Early Season Insects in Corn and Soybeans

Michael Sindelar with Robert Wright Author

06/12/2019 Added

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In this episode of the Nebraska CropWatch podcast, Michael Sindelar is joined by Robert Wright of the University of Nebraska-Lincoln Entomology Department. Topics include how to prevent, identify, and manage early season insects in corn and soybeans.

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[00:00:00.867](bright acoustic guitar)
[00:00:02.950]Welcome to CropWatch Podcast,
[00:00:05.410]a production of Nebraska Extension.
[00:00:12.120]Welcome to the CropWatch Podcast.
[00:00:13.750]I'm Michael Sindelar, Cropping Systems Extension Educator.
[00:00:16.810]For today's CropWatch Podcast we'll be talking
[00:00:19.090]about early season insects in corn and soybeans.
[00:00:21.636]Today I'll be joined by Dr. Robert Wright,
[00:00:24.246]who is a Entomology Specialist
[00:00:27.100]for the University of Nebraska,
[00:00:28.760]Dr. Wright, would you like to introduce yourself?
[00:00:31.230]Good morning, Mike.
[00:00:33.530]Yes, I'm a professor of entomology at UNL.
[00:00:37.970]And I have a joint research and extension appointment.
[00:00:41.610]I do a lot of research on corn and soybean insects.
[00:00:45.582]Historically, mostly in eastern half of the state
[00:00:50.380]is where I've done most of my work.
[00:00:52.760]So, I guess I've been in Nebraska for over 30 years
[00:00:56.440]so I've seen a lot of different situations come and go.
[00:01:00.352](chuckles)
[00:01:03.410]Well, the first question that's on my mind
[00:01:06.574]Dr. Wright, is why should I even be checking for
[00:01:10.570]early insect issues with all these nice traits
[00:01:15.030]that I can stack in corn?
[00:01:19.950]Well, there are some traits
[00:01:22.335]that can help with early season insects,
[00:01:24.930]but the current traits,
[00:01:26.419]even the insecticide seed treatments
[00:01:30.776]or insecticide planting time treatments
[00:01:35.490]don't control all insects,
[00:01:37.700]and there's always the potential
[00:01:39.160]of high populations of insects
[00:01:41.080]that could overcome even the best products.
[00:01:45.600]So it's always a good idea to check
[00:01:47.385]your corn and soybean fields as the crop emerges
[00:01:50.966]to assess stand, see if it's going to be adequate.
[00:01:56.090]If there is something damaging the plants,
[00:01:58.080]try to identify what it is and make a decision
[00:02:01.930]how to respond to that.
[00:02:05.640]So as corn is emerging right now,
[00:02:08.266]what I would be out in the field looking for
[00:02:10.630]is as I look at my stand, I would be looking for areas
[00:02:13.320]that have skips or have whole swaths missing
[00:02:18.090]or what would I be looking for in the field right now?
[00:02:21.070]Yeah, that'd be two things,
[00:02:23.194]would be if there's a gap in in the plants,
[00:02:27.320]and in that case, you'd probably want to dig around
[00:02:29.500]and see if you could find the seed,
[00:02:31.320]or the germinating seed
[00:02:33.750]and see, either it didn't germinate at all,
[00:02:37.240]or it potentially, could be killed by a disease.
[00:02:42.810]It could be killed by an insect, like a wireworm,
[00:02:45.584]or a seed corn maggot.
[00:02:48.030]And so that'd be one thing if there's gaps,
[00:02:51.264]try to identify what caused the lack of emergence.
[00:02:54.570]The other thing is with emerged plants,
[00:02:57.410]if they're obviously stunted compared to the other plants
[00:03:01.340]in the field, or wilting or have damaged of some sort.
[00:03:05.743]That would be another reason to dig around
[00:03:08.873]and look for insects and see
[00:03:10.580]what might be causing the problem.
[00:03:13.490]So when I'm out, I find an area that's damaged.
[00:03:16.061]What kind of damage, am I looking for
[00:03:18.392]and what are what are the culprits right now,
[00:03:20.710]what are the insects that we're looking to scout for?
[00:03:24.365]Okay, in terms of plants that have already emerged,
[00:03:28.080]if they're wilting, or they're dead
[00:03:31.360]to have to they emerge, it could be a soil insect
[00:03:34.110]like a wireworm or white grub that's damaged the plant
[00:03:38.020]or killed the plant by feeding below ground.
