Soybean development and management in hot, dry conditions

Dr. Seth Naeve and Dr. Robert Koch join us to talk about Soybeans. What's happening to them, growth and development-wise, and little bit about, some of the insect pests that can be exacerbated during dry weather conditions.

We want to thank our sponsors the Minnesota Soybean Research and Promotion Council, along with the Minnesota Corn Research and Promotion Council.

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Angie Peltier: 00:00

Well, good morning, everybody. Welcome to our Strategic Farming Field Notes program. These sessions are brought to you by University of Minnesota Extension and generous support from farm families throughout Minnesota through the the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council. We are happy that you have joined us for today's session that's all about, soybeans during this dry weather, what, what's happening to them growth and development wise, and we will also talk a little bit about, some of the insect pests that can be exacerbated during dry weather conditions. My name is Angie Peltier.

Angie Peltier: 00:49

I work for University of Minnesota Minnesota Extension up at the Northwest Research and Outreach Center in Crookston. Today, we welcome doctors, Seth Naeve. He is our soybean agronomist for University of Minnesota Extension. He is a professor in the department of agronomy and plant genetics as well. We also welcome doctor, Bob Koch or Robert Koch.

Angie Peltier: 01:17

He is a soybean entomologist and is the soybean entomology specialist for University of Minnesota Extension. He also is serving as the department chair of the department of, entomology on the Saint Paul campus. And so, in the interest of time, we have a half an hour here today, gentlemen. Seth, if you would like to start us out talking about the dry weather and what that means for soybean growth and development and yield potential.

Seth Naeve: 01:52

Sure. Thank you, very much, Andy Angie. So, yeah, I just wanted to spend a a few minutes. And I know not you know, there's some folks that have been able to capture some real timely rains and some big soakers in a few cases, but there's a whole lot of the state that's still either dry or very dry. And so there's some questions about soybean development at this time.

Seth Naeve: 02:14

And so I think that's all I that all predicates some of the stuff that Bob will talk about related to insect management in just a second. So let's talk a little bit about soybeans. I'll go back to the very beginning and talk about drought stress on soybean and what you see and what's happening in the plant. As the as the soil dries and we have we have low relative humidities in the atmosphere and warm temperatures, the soybean starts using more moisture than it can can utilize. And so the soybean basically closes the stomata.

Seth Naeve: 02:45

When those stomata are closed, that reduces the gas exchange so that the soybean is not losing water to the atmosphere. The problem is it's not taking in any carbon dioxide that it needs to then photosynthesize. It's a C3 plant, so it's much more sensitive to this than corn. And so what's happening is as soon as that happens, that doesn't necessarily hurt the soybean plant, but that is lost capacity to photosynthesize. That's lost yield potential.

Seth Naeve: 03:14

We're not necessarily taking any yield away. But in any moment that those somat are closed, then that soybean plant is not capturing a new energy that it can either use to grow the vegetative part of the plant or seed in this case this year. The next thing that we see is the plant stops growing basically or stops developing early, and now it's basically stopped filling seeds once that happens. The canopy then starts to warm up, and so what the soybean does is it basically flips over its leaves to the white lighter side underneath. And a little trivia here.

Seth Naeve: 03:52

The soybean uses a really interesting little organ at the base of the petiole called the pulvinus. It's a little bulge in that little petiole right at the stem. You'll notice on every soybean that where the petiole joins the stem, there's a little bulge there. This is a really specialized organ that basically helps that those leaves flip over and turn over, and so we get them get the lighter side of the plant. It's interesting from an evolutionary standpoint.

Seth Naeve: 04:24

The the corn plant has bulliform cells that follow the midrib. Those collapse when it gets hot and dry, and that makes the the corn corn leaf roll up. So we get completely analogous symptomology in corn and soybean from completely different organs that, you know, evolved separately at separate times. So clearly, this is an important strategy for plants to to deal with heat and and drought stress. And the drought stress is happening first.

Seth Naeve: 04:54

And because the soybean plant is not using evaporative cooling, that's why the leaves are heating up. So I missed that part. So as as as we have gas exchange with the leaves, we're losing the water. That that water that's leaving the soybean leaf then is is providing some evaporative cooling. So when that stops, that canopy starts to increase in temperatures, and that's why the leaves flip.

