Soybean gall midge, spider mites, and corn rootworm in southern Minnesota

Show Notes

Episode 15: Soybean gall midge, spider mites, and corn rootworm in southern Minnesota

 

What is UMN Extension Field Crop IPM Podcasts?

Hosts Anthony Hanson, Dave Nicholai, and Bill Hutchison at the University of Minnesota alert growers, ag professionals and educators about emerging pest concerns with Minnesota Field Crops, including corn, soybean, small grains and alfalfa. They offer useful, research-based pest management solutions.

Dr. Anthony Hanson, IPM Educator - Field Crops
Dr. Bill Hutchison, Coordinator of the MN IPM Program
Dave Nicolai, Crops Extension Educator & Coordinator of the Extension Institute for Ag Professionals

Anthony Hanson: 00:05

Welcome everyone to the third episode of twenty twenty for the U of M IPM podcast series for field crops, where we talk about different issues in the state we have with pests and different specialists and experts who have advice on how to either handle current problems or what problems to look out for. I'm Doctor. Anthony Hansen hosting today's episode and today we have Bruce Potter, IPM specialist in Southwestern Minnesota based out of Lamberton at the Regional Outreach Center there.

Bruce Potter: 00:32

So welcome Bruce. Thanks for having me.

Anthony Hanson: 00:34

Do you wanna introduce yourself a little bit? What's your background in terms of what you do down in Lamberton and kind of the different areas that you cover in terms of outreach because you have kind of a diverse background in the things you cover.

Bruce Potter: 00:46

Well, my training has been in entomology. I did spend seven years in the plant pathology department up on campus working on pest surveys and I have been a crop consultant for a good part of my early career. So that's pretty much given me a need to be well rounded on pest management. I think that's why I get involved in so many different aspects of crop production.

Anthony Hanson: 01:09

Now in Southwestern Minnesota, probably the biggest pests you've been running into lately in terms of kind of new and challenging things is soybean gall midge and that's kind of a fairly new pest. So for those who haven't heard about it in previous episodes, you want to give a brief introduction on what soybean gall midge is and why it's been a problem down there?

Bruce Potter: 01:29

Sure. It's fly, a larvae of a fly is damaging stage. These are gall midges, border sesetomidae. They're very tiny. It was first confirmed in Minnesota in 2018, but looking at yield monitor maps on some of these farms that have been infested has probably been there quite a bit longer and they have observed it in South Dakota, Nebraska, Iowa over a longer period of time as well.

Bruce Potter: 01:53

It was first identified and had a as a new species and been named in in the 2019. So it's a very new insect to science. We basically are having to learn everything about the insect from basic biology first and then also how it's interacting with soybean crop and causing damage. You know, for the most part in Minnesota, it's limited to the Southwest part of the state. Farthest north we've found it is Swift County and Big Stone County.

Bruce Potter: 02:25

I have a report this morning from up by South of Hancock, so that may be a new county in Stevens County once we check that out. So that's that's probably where it's at and when it's at. The damage to soybeans is pretty intense. It kills soybean plants and right now we're seeing Wilton's dying soybean plants in some of these infested fields. We don't have the problems in Minnesota at this point.

Bruce Potter: 02:49

Some of these more heavily infested areas, for example, Eastern Nebraska do where where a lot of fields in individual areas of the state are experiencing a lot of damage.

Anthony Hanson: 03:01

So especially in those areas of Nebraska where it's much more severe, what damage are they seeing down there? Is it to the point where it's not harvestable plants even? What kind of yield loss are they seeing?

Bruce Potter: 03:12

Well on the edges of fields it can be totally yield loss and the field in Rock County, Minnesota where we're doing research last couple years. We had up to 60% yield loss on first 60 feet of the field and we still had 10% yield loss a 100 yards out into the field. So that's a pretty significant impact on yield for a field. We can have entire edges of the fields dead. There's always a few surviving plants, but basically it's it's dying plants.

