Insect Pests in Corn with Dr. Katelyn Kesheimer

Hi everyone, my name is Caitlin Kesheimer and I'm an assistant professor and extension specialist with Auburn University and the Alabama Cooperative Extension System. While the clock is winding down on the 2020 growing season and the harvested corn is being stored for the winter, we're facing some major potential changes in corn pest management in the near future. So today I'd like to cover some of our most consistent pests we see in corn, their associated IPM strategies, and also what these potential changes may look like for the future in corn pest management. When you're growing corn, there's no shortage of insect pests that can cause yield loss. There are pests from the time the seed goes into the ground all the way through post-harvest storage. A key to protecting your crop from any of these two dozen pests that we see in corn is having a good integrated pest management or IPM strategy even before the plants go in the ground. One of the most important decisions that you make as part of your IPM plan is what corn variety are you going to plant. It's important to use genetics that are specifically adapted to your area in order to maximize yield potential. Another good thing to consider is if and what type of insect control is needed. So do you need underground insect control for corn rootworm or corn borer control like corn earworm? In addition to insect control, we need to consider genetics that have good disease and yield packages. So it's really tailoring the the seed package, the genetics, to your particular farm situation. And finally, I'll cover some more of these in detail later, but it's always important to follow the refuge requirements if you're planting Bt corn. This is a crucial part of insect pest management and also insect resistance management. And while we do have insects and diseases each year, there are several steps you can take to reduce the risk of yield loss from these pests and really maximize your yield potential. Reducing risk is going to give your crop the best possible chance to, one, survive, and two, really thrive and maximize that yield potential of your corn. So things like crop rotation, timely planting and timely harvest, so getting those seeds into the ground before the season gets away from you, and then harvesting when the corn is done, using seed treatments as an insurance against early season pests, weed management, there's a lot of things you can do. So for today, what I'm going to cover is two of our most consistent pest groups in corn, stink bugs and caterpillars. We see these pests every year, so I'm going to cover identification, management strategies, and also some potential big changes that might be coming forward for one of the major ways that we control caterpillars using Bt proteins. When we're talking about stink bugs and corn, it's really a complex of four different species. They're all capable of reducing yield, but some are more difficult control than others, so it's important to know what you're dealing with. Many times you'll have a few, if not all four of these species in corn, but the one that's really hardest to kill is the brown stink bug. We also have a fairly new pest, the brown marmorated stink bug, which looks a little bit different, and I'll show some pictures here in a minute. There's also both the green and the southern green stink bugs. We typically see more southern greens than greens down here, but from a management perspective, it's not crucial to separate these two species like it is for the brown and areas around here. So think woodlands, leaf litters, and for some, like the brown marmorated stink bug, they're going to spend the winter in structures like barns and people's homes, and now is the time they're slowly moving into those structures and overwintering places. It's important to keep in mind that insects are cold-blooded, and so their entire life cycle is dictated by the weather and primarily temperature, and so if we're going to have a very cold and harsh winter, it may lead to higher stink bug mortality and also cause them to come out of overwintering slower in the springtime. On the flip side, if it's a warm winter like we've had the last couple years, it's going to both increase their survival rate over the winter, but also allow them to come out of overwintering earlier in the year to begin feeding and reproducing. So here are the major players as pests of corn from left to right. We have the brown stink bug, the brown marmorated stink bug, and the southern green stink bug. I didn't include a picture of the green. It looks very similar to the southern green, but from a management perspective, it's not crucial to understand or know the difference between green and southern green when you're counting stink bugs for thresholds and treating and determining an insecticide to spray. However, it is important to distinguish between the brown and brown marmorated stink bugs. So if you look at these pictures here, I've highlighted some of the major characters you can use to distinguish brown from brown marmorated stink bug. Both are brown, as their names imply, with a beige or tan and brown border around their abdomen. The pattern is slightly different in the brown marmorated stink bugs. If you look at this pattern here, you'll notice those beige or tan markings are very well-defined sharp triangles along the edge of the stink bug's abdomen. In the brown stink bug, they have a very similar pattern, but it's sloppy. You don't have that definition in those triangle shapes. The tan parts don't make those triangles as nicely as they do in the brown marmorated stink bug. Another key difference is in the antennae. The brown marmorated has these distinctive white bands on their antennae, and they're very pronounced. You can see them. They do not have these in the brown stink bug. And so one or both of these characters can be