[Podcast] Making Your Strip-Till Operation More Rootworm-Resilient

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On this edition of the Strip-Till Farmer podcast, brought to you by Environmental Tillage Systems, we’re tackling one of the most common yield-robbing pests in the Corn Belt and beyond — corn rootworm.

Ann Marie Journey, independent soil, wetland and stream health evaluator and founder of EntoVentures LLC, shares her wealth of knowledge about the “billion-dollar bug.” 

Journey, who conducted extensive research on corn rootworm, insecticides and herbicide-tolerant and rootworm-resistant corn at the Univ. of Minnesota, shares critical information strip-tillers should know about corn rootworm, and ideas that could make your operation more rootworm-resilient. 

The Strip-Till Farmer podcast is brought to you by Environmental Tillage Systems.

SoilWarrior® systems help you defend your land and improve soil quality. With a choice of durable models, features and accessories, your SoilWarrior helps you minimize erosion while creating precise, nutrient-rich zones.

Let us help you defend your land and improve soil quality. Check out SoilWarrior systems online or request a demo today at www.soilwarrior.com. 

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Full Transcript

Welcome to the Strip-Till Farmer Podcast. Great to have you with us as always. I'm technology editor, Noah Newman. Thanks to our sponsor, Environmental Tillage Systems, for making this series possible. And hey, today we're tackling one of the most common yield-robbing pests in the Corn Belt and beyond, corn rootworm.

Few people know as much about the billion-dollar bug as Ann Marie Journey does. The independent soil, wetland, and stream health evaluator conducted extensive research on corn rootworm at the University of Minnesota, and during this presentation from the Strip-Till Conference, she shares critical information that could make your Strip-Till operation more rootworm resilient. Ann, take it away.

It's really easy to think of rootworm as being a problem. It is a lot more difficult for people to think of rootworm as an animal that's expressing itself in its environment. So we'll take a look first at the source of the problem, which is corn, beautiful corn. Corn that makes such lovely sugar. Massive amounts of sugar has been noted a couple of times today.

Well, the sugar that stays above ground rather is the sugar that goes through the combine. The sugar that goes below ground is the sugar that brings us here today. So this is where corn rootworms hang out. This is the rhizosphere. And the carbon buck does stop here. It's a really much more complicated place than I understood when I was working at NRCS. I thought it was the rhizosphere. That's what all of our documentation said. That's what we were talking about.

Then I got diving a little deeper into the literature, and more recently they've started talking about three parts. So there is the endorhizosphere, which is here, the endodermis and the cortical cells of the root that's outside of the vascular tissues. Then you get the rhizoplane, which is the ectodermis of the root and the root hairs, which are all individual cells. And then you get what we call the rhizosphere, but is now technically called the ectorhizosphere, which is the soil within about an inch of the root. That is a very biologically active area, but that biological activity is a gradient. It's highest closest to the root.

So the area of greatest concern is within about a millimeter or two of the root. Because the roots and the fungi are transferring sugars out into the soil, this is an area of very high biological activity. So I have seen in the literature estimates anywhere from 10% of all the fixed sugars up to about 40% of all the fixed sugars. But there seems to be kind of an agreement on that it's about 20-ish, 21, 25, somewhere in there, of those transfers through the mycorrhizae account for most of them. 20% of the fixed carbon that the corn makes goes through the fungal network.

The rest is out in exudates, which are said in some papers to simply leak from immature areas or actively growing areas of the root that have not yet been colonized by mycorrhiza fungi. The mycorrhiza fungi extend the plant's reach for nutrients and water, which we've been hearing about a lot today enormously, and they get to places that roots can't possibly go. There are two kinds. There are the ectomycorrhizal fungi. These are primarily associated with woody plants. They do not invade cells of the roots. And then there are the endomycorrhizal fungi.

And the endomycorrhizal fungi include the arbuscular mycorrhizal fungi. They penetrate the cell wall in here. They do not penetrate the cell membrane. The cell membrane invaginates around them and surrounds them in what is then called the peri-arbuscular membrane. And what that is is the place of exchange. So the plant is handing over six carbon sugars to the fungi. The fungus is handing back water and then micronutrients and especially in the major nutrient, phosphorus. So also nitrogen and sulfur, but phosphorus, very important nutrient for the plant.

If there's too much fertilizer in the soil, the arbuscular mycorrhizal fungi don't grow that well. They just sit. They don't have a job, and then the plant, it can call them, it can send out more strigolactone, which is a hormone it releases into soil. But there's no need, so there's not a good response. So the rhizosphere is the area of the soil that has the greatest impact on soil structure as well, because this is where all the soil glues happen.

