What’s in the toolbox? Aphids and other soybean insects in 2020.
So welcome everyone to the U of M IPM podcast for the first 2,020 episode. This is also our third year of having the IPM podcast as well. I'm doctor Anthony Hanson, an extension postdoc with the IPM program in the department of entomology. And I'm filling in for our normal hosts, doctor Bill Hutchison and Dave Nikolai, and we might get them on later in the season. So you might have noticed that we're starting a little bit later in the season than previous years.
Anthony Hanson:Some of that has just been logistics due to COVID nineteen and getting some, better Internet access more or less for recording. From there, we'll be trying to get, you know, a couple more kind of condensed episodes in the season in for what's going on either right now or what we've seen kind of throughout the summer so far, but then also a bit of an outlook of what we can look forward to either next year or just kind of in general for this third season of the IPM podcast. So for our first episode, we have returning doctor Bob Cook. He is an associate professor of entomology and extension entomologist in Soybean, as well as the director of graduate studies at the department of entomology at the University of Minnesota. So, Bob, we've had you on a few times before, but do you just wanna briefly introduce yourself and what you do at the University of Minnesota?
Robert Koch:Yeah. Hi, Anthony. Thanks for, having me on today. My position is focused on research and extension, and in particular, we deal with the insect pests in soybean crops. So a lot of work looking at the biology and ecology of these pests and trying to advance, integrated pest management recommendations for these critters.
Anthony Hanson:Okay. Thanks, Bob. So especially the biggest pests that often comes up in your field is going to be soybean aphids. So we'll cover that one first and then talk about some other areas that you've been working on once we've kind of worked through what's going on with soybean aphids. So what have you seen for soybean aphids so far this year?
Anthony Hanson:Either earlier in the season, what's happened, or what are you kind of seeing kind of in this current week at the very July, August?
Robert Koch:Yeah. Those that's a good question, Anthony. So the season started out and seemed like all the conditions were lining up really well to be a big aphid year, and I was getting kind of worried from the farmer farmer's perspective and, kind of excited from my own kind of selfish research perspective because we like to have lots of aphids in our research plots. However, things changed a little bit, and the aphid populations kind of stopped growing or developing as as fast as I thought they were going to. It doesn't mean we're in the clear yet.
Robert Koch:Aphids are still out there in many of our fields. Populations are increasing. We're not hearing of many fields yet that are near treatable levels, but we have heard of some. And, you know, I think weather like we're having this week, this is pretty nice aphid growing weather. So those populations could definitely be increasing quickly.
Robert Koch:So I think it's something folks are going to want to be scouting for to get a feel of where the aphid populations are at right now in their soybean fields and to be monitoring those over the coming weeks to see if any of them are going to be approaching treatable levels where an insecticide might be required to protect yield.
Anthony Hanson:Okay. So just looking back a little bit, we talked about the weather where, one, we had some pretty hot humid weather, then we've also had a lot of rain across the state. So did that kind of save us a little bit? How are the populations looking before up to now in terms of what those two factors were doing? Like you said, now the current weather is pretty favorable for aphid growth, but were those other ones maybe pushing them back to a large degree?
Robert Koch:Yeah. You know, I think that stretch of real hot weather that we have there probably slowed down the aphid population growth. There's some pretty solid research out there showing that soybean aphids don't really like the real hot temperatures. So when temperatures get up into the nineties, survival and reproductive rates can slow down. But I just wanna caution people not to think that aphids will not be a problem just because temperatures get into the nineties.
Robert Koch:Right? So that's gonna potentially slow things down, especially during that hottest part of the day. But we gotta remember that entire day isn't up in the nineties and temperatures do drop, you know, at night where things could get into the more favorable temperature range for the aphid. The microenvironment or microhabitat where aphids are at in the soybean canopy could be a little cooler than the air temperature. So those hot days can definitely slow down the aphids, but it's not necessarily gonna eliminate the aphids from being a problem.
Anthony Hanson:So you mentioned also getting to treatable levels as well, and the cost of soybean aphid thresholds, it's getting to be pretty commonly known by a lot of people. Others maybe don't have as many details about it yet. So you wanna briefly cover how the thresholds work?
