and we try to think about what part of the state are they going to survive in and if they do, what impact might they have.

Tony Cortilet: 6:12

And that's exactly the same thing with plants and just to build on that, Minnesota, just our climate from, you know, if you, if you split the state into equal quarters is very different. And so even within the state, we have a lot of plants that are introduced either through accidental, you know, they're accidentally brought into the state and they can germinate, but will they be able to continue on a population of any type? And like Rob said, we're sort of sandwiched in the middle of the U. S. And so we do get plants from both the east and the west that end up in Minnesota. And so we're We're lucky in one respect that we can look to our coastal regions and see sort of how they've come in and how they've started to spread. Um, but it is interesting when we see how certain species do make it into the upper Midwest and certain ones don't. And there's certainly more to it than just cold tolerance, but that's one of the risk factors that we're always looking towards.

Jennifer Burington: 7:12

Thanks. And as you mentioned, there are stuff coming from the east and the west. So what changes are you seeing in the insect pests as our average winters are trending warmer?

Rob Venette: 7:23

So we're seeing a number of different effects from that. So one of the first things that we see are greater numbers of insects actually making it through the winter and that's fairly common across the board. Um, that's always a concern because damage that those pests cause are related to how abundant they are in the landscape. So the more insects that make it through the winter, the more potential damage they can cause. But as Tony mentioned, cold is just one factor and there are many other things that can come into play and affect that, that pest abundance. The other thing that we're starting to see is different parts of the state are beginning to see insects that they haven't had to deal with historically. And a really good example of that is the emerald ash borer. And finding it up, um, in Cass County near Riemer, Uh, was a real surprise, at least to me, um, because that's in a part of the state that we thought was going to be too cold, and it's one of our more northern vines so far. Um, and then the other part is that, um, we're also seeing, um, populations build up just generally more quickly. And I think that's, again, related to how many make it through the winter. It provides, it primes the pump, if you will, and allows those populations to really take off.

Tony Cortilet: 8:42

Yeah, and we've seen with plants, uh, just a couple that come to mind that are, that are pretty popular. One that's really popular, wild parsnip, which over there, I would say the last 20 years used to be sort of a far southeast Minnesota problem. Um, we had even done some surveys here at the MDA to document that, that it was kind of relegated to that southeast quarter of the, of the state. And now that's not the case. You can find it throughout Minnesota. Yeah. Um, and probably temperature has a, a bearing on that. Um, it, it definitely, uh, serves some role in the plant's ability to compete better with other plant species, uh, from what it was used to, exposed to 20, 30 years ago. Um, we've seen those kinds of effects with other plants too. Um, but again, you know, that's just one factor we're looking at when we're evaluating, Hey, is this going to be a problem up north? Is it going to be a problem down south? Uh, we've also had spotted knapweed move from the north to the south, you know, so how do you explain that one? But, um, overall, we have seen trends in more weeds moving to areas where they traditionally were not found. And so it is something of alarm that we look at in terms of when we're evaluating risk for a species.

Jennifer Burington: 10:02

And then are there certain insects or plants that we should pay attention to? You mentioned a few already, Emerald Ash Borer and Spotted Knapweed. And what can you tell us about that and how should we become familiar with it ourselves and what should we pay attention to? And in particular, like what life stages to look for? And that we don't want to survive over the winters.

Rob Venette: 10:26

Well, you know, and that's one of the things that's, it's a great question. Um, and insects are so adaptable. Um, and so different insects have different strategies for making it through the winter. Some will survive, um, in an egg stage. Some will survive, survive as larvae or the, almost a caterpillar stage. And even others will survive as adults, the full blown insect. And so one of the things that I always recommend to people, First of all, try to figure out, if you can and are concerned, what do the adult insects begin to look like? Because often those are the most characteristic, those are the most easily identifiable. If that's not your thing, if you're not really into learning what insects really look like, I also really highly recommend a lot of the smartphone technology. There are a lot of terrific apps out there that will really help you identify what is this bug and should I be concerned about it. Um, so I highly recommend that people start there. Um, and then when people find new things, things that haven't been in their part of the state before, or at least new to them, I always encourage people to tell someone. Um, and the Minnesota Department of Agriculture has made that very easy for new pests. And so I always encourage people to report things, um, either on their website or even a phone call will do. Um, but to say, hey, we found this pest. I don't remember seeing it here before, just wanted you to know. And that has turned out to be incredibly important information I think both the Department of Ag, for me and my own research, we really take a lot of advantage of those community science programs and reports that come in that way.

