Black Cutworm Alert
Well, good afternoon. Welcome to the first in a series of IPM Podcasts for Field Crops. This podcast is sponsored by the University of Minnesota Extension Integrated Pest Management Program. Our goal with these podcasts is to alert growers and crop consultants about emerging pest concerns on Minnesota field crops including corn, soybeans, small grains and alfalfa. My name is Bill Hutchison.
Bill Hutchison:I serve as the coordinator for the Minnesota IPM program. I'm co hosting today's podcast with Dave Nicolai, Regional Extension Educator and Coordinator of the Institute of Ag Professionals with Extension. We are also assisted today by Natalie Hoydl, an Extension Educator with the Pesticide Safety and Environmental Education Program. Today we have Bruce Potter joining us to discuss one of the more common early insect pest of corn and other crops that in the past few weeks has arrived in pretty high numbers the black cutworm. So this is something that we know the corn growers should be aware of especially in the next few weeks as we've had several delays in getting corn planted this year due to the weather and so young corn will still be emerging in the next few weeks which will be highly vulnerable to black cutworm.
Bill Hutchison:Bruce is an IPM specialist based in Southwest Minnesota at the Southwestern Research and Outreach Center in Lamberton. So with that, we would like to go ahead and get started and Dave, why don't you start out with first question?
Dave Nicolai:Well, good afternoon, Bruce. Certainly this has been an active year for black cutworm concerns, black cutworm moths, particularly in Southwestern Minnesota and all along Interstate I-ninety across the southern part of the state, but in other parts of the state as well. Bruce, the last couple of weeks, we've seen really significant moth captures and a number of the pheromone traps that are in a network that you're working with around the University of Minnesota and across the state of Minnesota. What is significant about this year in terms of the higher moth catches than we've had in the previous years in some of these pheromone traps? And in your estimation, what may be some of the causes as to why we're picking up higher moth catches this year compared to other years?
Bruce Potter:Well, Dave, one of the things we're seeing this year is some good weather systems that are bringing these moths up from the South. They can't winter in Minnesota. They winter in the Southern States, the frost line and below. So what's happened is we've had some ideal conditions, we've had some weather systems that have moved through the areas that the overwintering cutworms are at. They're actually actively migrating.
Bruce Potter:They'll pick up in wind and head north every spring. What we're looking at is a series of weather systems where we've had a high pressure to the east and then we've got low pressure systems moving from the West. It's a real efficient transport system. These moths are coming up maybe 300 feet up to 3,000 feet. It's a real efficient transport system.
Bruce Potter:They can make it basically from the Mexican border up to Minnesota in a couple days that way. The difference this year is where these weather systems are tracking. We've had some perfect conditions to bring those into the southern part of the state. The other thing that's happened is corn planting and field work has been delayed this spring and the younger corn is more vulnerable and also there is a relationship to when fields are killed and when those fields are attracted to cutworms.
Bill Hutchison:Okay, Bruce. You know, speaking of the numbers we've had this year, you know, one of the rules of thumb or action levels that we've heard about in the past is eight eight black cutworm moths per two nights in a trap. Is that still a good rule of thumb guideline in terms of a high risk situation?
Bruce Potter:Yes, is at an individual location and of course we've got over 60 cooperators running pheromone traps this year. Where you pick those up, when you pick them up kind of helps us dial in the risk and how widespread a problem is going to be and it also helps us time when these problems are going to start showing up in fields.
Bill Hutchison:Okay. And then I know as of your last report that stated early May, May 5 through the eleventh, it looked like it was a lot of the border counties just north of the Iowa border that were some of the hot spots is how has that changed in the last week or so?
Bruce Potter:Well, I think we've had some of the most active traps in the Southwest corner, the Iowa, South Dakota, Minnesota corner up into Lamberton that happened early that was actually we started picking up flights in the April, May. But since then, we've had trap captures that have been significant all the way to south of the city, Southeast Minnesota and then up into the Southern West Central part of the state.