used to distinguish which stink bug you're dealing with, because we'll see more issues and it's harder to kill the brown stink bug. The key to stink bug control is scouting early and often, so you know when and if you need to make an insecticide application. And stink bugs are sneaky. If they see or hear you coming towards the plant, they'll likely tumble down, fall into the collar of the plant, or fall to the ground. So what I like to do is go down a row and look at the plants one or two rows over from where I'm standing, so hopefully you're not disturbing these stink bugs, and it's easier to get an accurate representation of what's actually in the field. And because stink bugs are known for forming these clumped infestations, it's important to check multiple areas of the field. You may look at one area and not see any and think you're fine, whereas they are clumped, infesting another edge of the area that you didn't look, or an edge of the field that you didn't look at. And they'll also aggregate along the field edges, so make sure you sample plants in both at the edge and go at least 15 meters in to see what the interior of the field looks like. You can look for stink bugs, you can look for eggs like shown here, or feeding damage. We'll see some of this feeding damage early in the season, and it's distinguished from other pests because we have that discoloration around the feeding site that lets us know that that's from stink bugs. And the reason I mention scouting early and often is that stink bugs are capable of causing damage and severely reducing yield at multiple growth stages throughout corn's life. If there's substantial feeding during the early vegetative stages, the plant may become stunted and you'll see that buggy whipping in the field. The affected plant may also put out additional tillers, which are really just reducing your overall yield in the entire field. And as we move into pre-tassel, this is where we see that characteristic crooked or banana-shaped corn ear shown here on the slide. The stem will crook away, the the ear will crook away from the stem, and you may even have aborted kernels. And this damage really continues into the early reproductive stages. We can have damaged or smaller kernels, and we also run into the risk of disease. You can see on this corn cob down below, all those darkened kernels are where stink bugs fed, and now that's an entry wound for a pathogen to get in and cause disease and damage your corn even further. So scouting really needs to start early around V1, and I would not stop looking for stink bugs until after R2 just to be safe. The thresholds are listed here. You can see for the younger corn, we're looking at one stink bug per 10 plants, and these are whole plant counts. And then it drops to one stink bug per four plants as we move into the reproductive stages. And I would recommend looking at about 100 plants to get your average both on the edge and interior of the field, and then several places to get an accurate assessment and see if you need to trigger an insecticide application. The most important factors that will help you achieve the best control are one, coverage, and two, timing. So I mentioned that stink bugs are sneaky and may hide. So you're thinking about where you're finding them on the plants. They're going to be down below, hidden in that collar and towards the base of the plant, and we're going to be spraying contact insecticides to kill these bugs. And so the chemical has to go where the stink bugs are. It's important to penetrate the canopy with both the volume and pressure if you want to have good control over these stink bugs. And timing is also very important because we want to hit them right before they go into tassel. So before that ear really comes out is to protect them from getting that crooked neck or that crooked ear that comes out from the plant. In terms of what we are spraying, pyrethroids are the tool we have right now against stink bugs. And there are several to choose from. I've listed a couple here and they'll all provide control. But this is where knowing what species of stink bugs you have in your field comes into play. Brown stink bugs are harder to kill with insecticides. Female brown stink bugs are especially hard to kill because they're bigger than the males. So if after scouting you find that the majority of your stink bugs are brown, not brown marmorated, but the brown stink bug, then I would go with the highest rate of bifenthrin. Bifenthrin is the most effective because it can be applied at a rate that one allows for a higher active and it's more toxic to the brown stink bugs. So now we're going to shift gears and move to the other consistent suite of pests in corn. And these are caterpillars. So I'm going to spend the rest of time today talking about the caterpillar pests we have in corn, some of the technologies like BT that we use to control them, and some changes that may be coming in the not too distant future regarding these BT technologies. So when I say caterpillar tests, I'm talking about several different kinds including armyworms, corn earworm, southwestern corn borer. Here in Alabama we're primarily concerned with corn earworm and fall armyworm, and we do see increased damage with later planted corn. And so that's why I mentioned earlier about reducing this risk of insect damage and feeding by having timely planted crops. So of the two most important ones, corn earworm and fall armyworm, they'll often be found together in the same field, both causing damage. And so I quickly want to just cover ways to distinguish corn earworm from fall armyworm. Both species come in a variety of colors, and so that's not the easiest character to use when you're separating out these two species. The first place I always look is at the head capsule. So if we think about caterpillars, we know they have a hardened or sclerotized head capsule, and they