Bruce Potter: 03:41

We're not exactly sure, you know, how those plants that die later contribute to yield, but definitely the ones that succumb early in the season there. If there's large number of plants, you know, there's no yield off of those. What happens is the flies lay eggs and wounds in the plant towards the base of the plant, especially early in the season. The larvae are under that bark of the plant you can see them if you peel back peel that back and see the larvae they start out white they end up orange as they mature. On the outside of the plant you'll see some distortions on the system kind of a callous or warty appearance a lot of times, but between the infested tissue and the tissue above that, there's usually a dark line of tissue.

Bruce Potter: 04:25

It's almost black. It's pretty distinctive. Later in the season plants start wilting and dying you know that'll progress we've got multiple generations at least three larval generations and where we have infestations as the season progresses that insect moves further out into the field and the damage moves further out into the field. Initially those infestations are on the edge of this field close to where the previous year soybean crop was, they're moving out of that, adults move out of the last year's soybeans and into the adjacent soybeans for the current year.

Anthony Hanson: 05:01

So you mentioned early in the season when you can start seeing symptoms. When exactly would that be in the year in terms of either calendar or the growth stage of the soybeans when you first see symptoms? And then what would you see around now if you have a problem and you feel this August 18 that we're recording this podcast, so it's later in the season, but would that be the point where you're pretty much seeing plant death or are there still larvae out there you could find pretty easily?

Bruce Potter: 05:27

Usually you can start to see symptoms on soybeans that are the V3 stage a little bit later, depends on you know how big those beans are when the flies emerge and lay eggs. And about three weeks, maybe a little bit more after we start to see adult emergence, you can start to see some of the heavily infested plants wilting and dying. Those larvae pupate in the soil, we'll get another generation out, and those will usually infest new plants. Sometimes they'll infest plants that are already infested, but that infestation moves further and further into the field and you get a higher percentage of plants infested. So that wilting and dying will continue through the season.

Bruce Potter: 06:10

And right now we're just finishing the emergence of adults. We've got a generation of adults that have emerged in in Southwest Minnesota. And so this last batch of larvae and we're expecting is gonna overwinter. The overwinter in

Anthony Hanson: 06:25

the soil is larvae and the adults emerge in spring and June. It sounds like you also have some complications with a look alike species found in Minnesota that hadn't really been seen much before.

Bruce Potter: 06:37

Well, has been seen before and for many years it's associated with white molds. So in years where we've had severe white mold infestations in soybeans, we've picked up these larvae associated with fungus, they're fungal feeders. They're very similar, they're in the same family of soybean gall midge. The white mold gall midge does not feed on the soybean plant itself, it's not injuring soybeans, it's feeding on on the white mold fungus itself. That's been known before and identified in North America before, but it's after the soybean gall midge, this pest species started to cause problems that, know, people started noticing the secondary insect a little bit more.

Anthony Hanson: 07:19

So what kind of control measures either are being developed for soybean gall midge or are we hoping that maybe have some somewhat established efficacy already? What's kind of the toolbox looking like for IPM for this pest right now?

Bruce Potter: 07:33

Actually, this insect, we're gonna have to use IPM because the management tools we've looked at, none of them individually at this point seem to provide adequate control. There's some evidence that we can get some relief from a well timed foliar insecticide. Maybe we get by just doing some edge treatments as I mentioned those insects come out of last year soybeans and progress in from the edge and usually the borders are most heavily infested. Problem with that is these multiple generations come up and even though you get good control of first generation, a reasonably good control of the first generation, those subsequent generations still continue to increase and infest more plants. We've got some research out this year.

Bruce Potter: 08:17

There's some unlabeled compound that seems like it provides a little bit more control, but even there in our small plot studies where they can be reinfested, nothing's lasting season long, so insecticides by themselves aren't going to be the answer. We thought maybe planting date was around way around this infestation, but this year we've had really long adult emergence periods and that doesn't seem to be holding as true as it was in the previous two years. So even late plant soybeans have been infested. One of the things we're looking at may provide some benefit or hopefully good control of this insect is if we can find host plant resistance and we're not doing that in Minnesota right now but several other states are screening a large number of soybean lines looking for varieties that that are either more tolerant or resistant to this insect. So that's that's something that hopefully down the road is going to be if the insect keeps persisting as a pest doesn't disappear on its own, host plant resistance is a tool that I think we're going to be using a lot of.