Dave Nicolai:Would you say Bruce that given the later planting this year, are more corn acres going to be at risk because we've had delayed planting, especially in some of the wetter areas and we're still continuing to have late planting in Southeastern Minnesota, but is this going to be a contributing factor as far as cutworm feeding and cutting later into the season that producers need to be aware of?
Bruce Potter:Well, the disadvantage that the corn plants have this year is they're gonna be small relative to cutworm size. Those cutworms don't start aren't big enough to cut a corn plant off until they're probably about fourth instar. We're not quite there yet. We've got we're starting to see some leaf feeding but larger cutworms on smaller corn that corn is at risk for a longer period that way and it's easier for a cutworm to take a small corn plant than a large corn plant. One thing that we do see though is if in larger corn plants been up to as late as six collar corn, cutworms may not be able to cut those off but they can still kill a corn plant.
Bruce Potter:Large instar cutworms can kill a corn plant by burrowing into the growing point.
Bill Hutchison:So a related question Bruce, given the timing that we are looking at this year and with cutting could begin fairly soon. What are some of the early signs of black cutworm feeding on corn and is that early leaf feeding on the leaf stage? Is it different from the dingy cutworm or how can growers tell the difference?
Bruce Potter:Well, can't initially can't tell cutworm species apart by their feeding. One of the first things you're gonna see and we're starting to get reports of this now, were out looking at the research and outreach center here at Lamberton starting to see some leaf feeding on lambs quarters. On small corn, it'll be notches and small holes in the leaves these small instars. As those cutworms get larger, they'll start cutting leaves, cutting parts of the plant off. You might see small corn plants wilting where they've gone part ways through the stem.
Bruce Potter:One of the things that black cutworms is distinctive compared to some of the other species is it tends to feed more below ground. Things like dingy cutworm which are overwinter and are very common feed at the soil surface and above and we've got a lot of species of cutworms that you know are lumped as climbing cutworms. The black cutworm feeding below ground of course makes a little more dangerous to corn because they are feeding lower, are closer to that growing point, they are more likely to kill a plant by cutting at or below the growing point than the dingies or that feeding is above ground.
Dave Nicolai:Bruce, are there some tips that you would give maybe some of the farmers that are listening to this podcast about scouting perhaps marking an area in the field, the time of the day for scouting in terms of that and then maybe some kind of a phrase that we could use when we talk about second and third and fourth instar, you know approximately how long or how big those cutworms are when they are concerned in terms of cutting?
Bruce Potter:Sure. Well the cutworms as they hatch are very small. They're one to two millimeters long. They won't be able to start cutting corn till they're over a quarter inch, probably closer to three eighths of an inch long. As far as scouting, if you start to see signs of leaf feeding, think if you're out there and you see feeding on weeds or corn, marking some areas of the field, marking off some areas of row, counting how many plants, coming back and seeing if that feeding is changing.
Bruce Potter:Particularly if we're starting to see plants being cut off, that's what the thresholds are based on. But again, looking at signs of cutting on some of the weed species a lot of times is helpful. One of the problems we've got with this black cutworm though is because it feeds below ground, if you've got larger cutworms out there and you've got small corn that hasn't emerged yet, you can actually, some of that corn may not make it out of the ground. The cutworms will feed on it as it's emerging and you'll have some holes in the stand. Plants just won't come up.
Bill Hutchison:Okay, Bruce. That's a good overview from the cutworms perspective. We've also got some questions about the differences in corn fields at risk to cutworm feeding. What are some of the highest risk factors this year in terms of non BT corn out there or corn following cover crops, things like that.
Dave Nicolai:Or even corn on corn situations or corn soybean or what kind of rotations would Well they be
Bruce Potter:historically we've seen more problems following soybeans. Soybean residue tends to have more issues. That have a lot of early season weed pressure, winter annuals, some of the early emerging spring weeds, things like lambsquarters I've mentioned, ragweeds, kochia, those sorts of weeds tend to attract those egg laying females. Low lying areas, a lot of times they'll congregate in kind of protected shelter areas, you'll see more egg laying activity. So those are kind of areas that the cutworms are honing in on.
Bruce Potter:As far as corn varieties or hybrids rather, corn without the Cry1F or Viptera trait is susceptible but even these fields have corn with Viptera or the above ground Hercules trait, third tacked by big cutworms on small corn, they can get overwhelmed as well.