So there are bacterial glues, there are exudates, there is the mucilage, and then there is the action of the fungi too, all forming sand, silt and clay into aggregates and then microaggregates and macroaggregates and so on and so forth. So this is the way that soil becomes soil, something that a plant can hold onto. They also, when you're talking about the mycorrhizal fungi, are physically linking plants together into something that it really is the internet of soil things.

So plants can communicate with one another through these mycorrhizal fungal networks. They can say things like, "Hey, I've been attacked by an herbivore, get your defenses up," or "Whoops, I'm running out of water, close your stomates." Unfortunately, those are very vulnerable to us because they're really concentrated in that top four to eight inches of soil. So meet your competition, the rootworms. They are rhizosphere natives, as I mentioned. So this is going to have a few knock-on consequences as we go.

The beetles to which they belong, the Coleoptera are 350 million years old. As a group, they were around with dinosaurs. They have survived two mass extinctions. These are the most successful insects on the planet. There are 400,000 named species. There might be a million species of beetles. Chrysomelids, the leaf beetles, of which the corn rootworms are four, are among the most successful of beetle species. And this really should give you a warning signal because they eat plants, and that means that they are constantly engaged in chemical warfare with plants, and yet they are that successful. That must mean that they're a pretty adaptive group.

So right here I have the Western, the Southern, and the Northern pictured. I do not have the Mexican Corn Rootworm. Mexican Corn Rootworm looks like the Northern, but it has a black stripe on its femur. So for a reference on an insect, coxa, trochanter, femur, tibia, tarsus. So it would be that long segment of the leg that's out horizontally.

Larvae of Westerns, Northerns and Mexicans are oligophagous. That means they're picky eaters. If you've ever had a toddler around the house that would only eat two things or three things, that toddler was oligophagous. In this case, they are grass eaters. They're very well-adapted to grasses. Corn is a big grass. Both Mexicans and Westerns followed corn north. They are native to Teosinte, which was in Central America.

As corn moved north with cultivation, they followed it. As corn fields moved north, the Western really started to gain its footing in a way that the Mexican did not. So the Western goes straight north and then starts heading east. Northern Corn Rootworms are natives of the prairies. Corn came to them. So imagine being a beetle that gets along on skinny little grass roots and all of a sudden you've got these big honking, sugary, wonderful corn roots. You must have thought you were in absolute bug heaven.

So they had the advantage of waiting for the crop to come. They didn't have to follow it. Between them, the Northern and the Western, are called the billion dollar bug. That was back in 1986. Now it's more like two billion just in North America. Adults have somewhat wider food preferences. They are found with the Westerns on cucurbits. Northerns can be pests of sunflowers. The Southern is different. The Southern is not oligophagous. The Southern is polyphagous. It eats everything in sight.

There are more than 200 known host plants for that one. The larvae are root and stem pests of sorghum, corn, cucurbits, beans, peanuts, sweet potatoes and soybean. They can also attack the meristem of corn and cause stem loss. The adults are listed as pests of cucurbits, beans and can defoliate sweet potato and soybean. So that is a much broader range of crops than the others have.

There's another important difference. The Northern and the Mexican and the Western are all univoltine. That means they have one generation per year. They have a diapause, a mandatory waiting period. That's how they get over winter as an embryo. The Southern is multivoltine. It can have up to three generations per year. It has one generation per year up north, two in the middle, farther south you go, the more you get. What has limited its Northern establishment in the US Corn Belt is overwintering mortality.

So lifelong corn eaters, three larval instars. The thing that's kind of shocking is how quickly they do the damage they do. So first and seconds feed on the seminal roots and the early nodal roots of corn. They start with the root hairs and then they burrow into that cortex and then from there into the pith, which is where the vascular tissues are that bring the sugar down in the phloem and in the xylem, the water and nutrients up.

As I said, the Southern also attacks the meristem and can cause stem loss. Seconds and early thirds of all of them will concentrate around the base of the plant, and then they start attacking those later nodal roots as they initiate. So instead of saying that they ate the whole root back, what you probably should say is that you pulled this plant out of the ground, you looked at it, it has no roots, it didn't get the chance to have those later roots.

They were hit when they were hit. All of this happens within 20 days at a constant 24 degrees centigrade. So it's about room temperature. So you've got about three weeks of active feeding. Then they build a pupal cell in the soil. Pupation takes about eight days. Having raised four sons, I wish they all could have done that when they were adolescents. That would have been so much nicer than having to deal with human pupation in the household at the time.

