Corn Rootworm Activity & Bt Resistance Updates
To the Third IPM Podcast sponsored by the Minnesota IPM Program here at the University of Minnesota. I'm Bill Hutchison, coordinator of the IPM Program. And today we have Dave Nicolai co hosting our podcast with U of M Extension and the Ag Professionals Program. Today we have Doctor. Ken Ossley with us.
Bill Hutchison:He's an extension entomologist and professor in the Department of Entomology at the University of Minnesota. We're really excited to get some insect pest updates for corn, what's been happening so far this spring and early summer. To get started, I think we'll just turn it over to Ken and give us a little more background on his experience.
Ken Ostlie:Well, good morning, Bill and Dave, and welcome to our audience out there. My name is Ken Ossley. I've been an Extension Entomologist at the University of Minnesota since 1984. I grew up in Western Minnesota, so I have memories of some of the pests hitting my father's fields during the course of growing up. Adventures like picking up corn from European corn borer infestations, gleaning the fields as it were.
Ken Ostlie:So I was intrigued during my graduate training by the interaction of plants and insects. Combined with my farm background, it led me to an interest in integrated pest management. My focus over the years has been in corn and soybeans, most recently focused on corn only. But there's been a progression kind of of hot topics that we've addressed over the years ranging from how to manage European corn borer in Minnesota gathering data relevant to the state and different parts of the state to extended diapause biotype of a northern corn root worm to the development of new technologies beginning first with the seed treatments and later with transgenic traits and finally the development of resistance by insects to some of those insecticides or traits. The thing I like about insects is that they are always a challenge, every year is different, and adaptation of insects can be phenomenal to observe.
Ken Ostlie:Obviously a key complex or key pest that's
Bill Hutchison:been on everybody's mind in recent years is the corn rootworm complex. Both western and northerns of Minnesota have been common. But before we get into Western corn rootworm, I think Dave has a question about black catworm.
Dave Nicolai:Well, Ken, this has been a really unusual year in terms of the weather fronts and so forth coming through here. As Bill mentioned, talk about some of the other insects. Can you give us maybe a little brief overview of where we are right now with black cutworm in terms of corn height? We know that there are planting delays in very extreme Southern Minnesota resulted in later planting smaller corn, but again the heat units have rapidly progressed as well. So let's just take a minute or so here to talk a little bit about that before we move back and talk about corn rootworms.
Ken Ostlie:Well of course we've had multiple flights of black cutworm into Minnesota. What it really boils down to with cutworms and corn is essentially a race and it's a race that develops along two dimensions. The first one is the progress of fieldwork. Cutworm females are attracted to crop residue and early season wheat growth and so the pace of field work takes those fields out of the equation as attractive egg laying sites. The second dimension of the race is literally related to the temperature and corn development and how that relates to cutworms.
Ken Ostlie:Broadly stated, cooler conditions favor more damage from black cutworms as does earlier flights, whereas warmer temperatures like we've been seeing the last couple weeks favors the rapid growth of corn. And literally corn can outgrow the ability of black cutworms to cut plants. And I think that's what we're seeing right now. We've got, of course, Minnesota a wide range of planting dates out there, but anything that was planted around the time of the black cutworm flights slightly before it by this point is largely beyond the cutting ability of black cutworms.
Dave Nicolai:So the take home message here is if it's later planted corn particularly in residue situations those are probably the fields to be scouting and taking a look at. Well, let's move on to talk a little bit more about another complex of insects. And if you think back to this spring, we had really a cool late spring, you know, we had a snowstorm in April, and then there was a lot of planting delays. How does that line up with the historical and the expected corn rootworm hatch and growth and development of corn? Then we had of course, you know, those extremely hot temperatures.
Dave Nicolai:Now the last week in May are anywhere between 90 to 100 degrees. So I guess the bottom line is is there a mismatch here between some of the root biomass and the corn rootworm larvae and the egg hatch? How are things timing out here for 2018?
Ken Ostlie:It's kind of interesting over the years that the timing of, you know, a corn rootworm hatch and corn development seems to be synchronized fairly well. In other words, because both corn and corn rootworms have a fairly similar response to soil temperatures, no matter what disrupt things we tend to have more synchrony than you might expect. But where it comes into play with corn rootworms this year is we have such a wide range of planting dates. Earlier planted corn, you know, that's now around V5 V six, that corn is all producing a root system that will be easily colonized. But when we're starting to talk about some of those fields out there that may be V2 or V3, we can see a reduction in successive colonization.