Dave Nicolai:One question that comes up if people are scouting, if they are seeing cutworms bruises, what are your recommendations on economic thresholds especially when we talk these younger corn plants that are perhaps one to three leaf in terms of an action threshold and then for treatment etcetera?
Bruce Potter:Well, the thresholds are all on cutting. Leaf feeding is not harmful to corn as long as that corn is if it's cut off, as long as it's below the growing point, it can recover and do just fine. So we're looking at initially at cut plants, kind of a generic threshold is 2% to 3% of the plants cut. You have to be a little careful though because corn is sensitive to stand loss and if you've already got an impaired stand for emergence for some reason, you got to you may have to adjust that a little bit because you don't want to go below 29,000 or so you really start to have some yield issues just because of the reduced stand.
Bill Hutchison:So I guess along with that Bruce then what are some of the insecticides that that extension would recommend if a grower feels like he's above those thresholds?
Bruce Potter:Well a good news about cutworms in general not just blacks is there is a lot of times aggregated, sometimes you get away with treating just part of a field. It's an easy insect to go in with a rescue treatment to control. They're not that hard to kill particularly early if it's wet. Those cutworms tend to be and this is kind of a scouting tip that I forgot to mention. They tend to hang out at the border between dry and moist soil.
Bruce Potter:Under wet conditions, they tend to be further up towards the surface. The good news is they'll cut a little bit higher in those cases. So they're easy to kill with any of the labeled Perythroids. As those cutworms get larger, you may want to use a little bit higher rate. Other products like chlorpyrifos will work just fine as well and particularly if you're out there towards the evening with that spray, they tend to be a little bit more effective just because that's when the cutworms are active.
Bill Hutchison:So Bruce, I think another key point on scouting that I've heard at least with some of the cutworm species is that they're really most active at night and so how does that affect a grower or a crop consultants scouting schedule? You know to they actually want to see what species is involved or look at cutting more closely, you know how critical is that?
Bruce Potter:Well, mean you if you want to see cutworms up on the plants going out there in the evening or at night is very early in the morning is probably easier to do. Unfortunately people have important things to do at night like sleep. You can still find those cutworms during the day. It takes a little bit of effort. They're nocturnal, they like to hide under little pieces of dirt clod, under pieces of residue and again if you start getting into that moist area in the soil, they're not going to be too far down into that.
Bruce Potter:So it's conditions like we've got right now in a lot of Southern Minnesota where it's fairly moist, they're going to be up towards the top and not that hard to find.
Bill Hutchison:Right, so even during the day if you see some cutting in a field, you can just sort of dig down at the base of the plant to possibly see what species.
Bruce Potter:Right, and then a real important thing to remember is after those cutworms have cut a plant, they don't necessarily stay there. So if you go to the base of the plant and you don't find the cutworm, you may want to go up and down to roll a little bit and do a little exploring, see if you can pick up one of the larvae. Generally it's easier to find them if you've an area with more leaf feeding and cutting activity than a single plant.
Dave Nicolai:Bruce, maybe you want to tell the audience a little bit about the newsletters that you produce and so forth, on black cutworm. But along with that, in one of the more recent newsletters, you included a table of approximate cutting times and maybe talk a little bit about what those cutting time projections are based upon moth flight and how do you arrive at those in terms of the mathematics and the computation to make those projections?
Bruce Potter:Okay, well, once a week during the spring, we're putting out a newsletter that kind of tracks where we think the cutworms are as far as development, where we've had these significant moss captures and probably instead of going through a whole URL, the easiest thing to do is probably go do a search on Minnesota Black Cutworm Reporting Network. We've run that network for a number of years but 2018 information is up there. We try to get them out on Tuesday. We process process the data from the last week on Mondays and takes me a little while to type them up. The nice thing about black cutworm is they've got more or less the same development temperature threshold as corn.