They are then adults. So if you think about it, you have a one-month period between egg hatch and adults for corn rootworm. Larval feeding physiologically stresses corn because when you start disrupting those vascular tissues, you're taking away the plant's ability to draw up water nutrients. You're also disrupting its ability to make certain hormones. You also then see a mechanical stress to the corn. The plant becomes weaker.

If you have a one-node equivalent of roots off, that would be the upper nodes, not the lower nodes, you're probably looking at about a 15 to 20% yield loss. If it's really bad and the plant is not stable, it may lodge. And I say may lodge because I've seen corn standing that I could literally lift out of the ground with a thumb and forefinger that had not lodged because the wind did not blow yet. Of course, that was one of my research fields where all that lodging was. And it blew down some corn that had some pretty decent roots because I think that particular thunderstorm was 75-mile-an-hour winds when it came through.

So if corn does get blown down, if it lodges, then you'll get the positive phototropism because it's trying to restore its canopy to the sun and then it'll stick out more prop roots as it can. But then you get the secondary loss of leaf tissue because once the leaves are in contact with the ground, the earthworms come after them, especially nightcrawlers, and they pull them right into the soil. So you can lose that as well.

And then of course, when harvesting, it's very difficult to harvest corn that's laying on the ground. So there can just be the mechanical losses of the ears that the combine passes right over. But the other field I had when I had that storm happen was a silage corn field west of the Twin Cities. And the day before the storm, which was mid-late July, the corn was nine feet tall. It was beautiful. The storm came through. It all laid down. We couldn't see the rows anymore, the corn was in so many directions.

The stuff that didn't lay down, a lot of it green-snapped. So we had all these little stumps in the field. And then the farmer's father decided, it was a dairy operation, that he was going to come in and fix this as soon as he could. We had research going on in there. So we were kind of, "Not today, not today." But he came in one day and we had to go out really fast and pull emergency cages because he was coming against the grain with the combine to lift the silage while there was still some of it left. He's the one that taught me about earthworms.

So soil, it should be a lively place. When I say the rhizosphere, I'm talking about the largest, most diverse concentration of living biomass on earth. We think that about 60% of all life lives in soil. That doesn't include plants. So if there's a red check beside this group, there is a natural enemy of corn rootworms in that group. There should be a lot of things in that soil eating corn rootworms. So from the top left going down at the top, that's a Streptomyces, it's a soil bacterium, filamentous soil bacterium. That's the one that produces geosmin. So when you smell that beautiful soil smell, you're smelling their work.

Then we have earthworms, slugs. That is a nematode in the grip of a nematode trapping fungus. That happens a lot of the time in the endosphere of the root. Then from there we have a springtail, a Collembola. Now deserving special mention, they've recently been separated from the rest of the insects, but we've also come lately to understand how important they are as shredders. Collembola and mites, I have a predatory mite over to its right, account for about 95% of the soil arthropods. So anything that has a exoskeleton and jointed appendages.

So that includes the insects, but that's that last group down there. 95% of them are either Collembola or mites. And those two groups we have now discovered have the ability to digest plant cell walls. They have the genes for the enzyme cellulase that allows them to break it down. That tells you how important they are in shredding up and then getting the debris from a growing season incorporated into the soil and ready to be picked up by a plant in the next growing season.

Below them, we have ants called agents of insect extinction in one article that I read. They're about as old as beetles and they've thought to have taken out more insects than humans have, which is an interesting little bit. You look at the ant crawling across your kitchen floor, do you think of it as that scary? They're that scary.

Then I have a seedcorn maggot fly, and then down on the bottom, because I do live in Minnesota, Goldy Gopher. So when we're looking at these groups, the ones that are particularly interesting from the rootworm point of view and from trying to deal with the rootworm point of view, you've got soil bacteria. One soil bacterium, bacillus thuringiensis is responsible for the proteins that are used in transgenic crops for Bt-corn, either against lepidoptera or coleoptera, the moths and the beetles.

There are a lot of screens going on right now in Europe, especially looking for new insecticidal proteins from bacteria because they don't use transgenic corn in Europe. So they're looking for bacteria that they can commercialize against corn rootworm as bacillus thuringiensis was originally out there itself. In healthy soil, you might have five to 20,000 pounds of these organisms per acre in the top six inches. There should be a lot of these natural enemies to corn rootworms there. There could be 10 billion microbes in a single teaspoon of soil representing 11,000 species. But when I say microbes, I'm also including all the fungi and the protozoans that are in that group of potential natural enemies.