Seth Naeve: 05:16

So what's happening this time of year? We've done some recent studies, published a couple really nice papers on timing of stress that basically confirms what farmers have seen for a long time is that, yes, this late August time period is really critical for for yield potential. But our data really shows that the most critical point in time is really around r 5.5 for stress in soybean, which actually happens right about now. So today may be one of the most important days. The good thing is we don't have high temperatures for stress, but we do have do have droughty conditions in a lot of the The States.

Seth Naeve: 06:00

So this critical period lasts between R4 and R7, so there's still quite a bit of time. There's a lot of time to recover. There's a lot of time for that soybean plant to continue to put on to fill seeds. Up until this point, stress was really occurring through dropping seed numbers. So we were having pod abortion and seed abortions within the pods.

Seth Naeve: 06:25

Severe stress is gonna continue to do that, but the biggest problem we're gonna have from now till harvest is reduced seed number. And farmers probably don't really care much about seed size, and it's all about the yield. So the, you know, the multiplying seed number and seed size is what really produces our yields. So whether it's seed number or seed size doesn't really affect us. On a yield standpoint, it's it's the it's the multiplication of those two values.

Seth Naeve: 06:54

So, yes, we're at a critical time, but, yes, there's a lot of time for that plant to still put on really good yields. Certainly, we've we've chopped the very top of these yield potentials, And every day that we have droughty conditions, we continue to reduce that. It's it's basically you know, we we have the data that shows exactly what everybody kind of knows in their gut and from from conventional wisdom about the soybean plant, but there is still plenty of time. If we get good timely rains going forward, we can still fill those existing pods that are on that plant and still have very, very good yields. We're to the point now where we're not probably gonna get any more flowers, pods, and seeds, but we need to maintain what we've got.

Seth Naeve: 07:39

So that's kind of a summary of where we are for yield potentials and where it comes from, and maybe Bob can take over about maybe hopefully relieving some of that stress on the plant that might be affected by the insect side.

Robert Koch: 07:56

Yeah. Thank you, Seth. Angie, did you have any comments before I go?

Angie Peltier: 08:01

What are we seeing, as far as, insects? And then, the other question, and I I hate to say this, but I've already had the discussion where farmer up here, was saying that they were curious. They have a spider mite, infestation at at the treatment threshold, and they're they were already starting to question whether they had enough of a crop to protect. And so, if you could cover that as well and those sorts of considerations, that would be

Robert Koch: 08:35

great. Okay. So, you know, I guess today I wanna talk about spider mites for sure, touch on soybean aphids, and then if we have time, little bit on grasshoppers. And in any year, all of these pests are gonna be out in probably all of our fields, but it just depends how conditions line up, you know, to determine which of these pests could become could become problematic. So in a year like this year where we've had, you know, recently lots of heat and very dry, these drought conditions line up perfectly for two spotted spider mites.

Robert Koch: 09:14

And they're not insects. Right? They've got they've got eight legs. They're related to insects, but entomologists are the ones that usually cover spider mites. Why they do better under these conditions is due to several different factors.

Robert Koch: 09:30

The plants themselves, as they start getting stressed, that can increase the quality of the plants for the spider mites, it becomes a better food source. As surrounding vegetation, like the grassy ditches, things like that start drying down, the mites might start moving into our fields to find some greener, more lush vegetation to feed on. Their development rates continue to increase into these higher temperatures. They've got a much higher, developmental threshold than soybean aphids. Right?

Robert Koch: 10:05

So when we're outside and uncomfortable with the heat, aphids usually aren't doing very good under those conditions, but spider mites, keep doing their thing and doing it really well under those hot conditions. And then the final piece is there are some naturally occurring fungi that cause diseases and mites and can do a really good job suppressing their populations. But when it gets dry, those fungi don't do a very good job. You know? So that's why, you know, a lot of times we hope for, you know, in a spell like this where it's hot and dry, if if conditions change and we start getting some more rainfall, temperatures drop, get higher humidities, you know, then those fungi can start acting against the mite populations.

Robert Koch: 10:52

The spider mites feed by puncturing individual plant cells and killing those cells, so they're, reducing the photosynthetic area. The leaves will start yellowing. Infestations get bad enough. The leaves will start dropping. So it can certainly impact yields for scouting.