Bruce Potter: 09:24

And finally, we may actually be having some biological control going on already. We've detected DNA of a couple parasitoid wasp species in samples of larvae from in 2019 and we're gonna be we're looking again this year see if we can actually identify which species those are. So there's a lot of things, but we're gonna have to use all of them together in order to make this work as far as like sound management practice.

Anthony Hanson: 09:51

You mentioned needing season long control or at least management of soybean gall midge. So is this a similar case to soybean aphid in terms of if someone wants to use a seed treatment that that seed treatment generally isn't going to be at high enough concentrations when soybean gall midge shows up?

Bruce Potter: 10:09

I don't even know if they're very efficacious because eggs are laid as early as as V2 soybean plant stem as the plant grows, the stem expands and we get these cracks. That's one of the places that we believe these adults are laying eggs. It's a way into the plant. Any kind of injury will probably suffice as well. But we're seeing we're seeing infestations.

Bruce Potter: 10:34

It's fairly early. It's about this time the seed treatments run out where we've tested it. And in fact, this fields I'm looking at, they've always had a neonicotinoid seed treatment on and it doesn't seem to have slowed down the the infestations at all. So I don't think at least the current treatments and the rates we're using, I don't think that's that's gonna be all that helpful. We do in some of the research in Nebraska, they've seen some short term control, but again as the season progresses, it's not enough.

Anthony Hanson: 11:06

So for soybean gall midge, kind of as a final note, do you think this is going to be a major problem like the next soybean aphid in terms of either severity of a new soybean pest or possibly spread further across the state? What's your current outlook on this pest for years to come?

Bruce Potter: 11:22

Well, that's hard to say, Anthony. At this point, we don't have the the widespread infestations that some other states do. We're not sure what the reason for that is. It's something we're doing management wise differently with soybeans in this part of the world, or if it's an environmental factor. One of the things we are trying to understand is just where this insect can be found, and one of the things we're doing this year is we're conducting a survey of many counties as we can get through seeing if we can detect the low levels of larval infestation in some of these fields.

Bruce Potter: 11:59

And we've got we're getting some reports from consultants and farmers of of fields with infestation. So I guess right now, in order to answer that question, we need to know what the status is now and determine if if it's getting better or worse. So really if anybody does have does observe this insect in the field we just finished a little scouting video that people can find on the extension website but if they do observe insect in their fields, which are I sure appreciate hearing about it, and we can kind of we're mapping the distribution to guess get a handle on just where it is right now. If it does continue to increase, it's a it's a serious soybean pest. It's not gonna cause field wide damage like soybean aphid does, but it's also gonna be a lot more difficult to manage just a simple insect skeleton insecticide application.

Bruce Potter: 12:51

At this point, it doesn't look like that's gonna be enough.

Anthony Hanson: 12:54

I've been hearing reports that spider mites been a bit more of a problem this year. Do you wanna give a background on what these pests are and for people that maybe haven't actually seen them themselves yet, but maybe they've been in their fields and they've noticed the issues but haven't been able to identify that problem yet?

Bruce Potter: 13:13

They're not insects. They're they're actually related to spiders and they over winter, they're real common insects. The one we're dealing with in soybeans is two spotted spider mites. In corn, there's another species as well, Banks grass mite. But the issue we're having this year is two spotted spider mites and primarily in soybeans.

Bruce Potter: 13:34

Their damage actually functions as defoliation. They have when they feed on those plants, they burst the cells and those cells die. Symptoms of the infestation, at least heavy infestations, are edges on the edge of the field, overwinter in in perennial vegetation grass, alfalfa when you cut those those plants or the crops, they'll move into the field little ways. But that damage initially looks like from the road looks like soybean plants are turning kind of a bronze ish yellow color. And if you look at the plants themselves, you'll see little yellow dots on the upper part of the leaf.

Bruce Potter: 14:12

If you turn the leaf over, it'll be dusty looking. You might be able to see the webs. If you tap those leaves onto a sheet of paper, you'll often see the mites moving, a hand lens you can see the mites and and their eggs on the underside of the leaf. Problem is that even though you don't see those heavy infestation signs further out in the field, there's a good chance the mites are out there already. You know, you need to scout the whole field to make a decision on how bad that infestation is.