Nematodes, we'll get to them in a minute. But one thing about nematodes, earlier we heard about E. coli in soil. So most nematodes are actually free-living bacteria eaters, and given a clear frame of reference to work in, they can chow down 5,000 bacteria a minute. So if you want to have less E. coli in the soil, you should allow for more free-living nematodes in the soil.

Earthworms, they are organic matter vacuums, and so I include them as potential natural enemies because they're not looking for anything. They're just moving through and ingesting soil. If they happen to go through rootworm eggs, the rootworm eggs are going into the earthworm. Now, we generally think of them as a marker of healthy soil, but that really depends on the context. In a hardwood forest in North America, earthworms are the worst thing that has ever happened because they don't belong there.

They are now the most important invertebrate by biomass in North America, which considering that they probably only got here about 400 years ago, is really impressive. You can have as many as 500 per square meter in temperate forests or grasslands. You might get 300 per square meter in a crop field, although frankly, I've never found anywhere near that many.

Because with NRCS, we generally were sampling a cubic foot, and I think the highest count we ever had was about 50 or 60 in a cubic foot. In a forest system, what they do is they take that duff and they turn it in, they eat it. They then have mineralized it all at once. That's supposed to be a slow burn system, and they speed it up and make it a fast burn system. So that's changing the way that plants deal with that space. It's advantaging grasses and invasive plants because they can handle those big bursts of nutrients.

But they then are taking over the forest floor from the slower growing shade-tolerant forbs and from the young trees themselves. It is estimated for reference that at any given time, there are 10 quintillion live insects on this planet. That's 10 with 19 zeros behind it. 90% of them spend part of their life cycle in soil.

Managing rootworm larvae is a saga of unintended consequences. We've been at this for over 100 years. And what happened really was that right after World War II, this is the break in the action, things got bigger about corn. Very suddenly you got bigger fields, bigger yields, bigger demand, bigger coverage because of irrigation. You've got bigger machines, you've got new hybrids, you've got insecticides, you've got synthetic fertilizers. There was all that wonderful industrial activity that came out World War II.

Consequently, the soil that I was talking about as being a very complex and complicated place became much simpler and rootworms loved it. So the first thing that happened is, "Houston, we have a problem." Soil insecticides. So these soil insecticides, the early ones were developed during the Second World War, the chlorinated hydrocarbons. Of those, the ones that saw the widest use against rootworms were the cyclodienes. They had a really long residual. You didn't necessarily have to treat every single year. You could get second and even sometimes third-year control over corn rootworm using these, but because they stayed around so long, resistance developed rather rapidly unfortunately in Nebraska.

Nebraska comes up frequently in this, and it's not that I'm picking on Nebraska, it's that Nebraska was in the right place at the right time and things happened there. So after a while, you started seeing enough non-target consequences. For instance, bald eagles disappearing from the land because their eggshells were thinning that these materials were banned. And so the next one up was the organophosphates and the carbamates, and they are more toxic, they're more expensive. So you started to see a real effort to instead of broadcast, go to in-furrow or banded treatment and to reduce the rates.

When I started working with rootworm in the mid 1980s, that's a lot of what we did in the summer, we did rate trials, we did placement trials with all the different chemistry that was out there to see how they worked. And in most cases, they worked pretty well, except in some experienced soils, the bacteria in the soil went, "Oh, that's food." And so you put the stuff in at planting, and by the time the rootworms hatch, six weeks later, it's gone. So you would get a control failure, but it's not because the chemistry didn't work, it's because it wasn't there anymore.

And then you've also had a narrowing of options for these materials under the Food Quality Protection Act. So then pyrethroids become the big kid on the block, and they work very well too as neurotoxins. They work fast, they get the job done, except now we're starting to see the beginnings of the potential of larval resistance. There is a population where the adults are resistant. They brought the eggs into the lab, checked the larvae, and yes, the larvae were starting to be resistant.

So then we got seed treatments and neonics have misfortune, in a way, of coming out right when Bt-corn came out. So they didn't get that big testing per se, against rootworms that the others had gotten, and they got put on seed, and that is creating its own problems. Because if you put it on at less than 1.25 milligrams kernel, it's really not going to do anything against corn rootworm. It probably won't if you have a lot of corn rootworms anyway at that rate, because those chemicals didn't get the field trials that the previous generations did. Also, we've found that they don't get taken up by the plant.

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So in one corn experiment, they found 0.2% of what was applied to the seed in the shoot, they found less than that in the root, and everything else went out into the soil. This is not where you want it. And because they're used everywhere, you're raising the risk of resistance. Also, the neonics have some pretty potent effects on non-target insects, particularly, which is raising eyebrows because of the extent of their use on seeds and especially among pollinators.