Robert Koch: 11:11

We usually wanna start on the edges of the fields. And if we start seeing mites and their injury there, though and that injury, we call it stippling where they're killing off those cells. You get those little yellow patches. If you see that on the edges of the field, you wanna move into the interior of the field to assess how bad it is. And Bruce Potter and Ken Ostlie developed a rating scale a number of years ago that we use as a threshold, and it's really based on how the mites and their injury progress through the canopy of of the field, they they usually start at the bottom of the plants and move upward.

Robert Koch: 11:47

Right? And first, the mites will be moving up, and then that injury will lag a little bit behind that. And the goal is to protect the upper two thirds of the canopy where you, you know, ideally have the photosynthesis going on. So kind of that trigger point is when you're getting mites up, you know, through the middle of the canopy into the upper canopy, and then that injury again is lagging behind, But you've probably got heavy stippling, maybe some leaf yellowing in lower canopy, but you're starting to get more of that stippling into the, into the middle canopy, maybe a little bit into the upper canopy. So if you if you were to look at, you know, one of our fact sheets or crop news articles, that would be that rating of a three on that rating system, and that that's our trigger point or the threshold.

Robert Koch: 12:35

We do have some different treatment options. We got the organophosphates like chlorpyrifos. There's a lot of different generic products there. A lot of people thought of it as Lorzban, but I don't think that's available anymore. But you got products like Pilot, and some are just under the name chlorpyrifos.

Robert Koch: 12:52

Among the pyrethroids, we've got bifenthrin. That's the only pyrethroid we wanna use. Any of the other pyrethroids are likely to make the problem worse by flaring the mite infestation. Within your organophosphates, just taking a step back, there's also dimethylate that can work pretty well. Beyond the the pyrethroids and organophosphates, there's agramac, which is abamectin.

Robert Koch: 13:17

And then my understanding is that agramac, bifenthrin, and organophosphates are all gonna be working against the adults and immatures of the spider mites, but not having much of an effect, if any, on the eggs. However, there's another product called Zeal, which is a toxazole, and that works against the eggs and the immatures, but probably not the adults. So depending on which of these products you choose, you're gonna have kinda the the situation will play out a little bit differently in in controlling the mites. Right? So if you use something that controls the adults and the immatures, you might knock that population down fast, but you're gonna have eggs there that can hatch, and you need to be scouting after treatment.

Robert Koch: 14:00

Because if you have enough of those egg hatch hatching, you could get a rebound in the population. If you use something like zeal that kills the eggs, those adults might be lingering around for a while, but eventually, they're gonna die. You know? So then it it just might be a little longer time period to get that control. Were you gonna say something, Angie?

Angie Peltier: 14:23

Yes. And so that, so it's not like, soybean aphids where if you wait for you wait until you reach that treatment threshold depending upon the point in the growing season, you might be able to have a perfectly timed application and only need a single application. You is it routine to have to make more than one application provided that, weather conditions stayed dry to manage spider mites?

Robert Koch: 14:57

Yeah. Unfortunately, it's not uncommon. I was just talking with one of my colleagues in Iowa, and she was saying that she's been in contact with a farmer, really struggling with mites, and they've had to treat three times. So the most important thing is scouting, right, for determining when to treat for any of these pests, but then after treating, to make sure that the treatment did what you want it to. Right?

Robert Koch: 15:18

We've got these challenges with mites, you know, where they can resurge. And remember, with soybean aphids, you know, we've got resistance out there to some of the insecticides. So we can't just assume that any of our treatments are gonna be a 100% effective.

Angie Peltier: 15:33

Sure. And there's so, with spider mite infestations, you, I imagine, Bob, when you're driving down the highway even at speed, you can often pick those out. Correct?

Robert Koch: 15:48

Yeah. That that that injury can be pretty pretty distinctive. Right? That stippling, and then you'll start getting the, you know, the leaves yellowing and dying. Yeah.

Robert Koch: 15:58

I wouldn't recommend doing all your scouting from the highway, but it it can give you a feel for the the the scale of the problem across the landscape for sure.

Angie Peltier: 16:07

You bet.