Tony Cortilet: 12:12

Yeah, I agree a hundred percent with Rob. I mean, we spend a lot of time here at the department answering questions throughout the growing season on what is this plant. My staff and myself do a lot of plant ID for folks. And so, On top of the, you know, artificial intelligence tools like Rob was alluding to that are out there, which are fantastic, PlantSnap, and there's a whole host of others people can go and look for. Uh, sometimes it is good to call somebody that this is all we do. You know, this is, we know plants. And, uh, uh, it, it's confusing what is a thistle sometimes to people. There are many plants out there with spines on them. And so they immediately put it into that category. And so I think that contacting the department, um, especially our, just speaking for our own weed program, but I know our insect program is the same way. Send us a picture. Everybody's got a phone in their, you know, pocket or their possession. Usually they can take a picture. And we always say, you know, make sure you take good pictures. It makes our job a lot easier than getting something that's blurred or hard to identify, especially for things that are really close, you know, looking together. But for us, it's important because we've found species that people reach out to us and say, hey, this is in my neighborhood. And we're like, what? That's only known to California. How did it get there? Is it surviving? How Those are the things we need to get out there on right away because they get here what I would say unnaturally and um, they can become big problems potentially if we're not on them. So we always ask people to be aware. If you see something that looks really, really strange to you or you just have never noticed before, let us know.

Jennifer Burington: 13:59

Yeah. And one of the ways that people can report in is the Department of Agriculture does have report a pest. So there is a website at www.mda.statemn.us/report a pest. And that has a lot of information on how to take a picture, what type of picture to send in. If you wanna send us a voicemail, if you wanna send us an email, there's also a really easy form, um, online that comes in and I actually triage them and send them to all of the experts to identify for you. So if you do see, do see a plant or an insect out there, um, by all means snap a picture like they're saying. Send it into us and we'll be able to identify it.

Rob Venette: 14:45

And, and Jennifer, if I could just jump in real quick, another free app that's available and has really proven useful both to the Department of Ag and to researchers is called iNaturalist. And iNaturalist is a great tool for anybody who likes to be outdoors because literally you can snap a picture, it will do its best to identify if it's a plant or an insect, and then it makes it very easy to report the location and with very good accuracy. And, um, it's those pictures that become so important and that's another thing I think people should be aware of is that we don't just say, oh, if someone reported it, it must be here in the state. Um, if it's new, that's treated very seriously and there are a number of folks that will work very diligently to make sure that that report is in fact correct. And those new reports, until they're verified, are not released to the public and for good reason. Because we don't want to confuse people about what's here, what's not here, and what do we need to be concerned about.

Jennifer Burington: 15:52

Perfect. And, As people are out and snapping pictures with all of their apps that they have that are free, um, and after, in the spring, we all like to get out and see what's growing right away. Um, so if we have a particularly warm winter again like we did last year, uh, what pest impacts can we expect to see on the, on the plants this coming year, if it's another warm one?

Rob Venette: 16:17

Yeah, that's a really good question, and it really depends a bit on the plant that we're looking at and the type of damage that it causes. So, one of the insects that I've spent a lot of time working on fairly recently is the emerald ash borer. And it overwinters in a larval stage, so it's a flat headed borer, it's living right beneath the bark of an ash tree. And that's a challenging one because to a homeowner, you don't necessarily see a lot of its feeding activity. You don't see, um, huge swarms of insects coming out at any point in time. But what you do begin to see is this, um, really sluggish growth in the plant. A lot of dieback, um, when spring comes, a lot of leaves are going to be malformed if they form at all. Um, We'll also see quite a bit of blonding on the tree that's caused by woodpeckers as they're trying to go after those larvae. Um, and sadly, a lot of times as the season progresses, what you actually begin to see is clearly that the tree is either dead or dying. Um, so oftentimes there are epicormic shoots, little branches that are coming off of the main stem. And that's just a sign that this, this tree is doing its very best to try and hang on there. Um, but it's a last ditch effort. And in all likelihood, that tree is, is going to die.

Tony Cortilet: 17:40

In the, in the plantscape, it's a little bit different. Similar, similar in the spring, but we be, we really look for disturbance. And that's going to lead to how plants establish or don't establish, especially weedy species. And if you're beginning to see sporadic new plants that are either listed noxious weeds or weeds of concern, which you can find those on the Department of Agriculture website, among other extension, University of Minnesota extension and other places. That's concerning, especially when they begin to become bigger monocultures and they start to grow and grow and grow where you see big clumps of the single species. Um, some of our native plants do that. They're naturally out there competing, but. When we see warm winters, like, uh, the past winter, certainly, and possibly this one, uh, we do see changes in different species that begin to pop up. And, um, that can really dictate how the rest of the summer goes, or the rest of the growing season goes. Because if they, certain species, are early bloomers, so to speak, They can get a handle on the landscape and outcompete other things and really begin to take hold. And the more that they begin to grow throughout the growing season, that could result in more seed that gets overwintered for the next year, begins to germinate, larger populations, and so on. So, we really look for those disturbance factors if it's just staring out at the landscape and try to warn folks about, hey, we're seeing a year where this species is really beginning to take off, something you should keep your eye out for.