Bruce Potter:So we can use a 50 degree base and we can project out how far they are in development based on degree days and we can use the same development models as we do for corn. Some of the numbers we're keying in on are when those eggs hatch that happens about 90 degree days after the significant flight. They'll be large enough to cut or they'll hit that fourth instar, little over 300 degree days so you should be scouting by then. Then the other important thing is when they're done feeding and that's a little over six forty degree days, they'll start to pupate. So got a kind of a good wind projection on when that damage is going to occur and then you can look at projected size of the larvae compared to corn.
Dave Nicolai:So the early June here, if I remember correctly, it was going to be a critical time but it's going to continue into mid June?
Bruce Potter:Yeah and actually this past week, most of those projections we've moved up a day or so. So yeah, basically the last couple days of May well into June is when those larvae are gonna be active. Fifth instar is when they do the most of their damage.
Bill Hutchison:Okay Bruce, this is great info. I think it'll be very timely for growers and crop consultants. Another group that I work with a lot are the sweet corn growers and processors in the state. A lot of what we've already covered I think is highly applicable to sweet corn as well. However, one question that might be a challenge for you and that is for our organic sweet corn growers in the state.
Bill Hutchison:Do you have any suggestions on an insecticide that's organic certified that would also be active on cutworms?
Bruce Potter:Not that it's going to be real effective. There's some like organic and neem and those sorts of things but as far as being able to kill cutworms quickly, I think they're going to be a little bit lacking.
Bill Hutchison:Right, I think that's going to be a challenge. Entrust is another product that comes to mind but those larvae get to be one inch in size, they're actively cutting that's going be
Bruce Potter:a challenge. The BTs will work but they work slowly so you're going to have an issue with those cutworms maybe taking another plant or two before they succumb.
Dave Nicolai:You know, Bruce, I know at the trap at Rosemont, we were continuing to get some moth flights, not very much but even up here the week before Memorial Day. Is that significant in terms of how long farmers will have to face cutting issues or feeding issues in corn? In other words, are we moving farther into the spring given the type of spring? And do these late moth lights, although they are lighter, are they of concern to extend this damage farther into the spring?
Bruce Potter:Depends on when the corn planting happens. Like I said, if you get fifth, sixth leaf corn, it's usually pretty safe from cutworms. But Bill mentioned sweet corn, a lot of that's planted quite late. That's going to be vulnerable for a longer period of time. They will take soybeans and some of the other late planted crops.
Bruce Potter:The advantage soybeans have compared to corn is they're more tolerant of stand loss. They're not as responsive to stand loss with yield as corn or sugar beets are.
Bill Hutchison:That is all very useful, Bruce. And this has been a great podcast to launch this new series in IPM for field crops. Before we go today, just curious if you've got any additional points that you would like to make that might be useful to farmers crop consultants.
Bruce Potter:Well, think just because we've got these significant moth captures doesn't mean that we're going to have a cutworm disaster but the risk is certainly elevated this year and the late planning a lot of these fields is kind of exacerbating it. I think people should always be out scouting early and cutworms should always be on their mind and on the radar. But I think this year, particularly paying attention to black cutworms. I think knowing which species you have is important. Dingy cutworms, I'm not aware of any time they really actually hurt cornstain because of their climbing habit.
Bruce Potter:So it's important to recognize those species and you can do it by looking at a little tubercles or bumps on the back of the cutworm but it's better to do that with a picture. There is a pretty comprehensive publication on the extension website and if you search on University of Minnesota black cutworms and corn, it gives a lot of advice on scouting degree days, how the things migrate and then identification as well.
Bill Hutchison:Okay, that sounds good and then one more quick resource before we go today. Today is Tuesday, May 22. I believe you said in the next few days you'll have a new update out on your website with the new cutting dates as well. Yes, it'll be out hopefully this afternoon. It's being edited right now so we try to
Bruce Potter:get those out every Tuesday.
Bill Hutchison:Okay, sounds good and a great thanks and shout out to all of your cooperators that are helping out with the network this year. It's really covering the state well and we just want to say thanks again and look forward to the next time we can get together.
Bruce Potter:Well thanks. I think it's been fun and hopefully hopefully people go out there and do a lot of scouting and don't have any problems to show for it but better to do the scouting. Okay.