Robert Koch: 16:08

In some areas, it it's it's pretty bad. For soybean aphids, their populations have been pretty low this year for the most part. I think that's due to early in the year, we had some pretty heavy rainfalls and quite a bit of it, and I think that took a toll on the populations as they were colonizing soybean. You know? And maybe at that time with that extra moisture, it was conducive to the fungi that that attack the aphids as well.

Robert Koch: 16:34

There's different kinds of fungi that attack the aphids. And then now these hot conditions, as I was mentioning before, the aphids don't do very well. But, you know, we're in a stretch now where we've got some cooler, more comfortable temperatures. So I think we still need to keep an eye on aphid populations in in our fields to you know, because because things can change really fast with either of these pest spider mites or soybean aphids at very high reproductive rates. You know, Angie was just mentioning her scouts encountered a field that was near threshold already for soybean aphid.

Robert Koch: 17:08

I've seen some reports from my colleague in South Dakota saying that they're getting near threshold, so we just need to keep scouting using the threshold to decide when to treat. And then the final insect that I wanna talk about is is grasshoppers, and, you know, they're feeding on the plants differently than some of these other pests. Right? They're chomping holes in the leaves, reducing the photosynthetic area that way, so defoliation. And these hot, dry conditions favor grasshopper populations as well because there are certain diseases that attack the eggs of grasshoppers and maybe some for some of the later stages as well that get suppressed by these dry conditions.

Robert Koch: 17:53

So if you get rid of the diseases that are attacking the pest, the pest population can increase. The vegetation drying it down in the surrounding landscape pushes the grasshoppers and some of these other pests in into our crop fields, and, you know, their development rates increase too with with the higher temperatures. So it's another insect to be scouting for. You'd wanna be thinking about the defoliation thresholds. So estimating how much leaf loss has occurred, what percentage by looking at some plants or leaves from the top, middle, and bottom of the canopy, and then multiple locations in the field to come up with kind of a a canopy wide estimate of the percent defoliation, and then the threshold that we use is is 20% defoliation for reproductive stage soybean.

Angie Peltier: 18:42

So we had a question here from a colleague, Liz Stahl, who who mentioned that during her interactions at FarmFest, just this week that somebody commented and had a question about whether or not it makes sense if if you see a soybean aphid infestation happening just around the edge of the field. Should one consider just spot treatment, or is it more like the the spider mite infestation where where you should really consider treating the entire field?

Robert Koch: 19:18

Yeah. You know, so so for spider mite, some people will talk about doing edge treatments because a lot of times, they they truly start on the edges of the field and progress inward. I think that could make sense if you very thoroughly scout that field and if you can be very confident that you don't have much going on in that interior. But these mites move around pretty readily by by what we call ballooning. They release a little silk.

Robert Koch: 19:40

Right? They live up to their name spider mites. They release a little silk, and the wind can move them around pretty easily. For soybean aphids, I think very early in the season, we see a much more patchy distribution, like colony showing up on the edges. But a lot of times, once you get, you know, into this time of the year, those aphids are developing winged forms, and they're migrating around to different locations and fields or between fields where I personally don't see many situations where spot treatments could work well for soybean aphids.

Robert Koch: 20:16

But, you know, it doesn't mean it can't happen. So I think, you know, if somebody's able to scout thoroughly enough and feel confident that, you know, that infestation truly is just on the edges or an edge, you know, maybe you could get by with it. But you certainly wanna keep an eye on on the remainder of that field as well.

Angie Peltier: 20:37

And another question too. So we were, our scouts were visiting fields, towards the basically south, right around Moorhead and south, and that's where in in the state, in my experience, you start to see bean leaf beetles. And, they weren't I mean, there weren't tons of them, but I know that with bean leaf beetle infestations that people, particularly this time of year, start to worry about pod feeding. And so would you comment on that, Bob?

Robert Koch: 21:14

Yeah. So bean leaf beetle, it's it's a pest that I haven't been seeing that much of lately. In the early two thousands, it was pretty abundant and getting a lot of attention. So it can cause problems in multiple ways where they they'll feed on the leaves causing defoliation early in the season and then later in the season as well. But as you mentioned, they'll also feed on the pods.

