Old and New Summer Soybean Insects
Welcome to the fourth episode in our twenty nineteen series of the IPM podcast. I am filling in for Doctor. Bill Hutchison and Dave Nicolai as co hosts. I'm Anthony Hansen, an extension postdoc at the University of Minnesota in the Department of Ecology. So today we'll be discussing summer soybean insect management.
Anthony Hanson:We have Doctor. Bob Cook with us today. So Bob, do want to introduce yourself briefly? You've been on the podcast series before. We'll probably have some people that may not have heard from you before.
Bob Koch:Yeah, thanks for inviting me to chat with you today. I'm the soybean extension entomologist stationed on St. Paul but have responsibilities for insect and mite pests in soybean fields statewide. So we've got no super pressing issues right now but some things could develop over the summer so I think our growers and agriculture professionals should be aware of some of these issues.
Anthony Hanson:So we have two main pests we'll talk about today. The first one common for most people know about already is soybean aphid. So want to give a general update on what's happened so far this year or general outlook of what's happened maybe over the winter up to basically current day?
Bob Koch:Yeah, Anthony. It's actually been a pretty slow start to the year for soybean aphid. We can find them in scattered fields throughout much of at least the southern half of the state, but the percentage of plants infested in any given field is pretty low yet and the actual numbers of aphids per plant is still very low. We have not heard of any fields anywhere near a treatable level. Remember that the recommendations are to be scouting the fields regularly and using that threshold of two fifty aphids per plant to decide when to apply the insecticide.
Bob Koch:And that two fifty aphids per plant is not where we're experiencing any economic losses. That's a trigger point to try to knock that aphid population down before it reaches higher levels where they would actually be causing that economic damage. And again, we're not aware of any fields in Minnesota that are near those levels yet.
Anthony Hanson:So how long should growers continue scouting throughout the year? Know it can get to the August, we should be scouting for it. There's a general point when the aphids leave too, right?
Bob Koch:The aphids will eventually leave the soybean fields and go back to Buckthorn. That's more so driven by temperatures and day length. Recommendations are to continue scouting through R5, maybe even into early R6 over that period of time using that threshold of two fifty acres per plant through R5. And that should help you head off any of the trickier management situations which occur when you get large aphid populations in R6. We don't have a very solid threshold for R6 soybeans But we do know large infestations well above that economic threshold can cause losses at that point.
Bob Koch:So kind of hopefully if scouting regularly using the threshold through R5 you can knock those populations down and prevent those tricky R6 infestations. But if you get into R6 and if you have a large infestation and maybe some other stressors going on with those plants, it might make sense to treat even into early R6. Once you get into later R6, yield is already pretty much set in those beans so it doesn't make much sense after that.
Anthony Hanson:So for the current time in the summer also for these thresholds, about how long do growers have once they reach that two fifty threshold to go out and spray?
Bob Koch:Yeah, so it's typically probably figure five days to a week or something like that under average environmental conditions and the aphid population growth rates are based on those environmental conditions. So if you hit two fifty aphids, you've probably got five days or so to make an application to that field and knock it down before it grows to damaging levels.
Anthony Hanson:So if we do get aphid population growth high enough where we have economic damage potential, what options are currently available? Because I know we've had insecticide resistance issues in past years. Are there any indications of what's going on this year or new options available to growers?
Bob Koch:Yeah, so that's a good point there Anthony. We've detected insecticide resistance in soybean aphid populations in Minnesota for four years now. So going back to 2015 and it's been issues with the pyrethroid insecticides mainly Lambda cyhalothrin and Bifenthrin. It's too early yet this year to say what the situation is going to be. But due to the fact that we've seen these resistance problems for four years consecutively and over fairly broad geographies in Minnesota and neighboring states and provinces, there's really no reason to believe that the problem has gone away.
Bob Koch:So I'm going into this aphid season assuming that we're going to have pyrethroid resistant aphids that we'll have to be dealing with. And that really complicates things, right? Because we've been relying a lot on pyrethroids for management of this pest over the years and getting some use of the organophosphate chlorpyrifos over that time. And now with the pyrethroids not working as well, lot of people are switching over to using chlorpyrifos more. And that's a concern to me because that greater use there could potentially drive resistance to that insecticide.
Bob Koch:Other than those two groups of insecticides, the pyrethroids and the organophosphates, previously we really only had the neonicotinoids. Those are mainly used on the seed treatments but there are some foliar formulations so they do get some use as well. But fortunately for our growers, we've got a few new products from different insecticide groups or subgroups that are now available for use in soybean. So I'm not sure how many years ago it was. Several years ago we got access to Flupirodifferum or Civanto.