Robert Koch:What we call the the economic threshold, that's the point at which you wanna start lining up an insecticide application to prevent the aphid population from reaching an economically damaging level. So ideally, we're going to be out scouting our fields on some kind of a regular basis, maybe every week or so, kind of documenting where those aphid populations are, The research based economic threshold that continues to be validated by research at the University of Minnesota and other universities in in the region is two fifty aphids per plant. So if you're scouting your field and you get to a point where you've got the majority of the plants infested across all those plants that you're inspecting, you have an average of 250 aphids per plant, and those populations are increasing over time, that's when you want to start lining up that insecticide application to knock that aphid population down. Because when they get to two fifty aphids per plant, it's very likely that those populations are going to keep increasing up to a level where you'll get economic losses, which would be 700 aphids per plant or so. So keep in mind that that two fifty aphids per plant, just to restate it here, that's that trigger point for lining up the insecticide application.
Robert Koch:That is not where we're getting economic losses from the soybean aphid. And I know some people can get that mixed up a little bit. Soybean is a very resilient crop, and it can tolerate some number of pests on it, be it, defoliating pests with chewing mouthparts or sucking pests like soybean aphids. So those two fifty aphids on those plants, you know, that's not gonna be causing economically significant losses to to that crop.
Anthony Hanson:So let's say I've found about 50 aphids per plant on average then in the field, and I have heard about some fields where they're maybe getting close to that, but if you have about 50, maybe a 100 aphids per plant, they're not necessarily causing damage at that point?
Robert Koch:That's right. So that's the research that we have from multiple states multiple years still shows that we should not be applying insecticides for soybean aphids at that low of densities. The soybean plant should be able to tolerate that abundance of aphids there. I personally have seen multiple fields where the densities get to 5,100 aphids per plant, and then they just flat line there. They don't increase anymore over the season.
Robert Koch:So if you were to have treated one of those fields, it would have been a wasted application, right, because that population stayed below that damaging level. However, those are fields where you're gonna wanna get back out there, you know, within maybe a week or so and scout again to see if that population is increasing. And then if you get to that threshold of 258 if it's per plant, then you definitely wanna line up that insecticide application.
Anthony Hanson:So how far does that stretch into August now? Because we're literally just on the cusp of August and eventually soybean aphids are going to migrate back to buckthorn or the plants are going to be maturing as well. So kind of how much of a window do you kind of expect where if someone hits say 200 aphids per plant, but it's also the August, is that kind of the situation where you keep an eye on it, but at a certain point, you kind of expect that you may not have to treat?
Robert Koch:I wouldn't base it just on the calendar. The insects aren't using calendars, so focus more on plant growth stages. So that threshold of 250 aphids per plant should be used through the r five growth stage. Once you get into r six, things get a little trickier where there's a little bit of data from the University of Minnesota showing that if you're in the early r six growth stage of soybean, large populations of soybean aphids could cause, some yield losses to soybean, but we don't have enough research to really have a valid threshold for that r six growth stage. And then once you get into the later r six growth stage, those yield components are, you know, kinda set where the where the aphids aren't gonna be having any more impact.
Robert Koch:But if you're scouting on a regular basis and using that threshold through r five, you should be able to prevent that tricky early r six situation, you know, where you could be able you should catch those aphid infestations earlier on and prevent those large numbers in r six where it kinda leaves you wondering if you should really treat or not.
Anthony Hanson:So I was kinda curious about the concept of basically there being a value of not treating your plants. Because sometimes it could feel like if you're out there scouting and you're thinking, well, maybe I should be getting ready to spray soon even if I'm not quite at two fifty, but it kinda seems like a situation where you could remind yourself that also not treating is also kind of an action in a way where you're potentially saving yourself money. Do you want to, comment on kind of how that works essentially?
Robert Koch:So if you're at 58 if it's per plant, like I said before, just keep scouting that field and see if it's going to increase. That decision to not treat the field at that point, I think, a good decision because you're not making an investment in the crop that is probably going to turn out to be an unnecessary investment. And those insecticide applications, any insecticide applications that we make, whether they're unnecessary or not, those are all creating selection pressure on soybean aphid and other insect pests in that crop to develop resistance to the insecticide. Right? So whatever insects are in there that are susceptible to the insecticide would get killed off, but there's generally some tiny fraction of whatever insect population that's resistant to the insecticide.