Jennifer Burington: 19:17

Perfect. And there's a couple, couple species that I know are, that come and go in population, um, highs and lows as the weather changes. And ones that come to mind are like brown marmorated stink bug and, uh, Japanese beetle and some of the lady beetles, box elder bugs, like everybody remembers from the fall, um, invading their houses. Um, Those aren't very risky, but how, how do we determine how risky a pest may be for Minnesota?

Rob Venette: 19:49

Boy, and that's a really great question, because I wouldn't be so quick to dismiss some of those as not being risky pests, because, because it really depends on who you are and how you're looking at the situation. So, at the Minnesota Invasive Terrestrial Plants and Pest Center, We've been asked to try and look very broadly at all kinds of new insects, diseases, weeds that might be coming to the state and trying to identify what are the greatest threats to the state. What are the greatest risks? We look at 17 different criteria that affect the threat level that they pose. What's really interesting to us is they're not all equally important. Um, But, what's curious is that the impact that they might have is the overwhelming factor that drives how people feel about a particular pest. And they're concerned about what it's going to do to the ecology, if it's going to affect our native species, that's the number one single factor. Then it goes to economics. Then it goes to other social impacts, and then it's a whole slew of questions about whether or not the insect will actually make it in the state. And we were actually quite surprised when we put that together. Um, as someone who works on cold tolerance, I thought cold had to be the number one answer, but it turns out it's not. And, um, and what we've also learned is that, of course, depending on who you are and what you care about does affect your health.

Tony Cortilet: 21:23

Yeah, and we have the same thing on the weed side, and Rob shares time over on the Noxious Weed Advisory Committee too. Uh, and there's a similar, uh, risk assessment process that every one of our listed species goes through. Um, we also are evaluating all species that, uh, you know, are either reported to us from people being concerned, or concerns, again, to the East Coast, West Coast, South, North, and South. Um, we look at anything that's potential threat to moving into Minnesota and, um, it's a decision tree process. Uh, and it's based on fact, uh, to start out with, but because we are a state agency and that committee are constituents representing, uh, organizations throughout the state of Minnesota, there are also politics that come into play, believe it or not, uh, that determine these things. So one person's weed might not be another person's, um, and it becomes sort of a democratic process in a way to, um, try to take the best facts we have on a species and determine, Can we regulate this and back to what some of Rob's criteria, you know, what are the factors that pose the most importance to this species and why we're going to list it and regulate it. The whole purpose of the ranking of a species is to let folks in Minnesota know we didn't just pull this out of our hat and say, Hey, we're government and we know what's best for you. We're trying to look at the most science we can, uh, but also be considerate of our industries, um, especially farming, which is done on land and a lot of land in Minnesota. So we want to be considerate of that and we want to try to find the best way that we can list things, regulate things, or just point out awareness to things. Uh, that we don't necessarily want to list because there may be real hardships with that. Uh, but the main fact is that we're, we're looking out for things that could potentially become species that are dominant and affect our industries, our natural resources, you know, a whole host of things, and putting some, uh, risk level to them.

Jennifer Burington: 23:33

Thanks. And one of those, actually two of those species would be like, you Uh, Spotted Lanternfly and Tree of Heaven. Do you guys want to talk a little bit about what Spotted Lanternfly, what kind of risks that poses, and then how it's tied to Tree of Heaven, so you've got both the insect and the plant side of things.

Rob Venette: 23:49

Yeah, so the spotted lanternfly, it's a leafhopper that showed up in Pennsylvania just a few years ago, but it has made a media splash. And you may have heard all of the campaigns to squash it, stomp it, kill it wherever you find it. Um, and we in Minnesota have been watching those stories with a lot of interest because this is an insect that is not here in the state. And we were very, we're not sure what it will do when it gets here. Um, it's an insect that absolutely loves the sun. tree of heaven, which is a weedy plant that's very common in the eastern United States. Um, but as those populations build up on tree of heaven, it moves off onto other plants. So it affects grapes, it affects apples, it affects a whole host of other hardwood trees, and in the process causes considerable ecological and economic damage. And so this is definitely an insect that we don't want to have in Minnesota. The challenge we have, though is that, um, early forecast models, national forecasts, comparing the climate where spotted lanternfly is native in parts of China and new populations in Korea, comparing the climate to that in the United States, those forecasts say that Minnesota may not be a very good environment. In fact, suggesting that cold might be a primary limiting factor for it. But we want to be very, very cautious about that. And we know that that's the best available information, but we need more information in order to really have a good strategy and be prepared for it. The tree of heaven is really a critical element in the life cycle of that particular insect.