have these different plates on that head capsule that are held together by sutures or seams. And on the fall armyworm shown here, these sutures make a very obvious upside down Y on the top of their head. You can still see these sutures or these seams in the corn earworm head capsule, but they're not nearly as pronounced as they are in the fall armyworm. Another way to tell the difference is flip it around. So you've now looked at the head, now look at the very back of the abdomen. On fall armyworms shown here on the left-hand side, you have four dots. They're equally sized, arranged in a square on the tip of their abdomen that you don't see in corn earworm. So the primary way that we control caterpillars is with Bexillus thuringiensis or BT. BT toxins are proteins that are toxic to insects, and these were originally isolated from naturally occurring soil bacterium. Now these toxins are being produced by BT crops or genetically modified crops that have been modified to produce this BT toxin. The genes in the bacterium to create these proteins were transferred to the DNA of the crop plant, in this case corn, and the plant, the corn, now produces these toxins. And since the initial discovery of BT, there's been several different toxins that have been isolated, and they're active against different pests. We have ones against beetles, caterpillars, mosquitoes, flies. In corn, however, we have two main categories of transgenic insect traits. Ones that provide above ground control against corn borers like corn earworm, and then we have a different set of traits that provide below ground control against corn rootworm. Here in Alabama and Florida, we are not concerned with corn rootworm. There are some parts of northern Alabama that still see issues with corn rootworm, and these are primarily in fields that plant corn after corn without rotation. It's very important to note that susceptibility to BT toxins varies both between different groups of pests, so like caterpillars versus flies. So the ones that are going to kill caterpillars are not going to be the same ones that you can use in your backyard to control mosquito larvae. Similarly, there's also variation within groups of pests, and so even within caterpillars, these BT toxins are going to have a different effect, and that's why it's important to know what caterpillar species you're dealing with, and also how a particular toxin is going to perform against that pest. The easiest way to understand which BT toxins are present in which corn varieties and what pests they control is using this BT trait table. So it's called the Handy BT Trait Table for U.S. Corn Production. It's developed by corn entomologists in Michigan and Texas, and it's updated every year to include this information I just mentioned. What products are available, what pests do they control, and then also what issues have arisen with these traits in terms of resistance to the target pests. So the first page shown here of the Handy BT Trait Table lists all the different events for BT protein, and so this is each time a gene is successfully inserted into the crop, and then each of these events has their associated target pests, and a lot of the times herbicide tolerance for weed control. The second page of the table shown here lists all the varieties on the left that are available for insect corn control, and we're looking at over 30 of these traded varieties, and included in the chart you have the name, so Acromax 1, what different BT toxins are included in that package, the pests that they are supposed to control, so corn earworm, fall armyworm, western bean cutworm, if you're in a region that has that as a pest, southwestern corn borer, and then this column is very important over on the right hand side, which pests have been found to be resistance to some combination of these traits. There's also a note to check the local situation, so this is a U.S. trait table, it's also recently included Canada, so there's going to be a lot of variation in what your risk is and levels of resistance in different areas, and so check with your local extension office or your local seed dealer to find the best fit for your particular region. As we talk about BT and resistance management, I thought it would be helpful to go over some terms that are unique to this environment in this conversation. So the first one is a word that I've already said before, it's event. I mentioned this when I was talking about each event with a particular trait being successfully inserted into the plant. So when these genes, the genes that create the soil bacterium that we're using are inserted into a crop plant, each successful transformation is referred to as an event. And so even if the same two genes are inserted into the plant's DNA but it's two separate times, this could differ in how these genes are expressed in the plant. And then this difference in gene expression can lead to different effects on the pest, and some of these may not be that we're killing the pest, which is the goal of putting these toxins in the corn. So that's why we refer to each separate event in terms of the traits and packages, because it may affect the pest differently. And we're looking at traits, there are now several products on the market that contain multiple Bt toxins, and these can either be stacked or pyramided. Stacked traits have toxins that are active against different pest groups combined into one hybrid. So for example, you have within the same corn hybrid, you have Bt toxins that will attack an above ground pest like corn earworm and a below ground pest like corn rootworm. And this is different than pyramided traits, which have toxins, multiple toxins that are active against the same pest group. And so think multiple Bt toxins are working together to kill the caterpillars. And the reason that I'm covering all this background on Bt is that we are currently fighting against