Ken Ostlie:So planting date can have an effect and the later planting date typically occurs. The smaller the root system the poorer the colonization and the threat of damage is reduced in those fields. Getting into corn rootworm biology and management for 2018 in a little more detail, I know one of
Bill Hutchison:the questions on the minds of a lot of growers and crop consultants is where we might be at in the state for BT resistance in corn rootworm this year. And so we're just curious if you can give us an update. Obviously Minnesota has been one of
Ken Ostlie:the states that's been hit pretty hard in recent years with with BT resistance. Can you give us an update on that and what you might expect to see this coming year? A quick sketch on where we're at in terms of BT resistance development would be that evidence of problems probably peaked around twenty thirteen-twenty fourteen. Since then we've had a decline in the frequency of observable problem fields in large part to shifts in grower behavior becoming more aggressive about corn rootworm management switch from single rootworm trait hybrids to pyramids of two and nowadays even three rootworm active traits and increasing use of soil insecticide and probably the biggest factor is increasing use of crop rotation. For all of those reasons we've seen a reduction in the frequency of issues and the thing that growers may not be thinking too much about but that's been incredibly important are weather effects on this whole situation.
Ken Ostlie:The '14 was extremely cold somewhat reminiscent of what we saw this last year, and it had a big effect on Western cormorant worm populations so that reduced the threat. Just to backtrack slightly and remind our listeners that we've seen no evidence of resistance development yet in Northern corn rootworm. It's only been Western corn rootworm where we've seen the problem. So that means we don't have to worry about the extended diapause aspects when it comes to BT resistance problems and instead we're just looking at corn on corn. The one caution I give everybody is that while the appearance of problems has diminished, the intensity of resistance in rootworm populations has not diminished, it's still extremely high.
Ken Ostlie:All it takes is two or three years of conditions favoring buildup of root worms and we could see problems appearing once more.
Dave Nicolai:Ken, without getting into the details of individual traits, can you just maybe make a comment or two about some of the things that are out there in terms of traits that people might have that they should be watching a little bit more closely?
Ken Ostlie:In terms of looking at the spectrum of traits that you might have in the portfolio you're planting this year, the biggest advantage we've got at the moment is the single traits, rootworm traits, are disappearing. The pyramids are being widely planted nowadays. Of course cost issues with seed and production of corn have created a pendulum swing back towards reducing seed costs. Growers have been able to get a hold of single trait seed, we've seen it embrace single traits as well as Roundup Ready only or above ground traits only and even conventional. So we've got a lot more of the landscape which is being planted to corn with little or no to BT resistance resistance to corn rootworm.
Bill Hutchison:So Ken, one thing you just alluded to regarding current corn rootworm conditions in Minnesota and the BT resistance, You have been doing some monitoring for resistance in the state. Can you just give us a little more detail on how you're doing that and what you're finding so far?
Ken Ostlie:We're looking at a variety of fields, some of them have had performance problems reported to us, some of them are just fields with heavier corn or worm pressure that we want to get a feel for how that population handles the various BT traits. So we'll go into the field, put out a study with all available traits, and then we monitor what happens in terms of root injury, lodging, yield, and probably the most important thing is putting in cages that collect emerging corn rootworm beetles so we get an idea of how effective the treatments are. In other words, how effective these trait and trait combinations are. The interesting thing in those ten years of study we've been doing, and especially since resistance really blossomed, is that corn rootworms populations, once they develop resistance, don't backslide and suddenly lose that resistance. In fact, no matter what the population level in the field, they're still maintaining resistance to these traits.
Ken Ostlie:So that means that when populations decline and the growers see the problem disappear that they could fall into a trap of erroneously thinking we've managed our way out of this problem that multiple traits or insecticides or mother nature have eliminated the issue and we don't have to deal with it going forward. The sad thing is that all it will take is two or three years of favorable weather to build that corn rootworm population and we could see performance related resistance problems again.
Bill Hutchison:So one thing you're also saying is that really a comprehensive approach is needed to continue to monitor populations in any of these Midwest states to really look at where the populations are at. For example, just looking at route ratings in mid summer may not be enough, just watching for lodging may not be enough. It sounds like looking at the adult activity at the end of
Ken Ostlie:the year is really a key. Bill, that point is very crucial. It's the one piece of information that going to allow growers to decide how they really need to manage that field. It will tell them if an issue is developing and it will also potentially point them to what direction they need to head. For example, if their unconventional corn populations are building up it might be time to add rootworm control to the equation.
Ken Ostlie:Or if they've got Bt traits in the field and they're seeing a population building, that could be the tip-off that there's a resistance issue looming down the road. Scouting is critical.
Bill Hutchison:I know you may not have a specific number on this, but do you have any idea from what you've heard of winter meetings this year in some parts of the state in terms of less? What percentage of non Bt corn might be in the ground this year? Is that still up for speculation?