Robert Koch: 21:37

Alright? So they're chewing through that pod tissue sometimes into the seeds themselves, opening up those tissues for, you know, secondary fungi and things like that to get in there. Sometimes some of these pod feeders, grasshoppers included, will actually clip the pods from the plants. You'll start losing losing pods. And, you know, if if you see that kind of scenario, definitely wanna take, you know, some more aggressive treatment action.

Robert Koch: 22:04

Yeah.

Angie Peltier: 22:07

And and, also, I'm I'm wondering, you know, we've taken a look at the the drought forecast map. It looks like drought is likely to persist in some of our some of the areas throughout the state. And I know you mentioned before that wet weather can take a toll on grasshopper eggs, and we we often think about, you know, fall, weather and grasshopper survival and threat for the the following growing season. Would you mind commenting on that as well?

Robert Koch: 22:46

Yeah. I guess, you know, I I think a lot of it comes down to, you know, some of the grasshoppers, I think, overwinter in different stages. But if if you've got those wetter conditions when the grasshoppers are in their egg stage, you know, I think you're gonna have more chances of getting some of these diseases acting against those eggs. I'm sorry. Not sure if I answered your question.

Angie Peltier: 23:10

No. That's okay. And then would you also mind commenting, Bob, on, where we are as far as, pyrethroids for soybean aphids? I mean, we, we expect that the soybean aphid populations, depending on where you are, depending on what they've been exposed to, that sort of thing over the years, you're still seeing that pyrethroid resistance being very persistent in the populations. Is that correct?

Robert Koch: 23:43

Yep. Yep. Exactly. So we we've been seeing pyrethroid resistance in soybean a fit over multiple years now across a very broad, broad geography, And it it's to the point where I I don't recommend a pyrethroid as that first application against soybean aphids as just a sole pyrethroid because we we just don't know yet if if the population we don't have a way to know if that population has pyrethroid resistance, and so many of the populations do. It it just doesn't make sense to me.

Robert Koch: 24:16

Not only due to the fact that, you know, we might not control the aphid infestation. Sometimes we can actually make that aphid infestation worse. Right? If we have that resistance there, we spray the pyrethroid. It's not gonna kill the aphids, but pyrethroids are broad spectrum, meaning that they'll kill off a lot of our beneficial insects as well.

Robert Koch: 24:36

Many of those are predators or or parasitic insects that work against the aphid populations. So you leave the aphids. You kill off the good insects, and then those aphids that remained can essentially have a free for all increasing in population. So we had a trial last year at the Rosemont Research and Outreach Center. And in our pyrethroid treated plots, we had 10 times more aphids than in our untreated plots.

Robert Koch: 25:03

And in the year before that, we saw a similar effect, but it was with, one of the formulated mixtures where it's a pyrethroid mixed with a neonicotinoid where we had more aphids in that in plots treated with that insecticide than in the untreated plot. Yeah. Which is scary. Right? Because, you know, my hope was that having the neonicotinoid in there would have provided some some coverage, so to speak, for the the noneffective pyrethroid component.

Robert Koch: 25:33

But, historically, you know, based on some data that my colleagues have, you know, the the neonicotinoids alone as foliar applications haven't been as reliable for soybean aphid control. Right? So the pyrethroid part probably wasn't working, and then the neonic, you know, maybe wasn't doing its thing very well either, but they certainly both would get rid of the natural enemies.

Angie Peltier: 25:59

About the mixtures, so you you mentioned that a lot of the the insecticide mixtures tend to have a pyrethroid with some other class of insecticide included. Can you just make a blanket comment about how to approach using mixtures or single site of action compounds?

Robert Koch: 26:21

Yeah. I think the mixtures are really good if, you know, you've got an infestation of maybe multiple pests. You know, if you'll see some products, you know, it's maybe more of an aphid focused chemistry combined with a broad spectrum chemistry, that might make sense. But if you think about Minnesota soybean fields, it's not that often that we've got multiple pests at threatening levels. It does happen, but it's not that often.

Robert Koch: 26:47

For resistance management, for insects, it's generally preferred to alternate individual active ingredients or modes of action. Right? So not not using tank mixtures or formulated mixtures using, you know, just an organophosphate or just, say, Saphena or something like that. And then if you need to treat, again, coming in with a different group, which is different from what you're gonna hear different than what you'll hear from our, you know, weed scientist colleagues, right, where they're talking a lot more about mixtures. Mixtures can work for resistance management in insects, but there's there's, you know, several different caveats that you gotta be thinking about.