Bob Koch:And that's proven to be effective against aphids. I think the price point's a little higher there. More recently for this year, we now have access to Afidopyropin, which is a new product from a new insecticide subgroup. And the product name is Saphena. It's proven to be very effective against aphids in our research across multiple states and pretty gentle on the natural enemies so you can apply it to kill the aphids and not have such an impact on the good predatory insects in the fields.
Bob Koch:And sulfoxiflor is another product that just regained its registration for use in soybean so that the product name would be Transform. And that's a product that we had available to us a couple of years ago but the registration was removed so we can no longer use it in soybean for a couple of years I think it was. But the EPA just announced that we've got access to that product again for use in soybeans. So with the resistance issues for the pyrethroids and some of the regulatory issues related to the organophosphates, I'm really happy to have some of these other insecticide tools available to us and I think it will allow us to have better insecticide resistance management programs where we can alternate among more insecticide groups and subgroups rather than just the two that we were mainly using before.
Anthony Hanson:So sometimes the question comes up, it's pretty perennial now where people will be out spraying for weeds and different herbicides and the question comes up, should I throw an insecticide in the mix as well? And there have been some recommendations on that. Don't know if you could reiterate what's been happening with recommendations for tank mixes whether it's herbicides or insecticides or potentially for resistance management when you have different modes of action.
Bob Koch:Yeah, so for the mixture there's kind of two different issues there. For insecticide resistance management, our recommendation is ideally to be using products with individual modes of action, active ingredients, so not the mixtures. Now that's different from what you're hearing in regards to herbicide resistance management where those mixtures are a pretty important part of those resistance management programs. But for insecticide resistance management, trying to focus on using the products with individual active ingredients or modes of action then alternating those as needed. As for mixtures of say herbicides and insecticides, you're going through the field applying the herbicides so why not just put in some insecticide just in case.
Bob Koch:And generally, we do not recommend that approach. I think the first reason would be that some of the volumes, pressures, nozzles, conditions that you're using for application of the herbicide are probably not ideal for application of the insecticides. And secondly, the timing at which those herbicides are being applied is probably not the optimal timing for application of the insecticides. So for soybean aphid, like I said, the density or numbers of aphids are still very low in the soybean fields. So if you're applying that insecticide now, thinking that you're getting some benefit against soybean aphid, it's probably really not going to be providing any benefit.
Bob Koch:Just because you have a few aphids in fields, it doesn't mean that they're necessarily going to increase to damaging levels. So it could be a potentially wasted application. And in addition to that, those additional applications are increasing the risk for resistance development. And on top of that, a lot of those insecticides that we would be using are going to be very toxic to the predators like lady beetles and the other good insects. So if you're knocking those good insects out while you're knocking out those few aphids that might be there, If that field gets recolonized by aphids, they could potentially have a free for all with no predators in that field to try to help contain or suppress their populations.
Anthony Hanson:So you mentioned some of the insects that prey on soybean aphids as well. I know there's been some mention of parasitic wasps especially in the last few years. Do you want to give a little bit of information on what's been happening there?
Bob Koch:Yeah, so these parasitic wasps, they're tiny little wasps and a lot of people freak out when they hear the word wasp thinking of yellow jackets and things like that that we see flying around late in summer and that can sting people. These are stingless wasps in the sense that they won't sting people. Their stinger is actually an egg laying tool. So they make their living by injecting eggs into aphids. The eggs of the wasp will hatch inside the aphid and the larvae will develop in the aphid, eating it and eventually killing it.
Bob Koch:And then that larvae will turn into a pupa and then an adult. And then eventually that adult wasp will chew its way out of the carcass of the aphid and start its life cycle over again. That remaining carcass of the aphid, that dead aphid body, turns kind of crusty. And for this one particular group of wasps, it's kind of a black coloration. And this new wasp that we've been seeing over the last several years is called Aethelinus certus.
Bob Koch:So it makes these dried crusty aphid carcasses which we call the mummies, it makes them turn black. So when you're scouting for soybean aphid throughout the state really you'll probably have pretty good chances of seeing some of these mummies from Aethelinus certis on the plant. So keep in mind that they're just one more type of beneficial insect that's working in your favor helping to kill off some of the aphids. But keep in mind that these parasitic wasps are very sensitive to many of the insecticides that we're using for soybean aphid management, especially the pyrethroids and organophosphates. So that's one more reason to be using that threshold of two fifty aphids per plant because it gives these good insects, the lady beetles, predatory bugs, the parasitic wasps, a chance to work against those aphid populations and potentially keep them from reaching outbreak levels before you actually have to apply an insecticide.