Robert Koch:And the more applications of insecticides we make, the more we're selecting for those rare resistant individuals to become more and more abundant. And sure enough, a few years ago, your very own PhD dissertation work, Anthony, documented insecticide resistance in the soybean aphid with the, pyrethroid insecticides. Back in 2012, 2013, some of my colleagues, Bruce Potter and Ian McCray, documented insecticide or miticide resistance in the two spotted spider mite out in Southwestern Minnesota. So we're definitely seeing some of those ramifications of all these insecticide applications to soybean with resistance developing in soybean aphid, the primary pest, and then spider mites, a secondary pest. In addition to that, the Minnesota Department of Agriculture has a pretty extensive monitoring program where they're monitoring surface waters and groundwaters around the state for pesticide residues.
Robert Koch:And in recent years, they've been finding concerning levels of insecticides in the surface waters, and that resulted in chlorpyrifos, one of the organophosphate insecticides. Lorisban is an example of it being declared as a surface water contaminant of concern. And they're also concerned right now about the neonicotinoid insecticides showing up in the surface waters. The the concern from my perspective here not only a health issue and an environmental issue, but if these insecticides keep showing up in the waters at concerning levels for the Department of Agriculture, they could increase their regulation or restrictions on use of these insecticides, which could potentially take away some of these management tools for soybean aphid and spider mites. And if we think about our situation right now for managing both of those pests, which are pretty important pests in soybean, we've got kind of a limited number of, insecticide groups available to us.
Robert Koch:So if we lose any
Anthony Hanson:more of those to
Robert Koch:resistance or increased regulation, our soybean growers are gonna be left with their hands tied, so to speak, in managing these pests.
Anthony Hanson:So you mentioned that pyrethroids are having issues with resistance. So what other options are available? You mentioned chlorpyrifos is one of them. I've heard there have been some developments recently on some other insecticides potentially available, but kinda how big is the toolbox now that we're essentially losing pyrethroids?
Robert Koch:Yeah. So previously, we've been managing soybean aphids primarily with the pyrethroids and the organophosphates. And I just talked about the concerns we have right now with both of those insecticide groups. What else do we have for chemicals in the toolbox? We've got the group four insecticides, which include the neonicotinoids.
Robert Koch:Those are commonly used on the seed as seed treatments, but also as foliar insecticides, a lot of times in mixtures where it's maybe a neonicotinoid mixed with a pyrethroid. And then within that group, there's a couple other subgroups within group four, and we've got some newer chemistries like Transform and Civanto, our separate subgroups within that group four. So, fortunately, we've had some new chemical tools come available to us, especially, good to see an entirely new insecticide group or mode of action with this group nine because that's what makes for more effective alternations or rotations of insecticides for insecticide resistance management and if we can switch or alternate between insecticide groups. So if you applied an organophosphate, which is a group one on the first application, then if you need to retreat that field, maybe you could come in with that group nine insecticide, Saphena, rather than rotating between subgroups. So think about that group four where we've got the neonicotinoids.
Robert Koch:Those are pretty closely related to, say, transform. They're distinctly different chemicals, but they're subgroups within the same groups that's not necessarily an ideal situation for insecticide resistance management to be rotating between those subgroups. Certainly better than using the same subgroup over and over, the same active ingredient over and over. But, ideally, you know, we should be trying to alternate or rotate among different groups of insecticides.
Anthony Hanson:So would you say that pyrethroids are essentially going out the door now, or they still have some use for soybean aphid? What should a farmer consider if they have pyrethroid available and they reach treatable levels? Is it pretty much default to the other insecticides now, or could you maybe still try it and hope it works?
Robert Koch:Anthony, in in in my opinion, we've seen and found insecticide resistant pyrethroid resistant soybean aphid populations now for five years, I think, and we've seen them across a very broad geography in Minnesota, pretty much the entire area of Minnesota where soybeans are being grown in one year or another. We've found insecticide resistant soybean aphids. We found them in the neighboring states. Because of that track record and our inability to distinguish right now a resistant population from a susceptible population before making a spray, I do not recommend using a straight erythroid for the first insecticide application for a soybean aphid infestation. I would look to one of the other insecticide group.
