5 - 2021 South Central Ag Lab Field Day - Corn Rootworm Research and Management

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10/14/2021 Added

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5 - 2021 South Central Ag Lab Field Day - Corn Rootworm Research and Management, Bob Wright, Nebraska Extension Entomologist

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[00:00:07.910]Hello I am Bob Wright,
[00:00:08.780]professor of Entomology and Extension Entomology Specialist at UNL.
[00:00:13.250]I do a lot of field work here at the South Central ag lab.
[00:00:16.760]And today we'd like to talk about some of the work we've done at this location
[00:00:21.260]and other locations in Nebraska on corn rootworms and their management,
[00:00:25.670]corn root worms have been a really difficult insect to deal with.
[00:00:28.610]Over the years,
[00:00:30.080]we have a long history of them developing resistance to different strategies.
[00:00:34.010]We've tried to manage them primarily insecticides,
[00:00:37.400]but also more recently BT corn. And, we have some,
[00:00:42.290]not so good history in Nebraska.
[00:00:45.290]We've been the first place where different types of insecticide resistance has
[00:00:49.220]developed in the late 1950s and early 1960s.
[00:00:53.390]Nebraska was the first place where with some of the old soil insecticides,
[00:00:58.220]we detected resistance. And then later in the 1990s,
[00:01:03.050]as we moved,
[00:01:04.940]away from those into organophosphates and carbamates,
[00:01:10.280]and in the 1990s, particularly in south central, Nebraska, we were using
[00:01:14.960]an organophosphate insecticide Penncap,
[00:01:17.630]or methyl parathion a lot. And we detected resistance to that,
[00:01:23.030]as well as some carbamate insecticides.
[00:01:25.940]And then some of those came off the market for other reasons.
[00:01:29.840]And we started using more pyrethroids and more recently,
[00:01:33.980]last 10 years or so we've documented resistance to one or more of the pyrethroids
[00:01:38.140]in Nebraska. In 2006,
[00:01:42.800]we had the first BT corns that are active against rootworms commercialized.
[00:01:48.170]And within a few years, several years in different parts of the Midwest,
[00:01:51.710]we started seeing resistance to some of the BT corns. And as,
[00:01:56.210]more BT corn acres for root worms were grown.
[00:01:59.570]We've seen more evidence of resistance in different locations.
[00:02:04.430]So we always have to keep on top of the root worm.
[00:02:08.750]That's part of why we do some of the work we do here to see how products are
[00:02:13.250]performing in this location to give information to growers.
[00:02:17.450]And we've also used that in our research program to document resistance.
[00:02:21.890]One thing I will mention is that crop rotation is still highly effective in
[00:02:26.510]Nebraska, for Western corn rootworm management.
[00:02:30.650]if we grow another crop, corn root worms primarily, or almost exclusively,
[00:02:35.570]feed on corn. So if we can grow a non corn crop, typically sorghum
[00:02:40.550]excuse me, soybeans, but it could be other crops as well.
[00:02:45.110]That really helps us manage corn
[00:02:47.180]Rootworms, it reduces their densities and makes all our other strategies work better.
[00:02:52.490]So just I'm going to hit this again,
[00:02:54.530]but crop rotation is a really important element in managing corn root worms in
[00:02:58.490]Nebraska. Some of the studies we've done in the past,
[00:03:02.890]different planting time treatments for larval control,
[00:03:06.190]whether it's seed treatments, liquid insecticides, or granular insecticides.
[00:03:10.960]And we've done not as many,
[00:03:12.340]but we've done a few studies looking at foliar insecticides for adult control
[00:03:17.800]of rootworms. That's a common practice nowadays, in some areas,
[00:03:22.690]if we have high populations, particularly in continuous corn,
[00:03:25.360]where we're trying to reduce the densities for next year,
[00:03:28.630]in terms of reducing the beetles that lay eggs that produce next year's
[00:03:32.560]rootworms. So one thing we do with our,
[00:03:36.810]our larval studies,
[00:03:38.470]we evaluate roots after the larval feeding is done.
[00:03:42.460]And most recently we're using this zero to three scale to evaluate
[00:03:47.350]root injury.
[00:03:48.070]And that's what some of the data I'm going to show you is based on and is based
[00:03:51.910]on the number of roots that are pruned to within an inch and a half of,
[00:03:56.310]of the stem or stalk.
[00:03:58.180]And typically we have three nodes or rows
[00:04:03.430]of a roots on a plant that are exposed to rootworm feeding.
[00:04:08.530]And so if one or more roots are pruned that counts toward,
[00:04:13.660]this score.
[00:04:15.280]If we have the equivalent of one nodes worth of roots,
[00:04:19.660]pruned to within one and a half inches of the, the stalk, that's a one.
[00:04:24.070]And then similarly for two and three,
[00:04:27.760]and if we have fractional numbers of nodes pruned,
[00:04:31.570]we have a fractional number,