Robert Koch: 27:30

So it's kind of a a finer line that we're walking, I think.

Angie Peltier: 27:35

Right. So, one last question before we wrap things up. When are we off the hook as far as scouting is concerned?

Robert Koch: 27:46

Yep. So, like, for soybean aphids, you know, we generally recommend scouting and using the threshold through R5, but we know from some data that some of my colleagues have that that soybean aphids can cause some yield loss in at least early R6. Unfortunately, we don't have a, strong research based threshold for that growth stage, so hopefully our scouting through those earlier growth stages and use of the threshold can suppress infestation before we get into those tricky late season situations. I guess I'd be curious to hear what Seth has to say, but, you know, I think, you know, a lot of people think of that that R5. Once you get past that, you're kinda in the clear, but, you know, I think Seth has a feeling that, you know, even in the R6, you know, we we can still get some losses as resources are being moved around within the plants.

Seth Naeve: 28:41

Yeah. My I do not have good research data on this, but I would say that the, you know, the aphids are hanging around, and they're doing something. And they're all on on petioles, and and they're moving around. And so they're getting something, I think. And so it's a really critical time for the soybean for yield development.

Seth Naeve: 28:59

So between this R5 and R6. So, I mean, I if if it was my soybean fields, I'd continue to be pretty aggressive through at least R6. And and then the questions are are what happens at R6 through between R6 and R7. I think that's I think that's up more for question in my mind how to deal with those. But I think at least at least through R6, we need to we need to battle these things back pretty hard.

Seth Naeve: 29:24

And, Angie, I think we should address the question you had at the very beginning about when to walk away from these fields. And I would just give a very broad summary on this is that if there's if there's enough yield out there, if the farmer thinks there's enough yield for them to take the combine to the field to harvest, that there's probably going to be gain from managing insects in these in these fields to to to get something beyond that, whatever that minimum is, 10 or 15 bushels that they need to to send the combine out there. So I think that's going to lead people to be a little bit to go ahead and spray some of these fields that are pretty bad. But I think it would have to in my mind, the field would have to be really, really in tough shape for a farmer not to want to treat simply because they think they're not going to get economic benefit. But that's that's one we need to go back to Bob for real quick and see what his thoughts are and my my agronomy kinda with my agronomy hat on there.

Robert Koch: 30:27

Yeah. I I know we're getting to the end here, but I just wanted to comment on some of these late season treatments. Things we gotta think about are preharvest intervals as well, which is gonna be listed on the label. And as we get later into the season, you know, for soybean aphids and some of these other pests, they might just naturally they they get to a point where they're naturally gonna be leaving the fields. Right?

Robert Koch: 30:47

The aphids are gonna develop that winged form and go back to buckthorn, and those chances increase. So, you know, you could get into a situation where you might be spraying a population that might might be leaving very soon on its own. And, you know, the probably that window of time during which the soybean plant is able to recover from or compensate for that injury is being reduced as well. Yeah. So it it just gets really tricky, and I always hope for not not having to deal with these late season situations.

Robert Koch: 31:22

Again, if we can scalp regularly, use thresholds, you know, through R5 , hopefully, we can prevent these these tricky R6 situations.

Angie Peltier: 31:32

That is wonderful information, that we've been getting here this morning. You know, let's let's hope for the best here. You know, if we get if we get some rain, if we get significant amounts of rain, like you said, some of those fungal pathogens might be able to do a a little bit of our management job for us. So, yeah, let's hope that that we get a little bit more rain here, as we finish out the growing season. Well, we would like to thank our guests here today.

Angie Peltier: 32:09

Thank you, Drs, Robert Koch and Seth Naeve for joining us here today. We also would like to thank our sponsors, the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council. And, next week, we will be back again next week. We are still working out the session topic next week and the speakers. And so if anybody has any suggestions, send us a line.

Angie Peltier: 32:38

We'd be glad to to hear from you about what your crop production needs are as far as education is concerned. And so thank you everybody for attending, and we hope that, everybody gets the rain that they need and not a drop more. Thank you. Bye bye.