So okay, insecticides, we've had a series of good ideas that got used really hard, and then they stopped working as well. So then we get crop rotation. So that's a quote from the first paper that described the Western Corn Rootworm as a pest. The very simple remedy will be don't grow corn twice in the same year if you have this insect. So very simple remedy.

Okay. So people at first had pretty complicated rotations. First sign of trouble with Northerns comes in 1932. There was a report of damage in Illinois in what was called a Short Rotation in the title of the paper. That short rotation, unusual, was just two years. So then you go a long way, probably getting a few of these reports every once in a while. In Minnesota, in 1965, Huey Chiang, who was at the University of Minnesota, went out and sampled eggs all over the state, and he found that less than 0.3% of those eggs could actually last another year before they hatched.

In 1983, his successor at the university went out and found that up to 38% of Northern Corn Rootworms in the state could last in that second year. So what was the difference in between those two years? Corn soybean rotation. It took over the state between 1965 and 1983. So now you have this particular problem spreading out from that bullseye. In 1987, you have a seedcorn production field go down in Illinois, and they think, "Oh, well, Northerns." It wasn't Northerns.

They collected beetles, they raised larvae that they found in the field. They're all Westerns, and that created a certain amount of angst, and they continued looking. They found that those eggs could not last an extra year in chill, although there is that capability with Western Corn Rootworm. They wondered if the females had been repelled because they were spraying permethrin for corn earworm in the seedcorn production field. Nope, that wasn't it.

What happened a few years later is they started seeing that showing up not in seedcorn production, which was rotated with soybean corn production and very clean and carefully monitored. They started seeing this show up in nearby corn soybean farms, and in that part of Illinois, it was really corn, soybean or bust. In 1995, you had the year that's called a perfect storm. You had 24 counties in Illinois and Indiana where they had up to 50% yield loss. There were people who didn't even have insecticide equipment, application equipment. They had never even thought of rootworm as a problem, and now they had 50% yield loss in that year.

In 1996, people went, "Oh, wait a minute. These things are eating soybeans." So you're seeing the beetles eating soybean leaves and then also laying eggs in soybean fields. So now we have a problem with crop rotation. So what's the next kid on the block? Next kid on the block is transgenic corn. That table speaks for itself.

The practical resistance means that 50% of the population is resistant and that you're getting protection failures. So within eight years of introduction, every single one of the Cry protein events has had at least one population somewhere, and usually more than one, develop practical resistance, and there's cross-resistance. So now the newest kid on the block that just came out is the mRNAi.

So the corn produces a double-stranded mRNA, which matches a short part of the messenger RNA for a housekeeping protein, which I call it Snf7, S-N-F-7. And that housekeeping protein is involved in protein recycling within the cell. And so what happens is the double-stranded RNA comes and it's picked up by the midgut epithelium and taken into the cell, and then it gets chopped up because that is an invader. The cell recognizes that as an invader. So it chops it up, and little pieces are loaded onto something called the RNA-induced silencing complex, RISC.

And once loaded up with these little pieces, those proteins scour the cytoplasm, and every time it matches up to the mRNA for the protein which the cell needs, it destroys it. So eventually, because this protein is involved in the recycling of other proteins, the cell fills up with protein junk and dies. Now both Bt-corn and mRNA corn attack or target the midgut epithelium. So in both cases, you're going after single cells at a time. This isn't like neurotoxins. This takes longer. And in both cases, if you rip up the midgut epithelium by poking holes in it, you can cause the insect to starve or to die of septicemia. Again, that takes a little time.

But the mRNAi can go systemic. So it's not just the midgut. However, the midgut is the line of defense. And this corn, after it was licensed, but before this technology was released, there was a paper published. It's out there in the literature by Monsanto scientists doing their due diligence and saying, "Okay, we found this mechanism. We don't think it's going to be that important." But what they did is they put a tent over part of a cornfield and they had just the DS Snf7 corn planted there. They got the survivors out of that tent. They took them into the lab. They bred them with a non-diapausing lab strain so they could keep things going constantly, and in 10 generations, they had a colony that just ignored double-stranded mRNA entirely.

It just went right through the gut with the rest of the frass and out. And so any mRNA technology that came up against that particular adaptation would be useless. It is a recessive trait. It is single gene in that colony. So they didn't think it was a threat given that you have it pyramided with two other Cry proteins. But it's there. It's lurking. So you can say that this technology also has an expiration date. So why does this keep happening? Well, it keeps happening because we keep throwing these existential crises at corn rootworm and we're dealing with an animal.