Anthony Hanson: 14:42

So what kind of thresholds are there for two spotted spider mite and soybean in terms of you're out there scouting, what's your next step in terms of treatment decisions or other options?

Bruce Potter: 14:53

Well, don't have any hard and fast guidelines like we do for soybean aphid, corn borer, some other defoliating insects for that matter. So the first part of that equation is do you have mites in the field? The second part is how far out in the field they are. So you need to get off the field edge and make that decision. Usually the damage starts from the bottom of the plant and progresses upward.

Bruce Potter: 15:18

Kind of a short version of the of the guidelines are if you're you're gonna see some damage on the field edge maybe some pockets out in the field where you can see those visual symptoms pretty pretty easily. But if you've got starting to get yield loss and you've got mites in the upper canopy out into the field, probably is gonna benefit from a treatment. Some guys are getting by with border treatments if they haven't moved too far into the edge. That's a little risky because the mites are able to move they spend little spin little prints of webbing silk and they're able to move on the wind that way so they can get quite a ways out in the field in a short period of time. So if you do do a border treatment again you have to you have to watch these things pretty close make sure that you don't have to come back and treat the rest of the field again.

Bruce Potter: 16:07

One of the things that we have observed with spider mites you know we've got soybean aphids that are resistant to pyrethroid insecticide. The spider mites we've got in 2012, we found populations at Lamberton in Southwest Minnesota and I believe also up in Northwest Minnesota, some populations that were resistant to chlorpyrifos, and we're seeing that again and particularly in West Central Minnesota, some of the chlorpyrifos applications did not work very well. And so we do we have selected for some resistance to to that insecticide in the spider mite population in Minnesota.

Anthony Hanson: 16:44

I've heard that some insecticides can also flare up populations of spider mites. It sounds like this is interacting with soybean aphid control as well.

Bruce Potter: 16:53

Sure. Parethroids especially have a tendency to flare spider mite populations. The exception of that would be bifenthrin lambdacylohathrin is labeled as suppression and but basically what doesn't make spider mites worse as much as some of the other Pirethroids. So you're doing two things, one is you're removing predators. Spider mites are usually controlled by predators and particularly a fungus disease that helps keep those populations down when we have good soil conditions.

Bruce Potter: 17:26

In drought conditions, there's two things that happen. The fun fungus doesn't work well, it isn't able to reproduce well, and also drought stress plants tend to favor the mites. They're they're more nutritious, I guess, is the best way to to describe it. Getting back to the pyrethroids, they remove predators, and the other part of it is they can actually some actually stimulate reproduction for within the spider mite. If you do have spider mites in the field and you're trying to make an eighth decision you have to you have to be a little clever on which insecticides to use.

Bruce Potter: 18:01

We have seen some interactions with neonicotinoids and spider mite populations both seed treatments and foliar treatments, you know, if you've got spider mites in the field, both of those have have some tendency to increase mite populations as well. So you're down to bifenthrin application, chlorpyrifos which may not work in all fields anymore, dimethylate that's not quite as efficacious as chlorpyrifos but but it's it's labeled And then you've got several compounds that are specifically miticides. They tend to be a little bit more expensive and and a little harder to obtain in this part of the world. But kinda like soybean aphids, options for management are starting to be reduced.

Anthony Hanson: 18:47

So as we're getting into the August, how much longer would we be seeing symptoms from spider mites or how much longer would they be causing damage that's worth treating?

Bruce Potter: 18:58

Well, you've got a field that's infested now and it's in its populations are pretty widespread in the field, they're gonna be there until either a fungal disease takes them out or the leaves fall off the plant. Okay? And you know the mite damage like I mentioned is form of defoliation and late into the season. Right now we've got r five a lot right now we've got r five and r six soybeans for the most part. And there's not much yield in those seeds yet.

Bruce Potter: 19:30

That's mainly water. And if you look at hail charts, this is still a critical time for defoliation. So we're gonna have to watch these until you start to see yellow pods, you start to see leaves turning, lower leaves turning. So it's gonna run a little later in the season. They're not gonna go out of the soybeans and move back to an alternate host like soybean aphids do with buckthorn.