resistance to these traits. The actual definition of resistance listed here is when there is a genetic shift in a population of insects that renders it significantly less susceptible to a toxin. So exposing these caterpillars, for example, to a Bt toxin does not in and of itself cause a genetic mutation that leads to resistance. However, what it does do is exposure to the Bt toxin allows survival of those few insects in the population that already have the genes that for some reason allow them to withstand the toxin and it's not going to kill them. And when we first introduce something like a Bt toxin, these genes, something coded in the insect's DNA, are rare in the population that are going to make it susceptible to the Bt toxin. Most of them, the majority of these genes and majority of the pests in the population are susceptible. However, over time and after repeated exposure to the toxin, the genes, the ones that allow for survival, become more and more common within the population. Ultimately, the population becomes predominantly resistant with a few insects left that are susceptible. And finally, this leads me to a refuge strategy. And so this is used to help combat the potential for resistance. And a refuge is where growers are required to plant some acreage, this is called the refuge, of non-Bt corn in addition to the Bt corn they are planting. And this is so we have insects coming out of the non-Bt corn that have not been exposed, they do not have any genes that allow them to be resistant, and these insects coming out of the non-Bt can then mate with the insects coming from the Bt plant and share those susceptible genes. And so the overall population remains susceptible to Bt toxin. So that's the idea behind the refuge strategy. And one way that we're trying to preserve the shelf life of these Bt technologies is with insect resistance management or IRM plans. And as these Bt traits are brought onto the market, as they're introduced and brought on by seed companies, they're required to present an IRM plan, an insect resistance management plan, to the Environmental Protection Agency or EPA. And then producers buying and growing the seed that has these Bt traits in it are required to follow the IRM plan to remain compliant. And then a big component of this compliance is planting a non-Bt refuge next year Bt to maintain the population of insects that are not exposed to Bt proteins. The refuge requirements change based on where you are growing your corn and what you are growing. So here in the cotton belt, we have a 50% refuge requirement for single-traded Bt corn. So anything that has one Bt gene active against insects is required to have a 50% refuge. And those that have two gene can go down to a 20% refuge. And this block refuge can either be planted in the middle of, next to, or adjacent, but as long as it's no more than one half mile away from the Bt field, and then you are compliant. So I mentioned that seed companies work with EPA to develop these IRM plans. They're also responsible for recording what percentage of follow these rules, specifically regarding refuge. And the reason they do this is because complying with refuge requirements is so important to extending the longevity of these traits. So if we have these IRM plans in place, we know that Bt toxin kills a lot of the pests we're dealing with, what's the problem and why are we seeing potential changes down the road? So it was recently announced by the EPA that they have released a new proposal for regulations related to Bt resistance management. And the goal of these regulations is to extend the lifespan of current Bt traits in corn and cotton, as well as any future products that may come on the market. And this proposal is going to change some of the current IRM strategies we have in place, as well as add additional ones. This proposal is specifically focused on corn earworms, or cotton bollworm, fall armyworm, and western bean cutworm. Of these, we only have corn earworms and fall armyworms as pests in the south. So the current IRM plan was created more than 20 years ago when Bt's first came on the market. And the original plan addressed some of the same current issues we're seeing today. How do we monitor for resistance effectively? How do we maintain compliance with block refuge plantings? And then how do we mitigate any potential instances of resistance? And this newly drafted plan is needed because of the documented resistance of caterpillar pests and the threat of additional resistance in the future. So currently, as we stand in 2020, corn earworms are currently resistant at some level to all of the Bt toxins on the market except for one. The VIP3A gene is the only remaining Bt toxin with no resistance for corn earworm across the cotton belt. We've also seen resistance for fall armyworms and western bean cutworm. So now with only one toxin remaining that's completely effective against corn earworm, the VIP3A gene, we're going to run into some problems. And so this draft proposal from the EPA is an attempt to help and extend the shelf life and longevity of these Bt traits. So what are the changes? So the proposal addresses several of the risk factors that led us to this point. How did we get here where we're needing to make drastic changes in the selection we have for corn and both cotton products? And so some of the risk factors include having only one trait or a very small number of highly effective traits. And so what this does is it puts all the pressure on single traits that will eventually be useless once we encounter resistance. We've also ran into a lot of issues with a lack of refuge compliance. Many fields are not planted with a required refuge, which means we don't have those susceptible moths coming out of non-Bt to mate with the ones coming out of Bt that might have survived and now have coded in their DNA, in their genes, the ability to survive a Bt