Remember, I said it does chemical warfare for a living. It lives in this really complex deadly place, and we keep throwing this, "Okay, we're going to do this. Now you're either going to go away or you're going to adapt." And every single time, it adapts because we keep throwing existential crisis at landscape scale, and we do the same thing years and years and years in a row. All of these technologies are still good as long as you don't use them year after year after year. It has to be used thoughtfully.

So what else is available? Well, this is my PhD. So I looked at entomopathogenic nematodes, try saying that three times fast, in soil. And this is, I got to tell you, one of the worst ways I can think of to go. The nematode gets in, it has a bacterial symbiont. It spits that out into the insect's blood space. The bacterium starts digesting the insect from the inside out, makes this nice swimming pool for the nematodes to swim around in. I told you it was gross. And then they breed and make more nematodes, and they keep that up until they've used up the inside of the insect, and then they form infected juveniles, that stage, and leave and go look for more insects.

So HB is heterorhabditis bacteriophora, SC is Steinernema carpocapsae. I was trying to use it as a biological insecticide. I wasn't figuring that they would be able to establish, because this was a conventional cornfield. It was going to get plowed up at the end of the season. It was going to be bare all winter long. So it was just, "Can I make this happen?" And after a few years of species and rate and application timing trials, I had it narrowed down so I could get better than insecticide adult population control. I could get comparable yield protection, lodging protection, and root injury protection with the nematodes.

Now, what I did not know then was that Western Corn Rootworms, in particular, can sequester and deploy chemicals that the corn root puts out in its own defense. They can do the same thing that monarch larvae do, that caterpillars do. They can make themselves repugnant. So that's one. And that repugnant means that some nematodes trying to attack them will be killed, others will be repelled.

Also, not known at the time. This research is really getting sped up right now because like I said, the Europeans are kind of interested in these things. When the corn is attacked, it releases a chemical called E Beta Caryophyllene into the soil that calls nematodes. Well, it also, if the level is correct, might call rootworms. Rootworms like to know where the good food is. "Oh, somebody's already eating that. That's cool." But we don't want too many someone's eating that because they don't want the competition.

In 2019, there was a study that came out using arbuscular mycorrhizal fungi and pseudomonas with nematodes, and at rates a third of my high rate, they got pretty close to equivalent results. Because they were supporting the plant better, and they were also adding pseudomonas, which increases the signal strength of the help-me chemical. How that exactly happens, I don't know yet.

So there were things going on that could have made this work better, that can now make it work better, but I didn't know them then. Soil texture, however, was considered because the nematodes can't handle high clay soils. The pore size is too small, they can't move through it. And that's an issue because my field was conventional, and it had been in continuous corn fall-tilled and spring-tilled for 45 years at that point.

And the year of that particular slide, it had been cultivated three times in the spring because it couldn't be fall-tilled. It was too wet. So did I have pores of the right size for the nematodes? Probably not. So this is just looking at some of the aspects of disturbance either by tillage or traffic and what can happen. My colleague here at the left is holding a radish that has encountered a plow pan. If you look, you can see that the radish has a clean part and a dirty part. When the radish encountered the plow pan that pushed, allowed the radish to grow up out of the ground, it fell over. It had to turn itself back up.

So about eight inches below the surface, it had that plow pan. Well, I had the same thing in my cornfield. I would occasionally have roots that went sideways. So plow pans are a major issue. Also, just the idea of having intact pores. These damages that happen when disturbance happens is the soil can linger. So those two cores, as you can see, are separated from an event that had happened 14 years prior. On this side, you have a goodly intact network of pores that water and air and nutrients and nematodes, earthworms, everything else can move through.

On the other side, you don't. But here's a little bit of cover crop and less tillage. These two are on both opposite sides of one cornfield. So A had been cover cropped in the fall for a couple of years and didn't get the fall tillage. B had gotten the fall tillage. You'll see how badly in a couple of slides. And the pore structure already in the two was completely different.

So that's one way to increase resilience. So another thing that happened with rootworm, we got stack traits. So this is one of my experimental corn fields. When stack traits were coming, I see a few knowing smiles out there, "Oh, that looks like terrible weed control." It was intentionally. When this started up, we started to ask questions about, "Well, what happens if there's an alternative host plant in there?" And the larvae that have to be on a host within three days of hatching or they die, and if they've been completely starved for 24 hours, you'll see about 50% drop off in adult emergence. What happens if they have more to go to than just the toxic corn?