Robert Koch:There are some of the mixtures available, like I mentioned before, where you've got a pyrethroid, say, mixed with a neonicotinoid, and our data shows that for the most part, those are still effective. So that could be an option for that first application against soybean aphids, and it should work pretty well. However, if you go in with a mixture like that, you're using two different insecticide groups in one application. So then that limits you even more in what's available for that next application if you need to make one if we're thinking about insecticide resistance management and wanting to come in with a completely different insecticide group.
Anthony Hanson:So do you have any closing advice for insecticide resistance management that maybe you haven't covered yet in terms of preventing another pyrethroid situation where it's sounding like we have a couple insecticides that are now available, but some might be leaving further reasons, whether it's regulation or whatnot, are gonna be back in the same situation we were just a couple years ago with our resistance.
Robert Koch:Yeah. So, I mean, I think we need to do what we can to protect the chemical tools that we have available for managing this pest. We know a severe infestation of soybean aphid can have big impacts on crop yield, and we need chemical tools to manage that pest. Unfortunately, right now, we don't have many other tools available for us other than insecticide for managing soybean aphid. So, you know, what are some of the things we can do to try to delay, postpone, or prevent development of further development of insecticide resistance?
Robert Koch:You know, only treating that crop when we need to. So make sure we're scouting, using the thresholds. If we need to make an application, make sure we're doing it right, using proper nozzles, volumes, pressure, making sure we're not spraying under conditions that are going to promote drift. Part of that insecticide application drifts off that field. Not only is that technically illegal, it leaves a functionally a reduced rate of that insecticide on the field.
Robert Koch:And we know that reduced rates can increase the risk of resistance development. And then after we make an application, which was hopefully, you know, a full rate of a labeled insecticide that that we think is gonna be effective, we wanna make sure maybe after four or five days, get back out there, scalp that field again to make sure that that application of insecticide was effective. If it was not effective, then we wanna be thinking about, you know, what insecticide do we wanna come in with for that follow-up application.
Anthony Hanson:Do you have any, other research in the pipeline you're working on with soybean aphids specifically? Well, that would be of interest for either insecticides or any other control measures that you're considering looking at.
Robert Koch:Sure. So we're continuing with our research related to insecticide resistance in the soybean aphid. We're doing some molecular work to try to figure out how the aphids are surviving insecticides. So looking at the their resistance mechanisms, we've got evidence that they're using detoxification enzymes to break down the insecticides that are getting into them, and we've got some more recent evidence suggesting that there could be some target site mutations in the aphids where the the places on the insect nerves where those pyrethroid insecticides are supposed to attach and do their thing to kill the insects, those places have changed. They've mutated, so those insecticide molecules are not binding as well as they should.
Robert Koch:So so far, we've found those two different, mechanisms of resistance in the soybean aphid. We're also doing work to try to increase the efficiency of scouting for soybean aphid. We know that trudging through soybean fields, counting aphids on plants is not a very pleasurable experience. It's, time consuming. Everybody's got a lot of other stuff they could or should be doing.
Robert Koch:So we're doing work with remote sensing, in particular, if we can use drones to fly over the fields and detect the stress that's caused by the aphids. And I've had a couple students working on that, and, it's promising. We're not at the point yet where we can be making management recommendations based on drones, but we're hopefully moving in that direction. We can detect soybean plants that are being stressed by by soybean aphid, and we can differentiate infestations that are below threshold and above threshold. We've got some follow-up work going on now, seeing what other stressors can affect our ability to identify these soybean aphid infestations from these drone based sensors.
Robert Koch:So that's kind of the the main research we have going on right now with soybean aphid. Well, there's also some work in collaboration with Aaron Lorenz, our soybean breeder, and that work is focused on trying to develop aphid resistant soybean varieties. Like I said before, we we don't have many other management tactics available to us for soybean aphid other than insecticide. However, there there's a lot of research showing that these aphid resistant varieties can be really effective at suppressing aphids and protecting yield, but there's just not very many varieties commercially available. So we were able to get some some funding, and we've got a postdoc working on this project doing crosses, trying to get multiple resistance genes into well adapted high yielding Minnesota soybean varieties.
Robert Koch:Hopefully, we can get more aphid resistant soybean on the landscape at some point, which should help increase the the sustainability of soybean production.