Bruce Potter: 19:54

They're gonna sit there and infest these plants until maturity or until a fungal disease takes them out at this or you take them out with an insecticide.

Anthony Hanson: 20:04

I know you also do work with corn as well, especially I've heard you have some plots by Lamberton with corn rootworm work you're doing. So you want to give a little background on what you've been doing there and what you've also been seeing especially in Southwestern Minnesota for corn issues.

Bruce Potter: 20:19

Yeah, historically had rootworm research at Southwest Research and Outreach Center, we've kind of continued that. We've got populations here of Western corn rootworm and continuous corn. They've documented resistant to at least a couple of the VT proteins and so it's a good opportunity to look at how insecticides function and traits function in this particular environment. Corn rootworm populations are very high this year. They haven't been this high since probably 2012.

Bruce Potter: 20:51

We started to see some issues in 2019 where, you know, we thought previous year the populations looked low, then all of a sudden 2019 in our continuous corn plots, we had quite a bit of Western corn rootworm pressure and guys are seeing that out in the field too with Northern and Western corn rootworm beetle populations. Populations had been very low, but this year we're seeing a lot of fields with significant beetle populations and damage to root systems.

Anthony Hanson: 21:22

It's kind of a two part question here is one is, do you have any inkling of where these pockets resistance are, especially in more Western Minnesota? Because I know some growers, they may be hearing about it, but they're wondering, is this near where my farm is? And the follow-up to that is if you suspect you have resistant populations, what are your next steps especially for next year when you're selecting varieties again?

Bruce Potter: 21:47

Okay, well basically across Southern Minnesota, you might as well assume that a pretty good portion of the beetles, western corn rootworm beetles in the field are resistant to the yield guard protein and probably cross resistant to the Durocaine protein as well. Resistance of those two proteins is fairly widespread in rootworm populations in Minnesota. That was first picked up in 2009 in Southwest Minnesota. I think that same year or the following year, they started to see some in Southeast Minnesota. A couple years ago in North Dakota, they found northern corn rootworm beetle populations that were resistant to the yield guard protein and the Hercules rootworm protein.

Bruce Potter: 22:31

And so that's another another species. We haven't confirmed that in Minnesota, but it's suspected that we've got northern corn rootworm populations resistant to some of the BTs as well. We've also got some populations that are resistant to Herculex in the western corn rootworm population. And there's also populations that are getting around the some of the pyramids. So that's happening with various pyramids, whether it's the Pioneer Pyramid or the Bayer Pyramids.

Bruce Potter: 23:00

They they're both working with the Hercules protein carrying the heavy load, and that seems to be failing, especially in we're seeing two things happen. One is really high beetle populations and it's overwhelming the traits and the other part of that is is actual resistance. So as far as do you have a problem in your field, you can assume that if you have high beetle populations and you're in the southern part of the state there's some potential for some resistance there. We're seeing more problems in conventional corn of course and we're seeing more problems and where where we've had corn growing continuously for for several years four or five year type corn rotation. So that's kind of the status of the outbreak right now.

Bruce Potter: 23:45

You know fortunately we're not seeing as many fields with that are rotated with any issues and we shouldn't be seeing that with western corn rootworm because that hasn't adapted to the corn soybean rotation in Minnesota. Northerns have that extended diapause trait. If you're wondering what to do for management, if you've got an issue where you've seeing a lot of beetles if you're catching them on the sticky traps or looking at them finding a lot of beetles on plants, the best thing you can do is rotate that field out of corn. Rootworms cannot survive on soybeans, so if you rotate it out of corn control volunteers, you're gonna starve the western corn rootworm population out completely and you're gonna starve out the non diapause and part of the northern corn rootworm population. In an area where you've got issues with some of the pyramids failing and you haven't rotated, you're going to probably have to use a pyramid plus a real a good rootworm insecticide.