[00:04:32.770]like one and a half would be one of the half equivalent of one and a half nodes
[00:04:37.030]worth of roots that have been pruned by rootworm feeding.
[00:04:40.930]And so this gives us a comparative way to evaluate root feeding.
[00:04:45.760]And then also, fortunately, this also is correlated with yield.
[00:04:50.590]some work out of Illinois several years ago,
[00:04:52.840]looked at a lot of trials over many years in many locations,
[00:04:57.400]and came up with this relationship between the node injury score and
[00:05:02.380]yield. As you can see, there's a fair amount of scatter about the points,
[00:05:06.160]but they found roughly 15% yield loss for
[00:05:11.110]every increase in a node injury unit,
[00:05:15.820]and some other research done at about the same time.
[00:05:18.340]Showed a 17% yield loss per node.
[00:05:20.890]So somewhere in the range of 15 to 17% yield loss for every
[00:05:25.450]increase in a unit of node injury scores.
[00:05:30.430]So we talked about insecticide resistance.
[00:05:32.920]One of the most recent things we've seen is,
[00:05:36.250]we've used a lot of foliar pyrethroids,
[00:05:39.640]particularly in Southwest Nebraska and in Southwest Kansas.
[00:05:43.490]Bifenthrin is a product that initially was labeled as Capture as
[00:05:48.490]a foliar insecticide now is Brigade and various other products.
[00:05:53.200]Initially in the late 1980s, it was used a lot for spider mite control.
[00:05:58.400]And then we saw that it has a very broad spectrum of activity.
[00:06:02.630]It's also active against Western bean cutworm, European corn borer,
[00:06:06.740]and rootworm beetles. And so in some areas, again,
[00:06:10.610]a lot of continuous corn in Southwest Nebraska and Southwest Kansas,
[00:06:14.990]for various reasons,
[00:06:16.220]they had been using a lot of this active ingredient over time.
[00:06:20.480]And we got reports of reduced efficacy and Nebraska entomologists with
[00:06:25.460]Kansas State entomologists did some work several years ago and evaluated the level of
[00:06:30.260]resistance to bifenthrin. So this,
[00:06:33.470]shows the results from the bioassays with bifenthrin,
[00:06:37.580]and you can see at the bottom,
[00:06:40.040]we have one hundred percent mortality and this dose was chosen.
[00:06:45.020]So this is susceptible populations would have 100% mortality.
[00:06:48.500]And if we have some degree of resistance,
[00:06:50.870]we would have less than a hundred percent mortality. So you can see at the top,
[00:06:54.710]we have locations in Southwest Kansas and Southwest Nebraska,
[00:06:59.090]where there's very little mortality.
[00:07:02.090]And as you go further east in Nebraska, we get more mortality.
[00:07:06.560]So there's a gradient across Nebraska.
[00:07:09.170]And once we get the east of the Missouri river, we,
[00:07:13.840]we typically had mostly susceptible populations.
[00:07:17.570]So we have a gradient of resistance in Nebraska to at least one of the
[00:07:22.520]Pyrethroid insecticides
[00:07:26.810]And a question that rose out of this is we also have some of our larval
[00:07:31.550]control soil insecticides or liquid insecticides at planting that are also
[00:07:35.390]Pyrethroids. Did the resistance in the adult stage had have an impact on the
[00:07:40.010]larval control as well. So Lance Meinke,
[00:07:44.810]Julie Peterson, and I did some cooperative work several years ago,
[00:07:48.920]where we looked at several of the planting time insecticides that are
[00:07:52.760]pyrethroids.
[00:07:55.550]And for each set of bars, the,
[00:07:59.180]the middle bar shows this location in clay county.
[00:08:04.130]the far left bar shows Saunders county,
[00:08:07.280]the Eastern Nebraska research and extension center,
[00:08:10.160]where we have susceptible populations and the general trend across all these
[00:08:15.080]different products is we had highly susceptible populations in Saunders
[00:08:19.820]county. And, actually it varied a little bit, but,
[00:08:24.560]either moderate or similar levels of resistance,
[00:08:28.820]we're here at clay center in terms of reduced efficacy of these soil
[00:08:33.110]insecticides. And so we have pyrethroid again, which is,
[00:08:36.890]Capture as a planting time treatment.
[00:08:40.550]we had tefluthrin, which is Force as a planting time treatment.
[00:08:44.750]And the last treatment is the active ingredients in Aztec granular
[00:08:49.490]insecticide,
[00:08:50.180]which is a combination of a pyrethroid and an organophosphate.
[00:08:55.440]So even though we initially
[00:08:59.820]had this resistance based on exposure to the adults that has implications for
[00:09:03.720]larval management as well. And again, as part of a statewide effort,
[00:09:08.670]this location showed the,
[00:09:10.920]the transition from Eastern Nebraska to Western Nebraska in terms of
[00:09:14.790]susceptibility.
[00:09:17.190]This shows one study we did in 2020,
[00:09:21.060]looking at several pyrethroid insecticides,
[00:09:24.540]FMC was doing some work evaluating foam formulations of insecticides.