Anthony Hanson:Well, sounds great. I worked on soybean aphid for my PhD, and one of it was insecticide use resistance, and then the other was this host plant resistance. So I'm glad to see that there's research going on with that still, get that pushed along. With that, soybean aphid is obviously the big topic, but what about other insects in soybeans? Sometimes we hear about defoliators.
Anthony Hanson:I know thistle caterpillar came up about a year or two ago. I haven't heard about outbreaks of that again, but has there been any rumblings about some of these other insects that pop up in soybean?
Robert Koch:Yeah. So the last couple of years, caterpillars were a pretty big issue. Thistle caterpillar and, green clover worm in Minnesota soybean this year. You know, we're seeing some of them out there, but I personally haven't seen any big infestations, and I haven't heard of any big infestations. But it's something folks, you know, while you're scouting, you're gonna wanna keep an eye out for defoliators.
Robert Koch:And remember, defoliators are insects that have the chewing mouthparts, and they chew holes into the leaves. So this would include things like these different caterpillar species. It could be Japanese beetle, bean leaf beetles, grasshoppers. These are all defoliating insects. As their populations increase, they're gonna be chewing more and more holes in the soybean leaves, and the way we manage them is based on defoliation.
Robert Koch:So we wanna be scouting the fields, sampling, or inspecting soybean plant from throughout the field, looking at some leaves from the lower canopy, middle canopy, and upper canopy, estimating what percentage of those leaves has been defoliated, and then averaging that percent defoliation across those three leaves on each plant and then across those multiple plants that you looked at in the field. And the economic threshold that we're using, again, that's the point where you should start lining up that insecticide application, is 30 defoliation before flowering, 20% defoliation after flowering. So this time of the year, keep in mind that 20% defoliation, and it's really important not to just focus on those upper leaves of that soybean canopy. Some of these pests like the Japanese beetle like to hang out at the top of the canopy. They'll chew a lot of holes in those leaves, so maybe you have some individual leaves that are over 20% defoliation.
Robert Koch:But if you factor in that entire soybean canopy, that average defoliation is oftentimes much, much lower than that 20%. So you've got the rest of that soybean canopy there that can compensate for that lost leaf area from maybe that upper one or two leaves.
Anthony Hanson:You mentioned Japanese beetle. I know I'm out in West Central Minnesota right now as of this recording, and we haven't really seen Japanese beetle out here yet. Whereas I know the Twin Cities area, they definitely have Japanese beetle. So what areas do you know of that, farmers have actually had concerns about Japanese beetle, whether it's parts of Minnesota yet, or is that kind of further east towards Wisconsin or other states?
Robert Koch:Yeah. Well, and that's a pest that first showed up on the East Coast Of The US, and it's been spreading ever since. It's been in Minnesota for decades now, really, in in the Twin Cities area and some of the larger urban areas where it's, a pest of turf, You know, in people's lawns and golf courses, the larvae feed on the roots of the grasses. And then the adults will come out and feed on all kinds of plants like roses and linden trees and lots of other fruits, vegetables, and trees. When I'm thinking about corn and soybean, this pest can feed on both of those.
Robert Koch:In in recent years, Japanese beetle populations seem to have built up enough, and they're spreading out into the the agricultural areas surrounding these larger urban areas. So we're getting reports of, not yet this year, but in previous years, of fairly heavily defoliation from Japanese beetle in some areas around the Twin Cities in soybean fields and then in other parts of Southeastern Minnesota, you know, down around Rochester and some of the other bigger cities down there. And I think it's, you know, just one of these things where over time, the populations are gonna continue building and spreading, and it's something that more and more of us are gonna have to have on our list of pests to be scouting for. But if you're out in Western Minnesota, it's I think it's very unlikely that you would see one. You know?
Robert Koch:So I think you can focus more on some of some of the other defoliators if you're out in those areas. If if you do happen to see a Japanese beetle and you're out in in Western Minnesota, we'd be interested in hearing about it because we're not really aware of infestations of this pest in corn or soybean out in those areas. You know, it has popped up in some nurseries and in places like that throughout the state. Areas where it's having, you know, noticeable defoliation in soybean has been, you know, Southeastern Minnesota.
Anthony Hanson:Alright. Well, thank you, doctor Cook, for taking the time to, talk to us at the IPM podcast here. Thanks to everyone who's been listening for all these years as well as we go into our third Thank you again.
Robert Koch:Okay. Thank you, Anthony.