This is important because only the neonates can be killed by the Cry proteins at the level of expression that has been achieved with the Cry proteins. They are not high dose proteins, so you have to get them right away. So I did a series of studies. I had a host choice study with a corn and a ring of grass or just the corn or just the grass. I put eggs in the ground, tried to find out what happened. That was a pain, didn't work well. I don't recommend it.

Then I had two studies in this field where one, I did a series of arrays. So I was looking at foxtail density, either seven and a half or 15 plants per square meter and seeing how that worked out, you'll see in this slide. And then this one was a grass removal timing study. So you can see there's some that look pretty clean in the background. And then there's one in the middle that looks recently dead, and this one had been sprayed earlier that day. So I either had pre-emergence or no weed control, or I took the grasses out at heights that were corresponding to different stages of corn rootworm larval development to see if I could make a move.

Now, this is the decision tree that we thought would exist for a corn rootworm. So the more plants are there, the more decisions a rootworm had to make. And we were trying to figure out how many of these could lead to resistance. So rootworms find corn by going up a gradient of carbon dioxide, but they have to accept the host by tasting it. So if they sink their mouth parts in and they go, "Okay, this is corn, this is good, I'll stay here. That's fine." If it happened to be transgenic corn, which I saw in the lab one day, they stick their mouth parts in, they start tasting it, they immediately pull their heads out, they start spitting all over the place. They could taste it, they didn't like it.

Now, if they then went to another host plant and survived that exposure, that could be a problem. If they went to a non-host plant and died, starved in the meantime, that's fine. If they just didn't have enough oomph to get to another plant and starved, that's fine. So we were trying to get a feel for this potential interaction, and this is the density study.

The thing that we found, we started seeing it the first time we planted rootworm corn, was that there was funky feeding. There was feeding in strange places. It was on the internode right beneath the soil surface, and we'd get scarring. And you can see here these parallel scars. I used to pull a lot of roots apart in the lab and look at the tracks, and I noticed that rootworms don't feed in each other's tracks. Guess they don't like walking through frass.

So what we found is that if you added grass, you might get a few more adult beetles. You didn't necessarily get a change in the feeding on the roots that we would call economically important. We didn't see that, but we saw this strange feeding. So probably what we were seeing was feeding in areas that weren't expressing as highly, but we didn't have the ability to test them. So that is a supposition. But it was so compelling that we started looking for that on all the roots, and we found that this would be called heavy feeding, stock feeding, was significantly higher on rootworm corn in the case of a light foxtail, which had bigger plants and more roots and may have carried more first instars.

So we were wondering if that made foxtail a lifeboat for corn rootworm neonates. Since then, there's been another study done with large crabgrass and giant foxtail, and they found that adult emergence from Bt-corn little bit higher, just as I have, a little bit higher if there's grass presence, not a significant difference, that you got more males out and that otherwise there didn't seem to be an effect. So if you have an alternate host there, maybe you get some more to survive, but the Bt-corn is still working as desired. It's just the question of resistance is there.

So here's another one, another practice. Consider increasing your plant diversity because you need to bring as many natural enemies as you can into a field, and you need to have as much support for the corn as you can have. So when you look at that field that's at the Menoken Farm in North Dakota. They've got the wall of corn, they've got all of that cover crop in between, and the place was just absolutely buzzing with insects. They were also running chickens in there, who were picking out even more insects and then actively scratching for them. What you have there is a case where there are lots of different roots feeding soil that are creating lots of different rhizospheres.

There are lots of different opportunities for natural enemies of corn rootworm to get established besides just on the corn. Unfortunately, what we don't have yet is a lot of data on how varied plant root systems can affect corn rootworm. But we do know that Western Corn Rootworm and Northern Corn Rootworm can both handle being on the only plant in the area, just not in the numbers that we get. We get the enormous numbers because we plant corn and only corn. Another thing is that because your inter-row soil is covered, you are helping the corn plant, another practice that you can do, to stay cool on a hot day.

So if you look at the temperature graph on the left, you'll see that between 95 and 113, you start losing soil bacteria. You're cooking it. Above that, you're autoclaving the soil, but you're also increasing the stress on the plants with that. The higher the soil temperature, the higher the stress on the corn plants. If the corn plant has already lost part of its root system, then that stress is exponentially greater due to the temperature.

On the right I have a temperature recording from that messy cornfield you just saw. And I was only monitoring during larval development, and this is just the first part of it, but you can see in the pre-emergence, that's the green and then the no-grass control. Now, the pre-emergence is the red and the no-grass control is the green, that having the soil fully exposed, the canopy hadn't closed yet, was allowing the soil temperature to spike on warm days. These weren't really hot days. These were just warm days.