[00:03:41.610]So that's one possibility, in corn also there is
[00:03:45.790]a potential for cutworms, to cut the plant
[00:03:48.958]at or below the soil surface,
[00:03:50.865]and some of these cutworms actually drag
[00:03:53.554]the above-ground part of the plant
[00:03:57.810]down into their tunnel and feed on it below ground.
[00:04:01.530]So there may not be a lot of evidence,
[00:04:03.754]other than maybe you can see where the plant was cut
[00:04:08.017]at the base, but you may not be able
[00:04:10.100]to find the rest of the plant.
[00:04:11.692]So, look for those types of damage symptoms.
[00:04:15.776]I guess the other thing is some of these insects
[00:04:18.231]may cause holes in the plant
[00:04:19.996]or distortions in the growth from the feeding below ground.
[00:04:26.790]And what are we scouting for?
[00:04:28.880]I know you said a couple, you said wireworms
[00:04:31.800]and white grubs and cutworms.
[00:04:34.399]What else are we out looking for here in the early season?
[00:04:38.710]Oh, potentially there's some other soil insects
[00:04:40.893]such as seed corn maggots.
[00:04:44.330]Those are the big three, are wireworms,
[00:04:46.840]white grubs and seed corn maggots in terms
[00:04:48.807]of the below ground insects.
[00:04:51.310]And then cutworms feed at or below the soil surface.
[00:04:54.810]It depends a little bit on the environment,
[00:04:57.830]if its moist like it has been recently
[00:05:00.850]they're probably feeding higher up on the plant.
[00:05:02.896]If it's dry, some of these will stay below ground
[00:05:05.604]where its moister and will feed at the lower part
[00:05:08.950]of the plant as far as cutworms.
[00:05:11.930]And so when we're trying to ID these,
[00:05:13.790]what are key features that we're looking forward to ID
[00:05:17.030]wireworms, white grubs and cutworms?
[00:05:23.298]Okay, well there are three different types of insects
[00:05:27.300]and there's good photos online.
[00:05:28.930]We have resources on the CropWatch.UNL website,
[00:05:33.990]but wireworms are usually long and slender
[00:05:38.090]often coppery in color,
[00:05:39.910]that's why they're called wireworms.
[00:05:41.901]They have small legs, but they may not be easy to see.
[00:05:45.972]White grubs are immature beetles.
[00:05:51.060]When you dig them up they they generally are C-shaped,
[00:05:54.138]typically are sort of white, fleshy in appearance
[00:05:59.020]with a dark head, and they have visible legs.
[00:06:03.760]And then cutworms are just a soil caterpillar,
[00:06:06.432]and they can vary in coloration.
[00:06:08.810]There's quite a few different types of cutworms
[00:06:11.610]we can see in Nebraska.
[00:06:13.770]So with the white grubs, there are two types, correct?
[00:06:18.011]Right, we have two different groups
[00:06:21.350]that vary in their life cycle.
[00:06:22.910]We have the annual white grubs which are the most common.
[00:06:25.710]They're the types that you also have in your home lawn.
[00:06:29.950]And we have another type that's less common,
[00:06:33.231]called the three-year grubs, and they take three summers
[00:06:38.004]to complete their development, and particularly
[00:06:40.980]in the last summer when they're larger,
[00:06:42.780]they can do a fair amount of damage if they're abundant.
[00:06:47.340]Typically the annual white grubs
[00:06:50.470]don't cause a lot of damage
[00:06:51.950]because they do most of their feeding
[00:06:54.080]during the previous summer.
[00:06:56.070]The eggs are laid in July and hatch out
[00:06:59.900]and they do most of their feeding
[00:07:01.510]their first summer that they hatch out,
[00:07:06.210]and then they come back after,
[00:07:08.740]as the temperatures get colder in the fall.
[00:07:11.440]They move down below the frost line
[00:07:13.314]and then as it warms up they come back up to the surface
[00:07:16.836]and feed again but they do most of their feeding
[00:07:19.530]in the previous year.
[00:07:21.720]So oftentimes, it takes a pretty high number
[00:07:23.920]of annual white grubs to cause economic loss,
[00:07:27.030]compared to the three-year white grubs which are bigger,
[00:07:30.410]and they feed longer period of time in their last summer.