[00:09:29.090]So we wanted to compare some of these newer products with standards.
[00:09:33.090]And if you look at toward the far right,
[00:09:36.960]we have the root injury rating score and
[00:09:43.260]we have Capture, and the Ethos product is also the same active ingredient.
[00:09:47.790]We also have Xylem XY way,
[00:09:49.770]which is a fungicide alone as a comparison.
[00:09:54.870]this,
[00:09:55.260]the fungicide is actually showing some benefits in disease control in some
[00:09:59.880]locations, but,
[00:10:03.120]we had similar results with the new formulations that we did with,
[00:10:08.010]previous formulations of the same active ingredient. And so that,
[00:10:12.270]that was worthwhile knowing
[00:10:15.180]we also did some work with different,
[00:10:18.420]BT corns and, with,
[00:10:21.840]and without a soil insecticide.
[00:10:24.840]And this shows that all the BT corns at this location were working,
[00:10:29.490]providing good suppression of rootworm feeding and adding a soil insecticide
[00:10:34.470]where the BT corns were working, did not have any benefit.
[00:10:40.380]And the last thing, again, we're looking at some, some newer products,
[00:10:46.470]and seeing that,
[00:10:47.880]some of the newer products again are compared to the standards.
[00:10:52.440]So we have some, some idea of how well they're looking, they're working again,
[00:10:56.220]concentrate on the root injury rating.
[00:10:58.860]Sometimes we don't see yield effects in our small plot studies because it takes
[00:11:03.660]a pretty large difference. We have a lot of variability in small plot research,
[00:11:07.800]and it takes a pretty large yield difference to,
[00:11:10.120]to have a statistical result in our, in our end points here.
[00:11:14.910]So the last thing we we'd done a few studies on adult control over the past few
[00:11:19.230]years. Again, this is data from 2020.
[00:11:22.500]We compared the several different modes of action of among the different
[00:11:26.430]products and to looking at,
[00:11:31.200]several days after treatment,
[00:11:33.030]you can see most of the treatments worked pretty well for at least a
[00:11:37.620]week or longer. Again,
[00:11:40.830]this was a little bit bigger plots than our larval studies,
[00:11:42.960]but still relatively small plot studies over time,
[00:11:46.170]the Beetles start moving around from the untreated plots or the plots,
[00:11:50.220]or even areas outside of our treated plots.
[00:11:53.290]And so it's hard to get a good handle on persistence in smaller plot studies.
[00:11:58.540]But, one thing Steward is a new mode of action,
[00:12:03.370]and that is working very well in our studies,
[00:12:06.400]as well as some of the older products that we've had for a while.
[00:12:10.330]Lorsban did not work very,
[00:12:12.970]long in this study and it may be related.
[00:12:16.660]It usually gives pretty good knockdown, but it particularly in hot weather,
[00:12:20.590]it volatilizes.
[00:12:22.240]And so may not have as long persistence as some other products.
[00:12:29.440]So these are some of the things we've been doing at this location in terms of
[00:12:33.490]rootworm management. as I said,
[00:12:36.940]one of the challenges we have with Western corn root worms is they're very
[00:12:40.270]adaptable.
[00:12:41.350]They've been able to develop resistance to our different insecticides,
[00:12:45.940]as well as BT corns.
[00:12:47.890]So we really need to continue to monitor individual growers,
[00:12:52.060]need to monitor how well things are working in their fields. And we have,
[00:12:56.950]testing programs at several locations at the state,
[00:12:59.750]the university at our research and extension centers where we're doing studies
[00:13:04.270]like this. So we can monitor performance of different products and,
[00:13:09.100]and the BT corns again,
[00:13:11.680]crop rotation is highly effective at reducing rootworm populations,
[00:13:16.300]and it really can help us play a role in extending the life of some of these
[00:13:20.170]other control tactics. If we have fewer Beetles,
[00:13:24.190]insecticides and BT corns work better,
[00:13:27.760]if we have very high densities,
[00:13:29.440]it's hard for any control tactic to work very well.
[00:13:32.740]So our main take home message is to try to use a variety of control practices
[00:13:37.690]against rootworms. Don't do the same thing repeatedly year after year.
[00:13:41.980]That's where we've had situations where rootworms have developed resistance.
[00:13:46.450]There are relatively few newer products coming down the road,
[00:13:49.870]whether it's insecticides or BT corns, or other transgenic corns.
[00:13:54.790]So we really need to take good,
[00:13:57.850]account of our current products and use them in a way that's sustainable,
[00:14:03.040]because we may not have a lot of new products coming out for several years.
[00:14:07.090]So that's, that's the main take home message.

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5 - 2021 South Central Ag Lab Field Day - Corn Rootworm Research and Management

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