toxin. And one of the other major changes that the EPA is considering is changing their strict definition of resistance. So previously if you planted a field to corn and it had a Bt trait and you experienced a field failure or the the pest got through the Bt trait, there would be a series of regulatory tests to 100% make sure that this is in fact resistance. And so it would take a long time, it would have to go conduct it in a lab, and that would have to be determined to be resistant or not before the EPA or any other growers in the region would be notified. And so this process, this long drawn-out process, has previously made it almost impossible to inform growers or the EPA of Bt resistance before it becomes widespread. And so this new proposal addresses the issue with a new definition of resistance. So if we have field failures, they're now considered cases of practical resistance. Similar to before, seed companies are still able to come and take samples and test them in the lab to see if they are in fact resistance. However, if we do have fields with unexpected injury, they'll automatically begin a resistance mitigation program while we are testing those insects in the lab. And so we are already on top of it, we're trying to get ahead of it as soon as we find unexpected injury. Additionally, these field failures will be communicated widely to growers, local extension personnel, crop consultants, and other stakeholders as part of an enhanced communication strategy against Bt resistance. So what does this actually mean in practical terms? So in addition to the changes in refuge requirements, there may be a option to plant refuge in a bag in southern states, but with a 20% block refuge. So this will be required in addition to planting your seed blend or refuge in a bag. And the block refuge, that 20%, is basically acting as the refuge of that crop until we gather enough data to know whether seed blends are a good or bad thing when it comes to accelerating resistance. In the meantime, there's several of us at land-grant universities that are trying to figure out if seed blends are safe, and if they are, then they may potentially drop that 20% block refuge in the future. And then the second big change is going to become regarding hybrid availability. So there's going to be a phase-out of all the products that we know are contributing to the resistant population. So this means is that all single-toxin Bt corn hybrids will be phased out over the next three years. This will be paired with a phase-out of all pyramids, so hybrids that have multiple toxins tarting the same pest, hybrids that don't have VIP in them. And so what this really boils down to is that over the next couple years, between three and five years, unless there's a new technology that comes out that is made available to growers, corn growers are going to have drastically less options to choose from regarding traits and varieties. The only options will be pyridamid traits, so again multiple toxins, and one of those toxins have to be the VIP3A gene. The other option is to plant non-Bt corn. And if you remember that handy Bt trait table that I showed earlier, we currently have over 30 options and we're going to be reduced to eight for Bt technologies. And so shown here are what could be available to growers in five years if the EPA finalizes their proposal. And that's an important note, this is only a proposal from the EPA. These changes have not been finalized and so that's where we're going to have need input from producers on what this means for you and your farm. So we are currently in mid-October and we have a 60-day public comment period open through November 6th. And the EPA really really wants to hear from growers to see how these proposed changes may affect their operations. They specifically are concerned and want opinions on the phase down of the single traded hybrids and the ones that don't have VIP. And they also really would like to have feedback on refuge compliance monitoring. So as the proposal stands now, the registrant, so the seed dealers, are required to implement compliance measures with growers that include on-farm visits, better record keeping, and measures for those that fall out of compliance. And so I've posted the link here that shows where you can read the document in its entirety and also make comments. What I just mentioned was a brief snapshot of the document. The important thing to remember is that EPA is genuinely interested in hearing how this will affect growers. These are big changes and so more input is really required moving forward. So in summary when it comes to corn insect pest management, there are lots of things that will cause yield loss in your corn, including a couple dozen insects. We know we'll see stink bugs and worms consistently in addition to a few other ones and so having a proactive plan in place to really reduce your risk from now when you're thinking about selecting varieties all the way to how you store your corn at the end of the season, you really want to give yourself the best possible chance to maximize outputs in this crop. We also really need to work together to preserve what technologies we have now. Talk to your seed dealer, talk to your local extension office, and find out what is the fit for your situation based on farm history, location, pest history, and that will really give you the best chance to maximize your crop next year. And finally, let the EPA hear what you think about their proposal. Every opinion counts and it'd be great to get opinions from especially the southern region and how this proposal will affect corn, which I've spoken about, but also cotton and cotton growers. And finally, I'll leave my contact information here. If you have any questions, feel free to get in touch with me via email or phone and also find me on Twitter. I hope you found this useful and look forward to hearing from everybody soon.