Ken Ostlie:Well it's clear that there's been a trend towards increasing use of corn without BT protection against real worms. That varies a lot from area to area, grower to grower, so I don't have a real good feel on an exact percentage. But what growers need to keep in mind is if they're in a pocket where either because of the amount of land they're controlling or that's going towards non trait protection or their neighbors, that as that percentage of non trait protection increases creating a scenario where there's less check on the corn rootworm population and a chance for build up to occur more easily. Two more questions about the VT resistance issue and I don't mean to put
Bill Hutchison:you on the spot, but also curious about between the single event for corn rootworm versus some of the pyramid events. Has any of your research or results from other states shown that there's any evidence of cross resistance in Western corn rootworm to two or more of the BT proteins that are on
Ken Ostlie:the market? From the first year resistance showed up, our studies as well as those around the Midwest have shown that there's evidence of cross resistance. If a field develops, for example, a problem with VT triple, that it potentially has problems with two other proteins. The thing that those proteins have in common is they're all what we call CRI3 proteins. That means that we've got in one fell swoop because of cross resistance we now have three of the four available traits could be having a problem in a field and that creates a challenge when it comes to the pyramid because every single pyramid has at least one and possibly two traits where we're seeing performance issues related to resistance.
Ken Ostlie:One final question I have about the Corn Rootworm Complex in Minnesota and given some of the dynamics you've already alluded to with 2013 being a tough year for Western Corn Rootworm. And we know that Northern Corn Rootworms are still present in the state with more corn being grown in Northwestern Minnesota up in the valley. What's happening with Northern Corn Rootworm right now? What have you seen in
Bill Hutchison:the last few years in terms of trends between the two species?
Ken Ostlie:Well, for about ten years, Northern corn rootworm populations have decreased dramatically. That decrease coincided with the rapid buildup of western corn rootworm populations as the westerns developed resistance to BT traits. So the challenge that we had was just trying to answer the question why in the world was northern corn rootworm populations declining? Because western corn rootworms aren't a threat, rotated corn. And as we were pondering that question about three years ago crop consultants started to report an increase in Northern corn rootworm presence and in fact in some areas populations became big enough to start to raise concerns about whether or not Northerns were developing resistance to BT as well.
Ken Ostlie:That's a question that's not been resolved at this point. Even though populations are generally low for corn rootworms in general in Minnesota right now, we encourage growers not to neglect their rotated corn. Look for evidence of northern corn rootworm population size and whether or not they're starting to see any buildup.
Dave Nicolai:We're closing in on the end of our time here, Ken, but I wanted to just briefly have you comment just maybe a little bit about the management strategies that growers might be able to implement and you can comment on when you think they're very successful or not or whether it's in realm of possibility and that is one is layering a corn soil rootworm insecticide, a traditional one along with a BT, and then also any other management strategies. And some of those could include actually doing a little bit of scouting, whether it's be at albedo and or even digging up corn roots later in the summer. Do you want to comment briefly about the opportunity to put a soil rootworm insecticide along with a BT? Is that sustainable or is that a short term strategy?
Ken Ostlie:In terms of corn rootworm management options, the one that still is kind of the bedrock is crop rotation. Where possible we still encourage growers to use crop rotation. It's a way to manage building populations in fields by dropping them down dramatically, but as we look at traits for example, of course pyramids offer a more effective performance than single traits. The challenge right now is the operating budget growers have to work with and that's where you see some growers making a choice to use either what remaining single trait seed is out there or go to no trait at all, that reduction in BT is a calculated risk. The reduction in BT traits is a calculated risk.
Ken Ostlie:They're banking on low populations. Many of these growers have not scouted fields. They don't have a real good feeling for population intensity in the fields and whether or not they could potentially get by with little or no protection. Of course soil insecticides could be option. The challenge there is many of these growers have literally gone away from soil insecticides in favor of seed based technology.
Ken Ostlie:And just
Bill Hutchison:a quick follow-up, going forward even this summer or into the fall for growers or consultants who would like to follow this more closely, get more information out of your research program, do you intend to have some new updated articles like in the Minnesota Crop News or other outlets?
Ken Ostlie:Well shortly we'll be producing an updated version of Scouting for Corn Rootworms that will focus on research advances that have taken place in the last couple of years. I've had a graduate student Tricia Leaf finish up her research on scouting with sticky traps and that's allowed us to update thresholds to look at the different types of sticky traps and create thresholds for each different kind as well as reduce by 33 to 50% the number of traps for our scouting effort that's needed. We're at a point where we're looking at this scouting in the sense of how much data do we actually need to make these decisions. Right, and that'll be really useful going into the fall and winter as growers are making decisions about which hybrids to plant, how much BT, how much conventional, etc. And so it's a tool to help us tailor rootworm management to where it's actually needed.
Ken Ostlie:Can we dispense with some of these costs? Hopefully it will help growers make a better decision about each individual field and in the process reduce pressure to develop resistance by the root worms.
Bill Hutchison:So all in all for corn root worm management this year, so far so good. It looks like growers can be looking forward to a productive season And it's good to hear that more and more growers have also gone back to crop rotation, I think is a foundational tool in addressing both the pest in general, but is also for the resistance management aspect. So with that, Ken, we will sign off for now and hope to speak with you again about some other corn pests as the year develops.