[00:07:36.150]All right, so I'm out scouting
[00:07:38.050]and I actually find these pests, what are my options
[00:07:43.725]for rescue treatments to start with,
[00:07:46.110]or is there a chance to use a rescue treatment?
[00:07:50.390]Now there's the cutoff point.
[00:07:53.335]The difference is that the wireworms and white grubs
[00:07:58.468]spend all their time below ground,
[00:08:01.470]and there's no good rescue treatment
[00:08:04.090]or post-plant treatment to control them.
[00:08:07.010]If you have a history of problems
[00:08:09.450]or particularly if you're planting early
[00:08:11.400]oftentimes growers will use an insecticidal seed treatment.
[00:08:15.025]And this can suppress some of these soil insects,
[00:08:18.140]but again, if we have high numbers
[00:08:20.220]we still may see damage,
[00:08:23.046]even with an insecticidal seed treatment
[00:08:25.370]or planting-time insecticide.
[00:08:27.648]So the decision, if you have stand loss
[00:08:31.600]or plant injury from white grubs or wireworms,
[00:08:36.890]the only decision once the plant, the crop is planted
[00:08:40.851]is whether or not to to replant.
[00:08:43.053]Is the stand going to be adequate
[00:08:45.387]or is it at a point where you probably want to replant.
[00:08:51.909]But it is important when we do see this damage
[00:08:55.100]to identify these pests, right, for future planning?
[00:08:58.430]Right, that's the other thing,
[00:08:59.630]and then the other point I didn't make
[00:09:01.990]about wireworms is the most of them
[00:09:04.310]have a multi-year life cycle,
[00:09:06.920]so if you have a high number of wireworms in the field
[00:09:09.780]this year, and you have a mix of different sizes,
[00:09:13.087]they're going to be around next year too.
[00:09:15.680]So you can be aware of that and maybe,
[00:09:18.580]if you didn't use a seed treatment
[00:09:21.700]or a soil insecticide at planting in that field,
[00:09:25.176]you're likely to have wireworms again
[00:09:27.570]if you plant back to corn or other susceptible crops.
[00:09:33.240]Again with the white grubs,
[00:09:35.500]if you know it's the three-year white grub
[00:09:39.189]those are going to be around
[00:09:41.910]for more than one summer or spring.
[00:09:46.300]So that would be something to be aware of for next year.
[00:09:50.300]And we didn't talk about cutworms,
[00:09:52.680]but they are something
[00:09:56.130]that you can use a post-emergence insecticide
[00:10:01.270]to control them if they're at economic levels.
[00:10:06.680]And so that is something you can respond to,
[00:10:09.770]if you get to it, that's a reason to scout
[00:10:13.310]as the crop emerges, before they have time
[00:10:15.640]to do a lot of the damage if they're present.
[00:10:18.500]So you were talking about cutworms,
[00:10:20.950]what are we looking for cutworms?
[00:10:22.360]What are the different types of cutworms
[00:10:24.320]and how do we treat these different types?
[00:10:27.570]Well, the thing that seemed to get all the publicity
[00:10:31.060]in the trade magazines is the black cutworm.
[00:10:33.630]This insect does not over-winter in the Midwest,
[00:10:36.610]the moths fly up each spring.
[00:10:39.530]And typically in Nebraska, black cutworms
[00:10:42.523]are only a problem in the maybe
[00:10:45.410]the eastern third of the state
[00:10:46.760]or the eastern quarter of the state they're most common.
[00:10:50.290]And then they're more common in the I states,
[00:10:53.202]the central Midwest.
[00:10:56.894]The issue with them, the moths fly up
[00:10:59.200]and lay eggs in the spring.
[00:11:01.330]And if the conditions or features of a field are right,
[00:11:07.090]particularly if there's a lot of winter annual weeds,
[00:11:10.230]or even heavy crop residue, this can be attractive
[00:11:13.280]to the moths to lay eggs, and the caterpillars hatch out
[00:11:17.139]before or just as corn is emerging.
[00:11:21.130]And you can have a very high population
[00:11:23.880]if they're high numbers of moths in an area.
[00:11:26.830]And again, if you're not scouting,
[00:11:28.799]you may not see the early signs of damage.