Tony Cortilet: 25:36

Yeah, and it's very interesting because the tree of heaven listing, uh, as a noxious weed happened well before we knew about spotted lanternfly. So this Kind of ties into this whole invasion issue. And, you know, as a cold tolerance, uh, is it just natural, um, movement? What is it? And tree of heaven is been moving from the East coast towards the Midwest at a pretty rapid rate. And so in Minnesota, we had a few locations. I want to say they were two where tree of heaven had actually been brought in and been most likely planted. It, it did not make it here on, on its own. Uh, both of those, uh, places, or both of those plants were, uh, cut down and eradicated, uh, because we didn't want it to establish just as a weedy tree species like Rob had pointed out. At that time, I don't even know if the risk assessment pointed out spot and ladder fly as being a problem. Um, so even in our best attempts to assess risk, Uh, there's so many factors involved. Yes, cold tolerance is one of them. Can something from the East Coast move to the, you know, the colder upper Midwest region? That's just one factor that we looked at. Um, but it appears to be able to move. And with it now, there's this insect species that's been introduced to the U. S. that is going to spread with it. Uh, because of that close developmental relationship, it needs that tree to develop. So, you know, We have a couple examples of, uh, plant species and insect pests that are like that, uh, and that's just one example. But, um, certainly the cold tolerance of Tree of Heaven is going to be very, very important for us to pay attention to moving forward.

Jennifer Burington: 27:21

Perfect. Thanks for that explanation. Um, and just to wrap up, is there anything else we can do to help minimize the effects warmer winters are having on invasive pest populations?

Rob Venette: 27:32

Boy, that's a really big question. It's a big one. And it's one that I don't know that I've got a great answer to. But it's one of those things where I really encourage people to become familiar with what's around them. And that often starts with the plants because the plants also affect what insects are going to be around. And having an appreciation, um, even if you don't know every critter by name, um, Um, but just having an appreciation for what's there is a really important start, because once you know that, then you can begin to see how things are different. And I think that's the thing, as our climate becomes different, we'll start to see differences in plants, and then new pest issues beginning to pop up. And so there's not one bit of advice, and I don't think one person in Minnesota is going to be able to solve all of the changing climate issues that we're dealing with. But that awareness and that willingness to share those observations with others really becomes important.

Tony Cortilet: 28:32

And I couldn't agree with that more. I mean, everybody should become sort of an amateur botanist. Uh, I think plants are a good key. Uh, they're some of the first things you're going to see after the snow melts in Minnesota. Uh, and they can tell you a lot about the landscape and some predictive nature of what may come later in the, you know, July and August months. Another thing that's interesting about cold tolerance from a plant perspective is a lot of our native plants that we want out on the landscape rely on cold. Uh, it'll break dormancy of seed. Uh, so it's not just something we look at to say, Oh, these bad plants moving in and are they cold tolerant or not? It's also if, Things begin to get too warm, uh, and that affects native species tolerance to cold and that requirement to have low temperatures. Um, the way the landscape works is if something else disappears, something else takes its place, right? And that not only affects the flora, but then the things that, uh, insect species and diseases that we've been talking about. Um, it affects what ones are going to show up and potentially thrive. So just. Paying attention to what's around you, like Rob said, is, is important, and there's so many tools out there where, uh, if you're not a, a biology oriented person, you can learn a lot. And, uh, cameras are certainly coming with all kinds of fun apps that will identify things for you, and, uh, if you, if you have any questions, you can always reach out to the department, too.

Rob Venette: 30:10

Yeah. Tony makes a really important point, and if I can just jump in real quick. Cold is important for insects as well as it is for certain weeds. And if some insects don't get enough cold, they don't do well either. And we know that, for example, with the eggs of spongy moth, as it's moving into northeastern Minnesota, the larvae of the emerald ash borer actually do need a period of cold for them to have their development be completed. And so, as Tony says, this is a very complicated situation. A warmer winter doesn't mean all things survive a whole lot better. And so that's where it's important also for us in the research community to tell this a bit of a complicated story. That the changing climate is going to bring all kinds of changes. Some of which we can foresee fairly easily and others that are going to be harder to predict.

Jennifer Burington: 31:06

Perfect. Well, thank you, Rob and Tony. And I hope that, uh, we can have some nice cold weather to make sure that we get some of those plants and insects that we do want, and maybe not some of the ones we don't want. Um, it was great having you both on to discuss how Minnesota winters affect plant pests. And I certainly learned a lot and I'm sure our listeners did too. So to wrap up, this has been Smarty Plants, a podcast from the Minnesota Department of Agriculture. Our producer is Brittany Revell. Our editor is Larry Schumacher and I have been your host. Uh, you can learn more about pests and other invasive species that affect our environment at www. mda. state. mn. us. And while you head to our website, we'll be working on the next episode of Smarty Plants. See you there.