Bruce Potter: 24:45

Some of the insecticides that do okay under low rootworm pressure are probably not going to be enough. So the simplest thing to do is rotate out of corn for a year. If you've got a bad extend diapause problem, a lot of northern corn rootworm beetles out there. You know, if you wanted to handle it all with rotation, you'd have to

Anthony Hanson: 25:04

do more than one year out of corn. So you mentioned insecticides. Are these soil insecticides for the larvae, or are you able to target the adults and have any decent management with that root?

Bruce Potter: 25:17

Well, if you're trying to manage rootworms and avoid lodging and yield loss, we're gonna be targeting the larvae. You know, by the time the adults are out, any of that damage has already happened to your corn crop. So the adult management is geared towards preventing egg laying. Some of these real heavy infestations if their beetles are out early enough you'll have some stilt clipping so you may try to prevent management just for minimizing egg laying it's a pretty dicey proposition because you have to get the timing right so you have to get it before a lot of the eggs were laid so you're talking seven to ten days after you see the beetle start to emerge and then none of these insecticides are gonna give you enough residual at least the perithroids and and organophosphate type comp aren't going to give you the residual that you're going to need to control that entire emergence period. So you're going to have to apply insecticide more than once.

Anthony Hanson: 26:20

How about European corn borer? Have you heard about any pockets of that popping up around the state or has it been pretty quiet this year?

Bruce Potter: 26:28

Sure. Corn borers have been pretty quiet. However, we're seeing infestations in the areas where we have in the past. These are areas that have had a history of large acreages or large percentage of the fields planted to corn without a corn bore BT trait. So those those areas are fairly limited in Minnesota.

Bruce Potter: 26:52

One of those is Northwest Minnesota, West Central Minnesota, there's some pockets historically had some corn bore pressure over the last few years and some in Southeast Minnesota, East Central Minnesota. It's basically areas where there's been, you know, less BT planted. Populations are maybe a little more spotty this year than last year from what I've had conversations with people, but there are still fields that that have been at threshold populations for for for corn borer.

Anthony Hanson: 27:20

And some of those fields, are they getting to a point where we gotta start thinking about other management tactics for corn borer now? You've relied a lot on area wide suppression for corn borer since BT traits came out. Have you heard about people kind of returning to the other IPM tactics that are available for corn borer if they need to?

Bruce Potter: 27:41

Well, there's people that are managing corn borers fairly effectively right now with scouting insecticide applications. I think that's going to continue just because of the reduced cost of some of these non BT hybrids compared to BT pyramids. I think if we start to see overall populations go up and we start to see, know, kind of like corn rootworm, we start to would start to see really high corn borer populations all of a sudden, then I think there's gonna be a move back to more BT just because if you've got large infestations you know it's hard to get good enough economic control just with insecticides. That's why BT was adapted so so widely works very well.

Anthony Hanson: 28:25

Alright, do you have any other pest issues in Southwestern Minnesota especially that either you have on your radar that you haven't seen anything happening yet or up and coming potential pests that you might want growers to keep an eye out for?

Bruce Potter: 28:41

Well, think we're getting towards the end of the season. So we're, you know, issues like with potato leafhopper, those are gonna cease pretty soon. You know, they're migratory just like a lot of other insects. We're seeing some pretty large populations of green clover worm adults right now or dark brown or black moths kind of aggravating people getting into barns and that sort of thing. So those moth populations are fairly high.

Bruce Potter: 29:06

There's still some time for those to do a little bit of defoliation in soybeans. We'll have to see how that develops. As we get towards the end of the season here, I think it's gonna be mostly guys looking at what problems they have in the field, whether that be weed escapes, if you've got particular disease in a field that you're gonna want to manage differently or nematodes. So right now other than a you know kind of the wrap up on some of the insects issues already should be starting to plan for next year whether that's picking out what hybrid or variety you want to plant based on what pests you have in the field or if you need to do some differences in how you're doing your scouting and managing pests in your field. Everybody out there is getting kind of worn out scouting right now, so it's gonna be over fairly soon and and we'll get to do it all over again next year.

Anthony Hanson: 29:56

Alright. Well, you Bruce for taking time to come and talk to us, and thanks to everyone for listening to the University of Minnesota IPM podcast for field crops.