[00:11:31.399]The black cutworm initially feeds just on the leaf
[00:11:35.587]and causes some holes in the leaves
[00:11:39.640]and then as the caterpillars get about half-grown
[00:11:42.110]they started feeding at the base of the plant.
[00:11:44.329]On small plants they can cut straight through the plant
[00:11:47.810]and cut it off at the ground level.
[00:11:49.372]And depending on the stage of the plant
[00:11:52.030]they may cut it off below the growing point and kill it.
[00:11:55.750]The other issue in Nebraska, we have half a dozen
[00:11:58.117]other types of cutworms that can damage corn and soybeans.
[00:12:03.831]And these all over-winter as partly grown caterpillars.
[00:12:07.857]So as the crops emerge,
[00:12:10.410]they're going to be bigger-sized
[00:12:12.450]than the black cutworms are
[00:12:13.650]which just hatch out in the spring.
[00:12:16.120]And so, they potentially can be
[00:12:17.900]a lot more damaging than black cutworm.
[00:12:20.380]Typically they're not as abundant.
[00:12:22.358]But individual fields can have problems
[00:12:24.816]with these other cutworms too.
[00:12:27.430]The other issue with the different types of cutworms,
[00:12:30.054]is that there are some Bt corns that have activity
[00:12:34.754]against black cutworm and have that on their label.
[00:12:38.360]And I think there's one other species of cutworm
[00:12:40.920]that's listed on the label of some of the Bt corns.
[00:12:44.810]But there are other types of cutworms.
[00:12:47.910]Either the companies don't have enough data
[00:12:50.070]to put it on the label
[00:12:51.510]or the fact that they're big caterpillars,
[00:12:53.515]even if they're affected by Bt corn,
[00:12:56.260]they have to take a bite or two
[00:12:58.450]to ingest the Bt toxin and be affected.
[00:13:01.880]And they can kill a plant
[00:13:03.090]with a bite or two on small plants.
[00:13:06.490]So that, even some of the Bt corns
[00:13:08.595]with these caterpillars that over-winter in Nebraska,
[00:13:12.190]they're going to be bigger in the spring
[00:13:13.940]and they're harder to control because of that.
[00:13:16.444]That's good information.
[00:13:18.200]A lot of good reasons have been presented
[00:13:20.470]to do some early season crop scouting,
[00:13:23.077]especially as you said,
[00:13:25.070]it's very important to get out there,
[00:13:26.610]to see what you have,
[00:13:28.240]to know what your field history is
[00:13:29.690]so that way you can plan what treatments to use.
[00:13:33.268]On that note, if you have
[00:13:35.830]one of the bigger caterpillar varieties of the cutworms,
[00:13:39.070]what are your options with that?
[00:13:41.820]Well, typically, they're more of a problem in corn
[00:13:44.440]than soybeans, but we have a rough guideline
[00:13:48.840]or threshold in corn.
[00:13:50.700]If five percent of the plants are injured
[00:13:54.290]or cut off the ground level by cutworms
[00:13:57.990]and the cutworms are less than an inch long,
[00:14:02.880]and still going to be doing some feeding,
[00:14:05.190]that would be a treatment level.
[00:14:07.270]So the issue, if the cutworms are all pretty large
[00:14:10.870]over an inch and a half long,
[00:14:12.176]they've probably done most of their feeding by now.
[00:14:15.100]And by the time you get a treatment on
[00:14:16.950]they're probably going to be done feeding for the season.
[00:14:20.820]So the size of the cutworm
[00:14:22.190]is another thing to look for as well.
[00:14:25.010]As I'm thinking about it,
[00:14:26.530]you did say that there are some resources,
[00:14:28.430]there are articles on CropWatch that have
[00:14:30.740]a lot of good pictures to help identify these pests.
[00:14:34.730]When looking to actually treat the pest,
[00:14:37.320]there is the Guide for Weed, Disease and Insect Management,
[00:14:40.810]that's published by the University.
[00:14:42.340]Are there any other resources that are really helpful?
[00:14:47.090]No, and there's a wide range of effective insecticides,
[00:14:50.950]can be applied for cutworm control,
[00:14:52.942]if we have economic populations.
[00:14:55.642]Well, thank you for your time.
[00:14:57.890]You have a good growing season.
[00:15:00.570]You too.
[00:15:02.246](bright acoustic guitar)

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Early Season Insects in Corn and Soybeans

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