So this is increasing the stress to the plants during a period of maximum feeding. So if you can lower the stress to the plant, the plant has a better chance of withstanding it. So now let's get to the adult question. Adults are their own separate issue. Really they are. Because the idea that you're going to control next year's rootworms by spraying this year's adults, you would have to spray so many times, it's not funny, to get all the females that could be coming into the field.

I know it has worked in the past to a degree, but it has also produced a whole nother set of insecticide resistance. So the first thing is scout, get out there and look. Find out how many beetles there are, where they are. You can tell males from females pretty easily. Check the yellow flowers, like I said, that's the international insect attraction color. The reason for doing this is to have an idea of how many eggs [inaudible 00:49:33] will have in a field. Do you want to treat that field the next year? Do you want to rotate out of it? If you have a ton of beetles, you might want to do both, or either rather.

So the follow-ons for this are that you can have effects of adult feeding choices on the larvae. So if the females are eating things like in the case of Northerns, sunflowers or in the case of Westerns, zucchini flowers, they are sequestering defensive chemicals from those plants and they're putting them in the eggs. The males can transfer those chemicals in the sperm. And so you've got eggs with a little protection from mom and dad that make the larvae a little bit repugnant to their predators.

With the rotation-resistant beetle, the thing that's happened there is that those Westerns actually eat soybean. They are changed in how they approach it. They have lost their ovipositional fidelity to corn. That's the fancy phrase for it. They just lay eggs wherever, but they eat soybean and they can actually digest soybean now. They have a different gut microbiome and they have different levels of expression of digestive enzymes, so they're kind of separating themselves.

But because soybean is not as good, it's junk food, it's not as good a meal for them, they tend to jump around a lot. So they've already left the corn because they've decided that the green silks and the pollen are not to their liking anymore. So now the females are jumping around to these other hosts and they are then spreading their eggs into other areas, and they are then also becoming something close to not there yet, a separate variety of corn rootworm.

Just to point out that, yeah, things are happening over a longer time span. This is the Soil Health Initiative that I did with the Minnesota NRCS. We sampled all over the state. Remember, I said you'd see that cornfield that the radish came out of? Well, that's the dividing line down that sweet cornfield, between the part that was involved, enrolled in the Soil Health Initiative and the part that wasn't.

So we had 25 farms that we sampled, we took 15 assessments in the field. So we looked at bare soil, cover crop coverage, residue coverage, soil temperature at a couple of different depths, infiltration rate, first, second, and third inch. We had horizontal fracture, penetration or resistance, 6, 12 and more than 12 inches, earthworm counts, soil smell, a visual evaluation of soil structure. And that really told the tale.

You could tell the difference between the fields where the cover crops were there and they were making an important difference and the one there wasn't. Now, the reason I bring this up is that we sampled in October and November of 2016. Do y'all remember how long the growing season was that year? It was in excess of 220 days in the Twin Cities.

By the time we went in and sampled in some of those fields, it had been two months since there was a growing plant. And so in all that time that you'd been trying to get along with whatever there was to get along with, if you're the soil biology writ large, including the invertebrates. So we have some evidence out there that having cover crops in the fall, over the winter increases the number of predators that are there in the spring that will take out corn rootworms. Jonathan Lundgren published a paper in 2010.

He found fewer third instars. There was less root injury and no till corn and development of Western Corn Rootworm larvae was slower. So he had created, by having this winter cover, a more hostile environment in the following cornfield for the rootworms because he allowed the predators to stay. They had a reason.

Here's another thing I didn't know when I did my nematode work. I knew they probably wouldn't stick around, but the persistence of those same kinds of nematodes in cornfields limited to weeks if they were under turf where there was a constant supply of sugar and activity and fresh things to eat, they can last for years. So when we're thinking about this system, what we're trying to do is to layer levels of natural control back into it by things like cover crops or disturbing the soil less so that you can have those natural checks and balances.

Great insights from Ann Marie Journey. Thanks so much for tuning in. Thanks to our sponsor Environmental Tillage Systems once again for making this series possible. And if you want to hear more great presentations like that one, you got to go to the Strip-Till Conference, July 31st, August 1st in Iowa City this summer.

We'd love to have you there. Head to striptillconference.com for more information or shoot me an email nnewman@lessermedia.com if you have any questions or if you just want to talk Strip-Till. All right, thanks for tuning in again. Have a great day.