Bob Koch:Plants are already in the ground but seed treatments come up a lot as well as resistant varieties. Do you want
Anthony Hanson:to give a quick update on that before we move on to
Bob Koch:other insects? Aphid resistant soybean varieties, the research still continues to show that those are very effective and those are resistant due to some aphid resistance genes called rag genes in those plants. Unfortunately, they're not very widely available. Right now their availability is more catered to the kind of organic niche market of soybean because those growers have even less tools for use in managing soybean aphid. But if a grower can find and get their hands on some of these aphid resistant varieties, I guess I would encourage them to at least experiment with them on their farm because they can be very effective for suppressing aphid populations and protecting yield especially if you can get varieties with multiple resistance genes in them.
Bob Koch:And the other tactic that you mentioned was the insecticide seed treatments. So that would be the neonicotinoids that are applied to the seed and those can do a great job protecting the soybean seed and seedlings from soil pests and other early season pests if they're present. However, for soybean aphid management, a lot of times the aphids are coming into the fields too late to really get much benefit from these insecticidal seed treatments. As the plants grow, as time progresses, the concentration of the insecticide in the plants that was taken up from that seed coat decreases and will eventually get to a point where it just doesn't provide any effective control of the aphids anymore. But if you have a situation where fields are historically colonized by aphids earlier in the season than other fields or maybe it's late planted soybean like soybeans after peas where you're planting closer to a time when aphids are going to be colonizing.
Bob Koch:Those are situations where you might get higher likelihood of a return on investment for those treatments.
Anthony Hanson:So we have a relatively new soybean pest here in Minnesota, at least it's a new problem in the last few years, soybean gall midge. What's been happening with soybean gall midge in the last few years that's become a problem now?
Bob Koch:Yeah, so the soybean gall midge, it's a tiny fly and the larvae of this fly are little maggots. They're kind of whitish colored when they're young and then they turn orange color, the maggots. And they occur under the epidermis of the stems of the soybean plants, usually just above the soil line. And kind of the thing to be looking for would be a darkened kind of discolored part of the stem. Sometimes it's swollen.
Bob Koch:And then those larvae are often right underneath the epidermis of the stem feeding away on the tissues of the plant. This insect really exploded in the Midwest last year. Large areas of Nebraska, South Dakota, Iowa and a bit of Minnesota were experiencing pretty heavy infestations. This year we have found it again in all these same states and in Minnesota in particular we're seeing infestations in Rock County, so down in the Southwestern corner of the state. This insect prior to this winter was essentially unknown to science.
Bob Koch:It just got a scientific name. So we really know very, very little about this insect in terms of its biology, its impacts, its management. So we're scrambling to do as much research as we can to try to get that information to be able to turn it around and get some recommendations out to folks. So this year, we're kind of watching infestations hoping to receive reports from folks to tip us off on additional infestations so we can figure out how widespread the pest is. And in addition to that, find some additional sites where we can perform some research.
Bob Koch:But right now, this year, all that I'm aware of is in Minnesota is some fields in Rock County.
Anthony Hanson:So if growers suspect a problem with soybean gall midge, what should they be specifically looking for and who should they send information to?
Bob Koch:Yeah. So the gall midge infestations, this pest it's overwintering in the soil. And in the spring it'll emerge from the soil from fields that were infested the previous year. And then they're going to fly to what we assume is going to be looking for the nearest soybean. So typically the fields that are of highest risk are going to be fields that are near fields that were soybean the previous year.
Bob Koch:A lot of times the infestations are heaviest on the edges of the fields. So you kind of see that pattern where they're moving in from that adjacent field that was soybean the previous year. But they have been seen to move in a pretty large distance into the fields. Again, what to look for would be when you're looking at those edges near fields that had soybean the previous year, look for that darkening along the base of darkening of the stem at the base of the plants. As the season progresses or as that injury progresses within the plants, they're essentially girdling the stem.
Bob Koch:So you'll see those plants starting to wilt and then they'll eventually die. And we're seeing that progression happening already in some of those fields in Rock County now. So I think those would be kind of the key things to be looking for in terms of scouting or trying to see if fields have infestations with this pest.
Anthony Hanson:So are there any options currently available for management of this?
Bob Koch:Yeah, so that's the challenging part now is we really have no management recommendations that we can give to our growers. Once those larvae are in the stems of the plants feeding, they're pretty well protected and probably pretty much untouchable in terms of getting insecticides to them to try to kill them. Seed treatments appear to be not effective. You know, some of these infestations that we're seeing are in soybean fields that were planted to insecticide treated seed. And you know, those observations were from last year and from this year.
Bob Koch:So the seed treatments appear to not be giving much, if any, protection. Foliar insecticides could potentially be an option for management of the adult soybean gall midge. However, the challenge is going to be timing. We really don't know when the adult flies are active. We don't know how many generations we're going to see throughout a year.
Bob Koch:And we don't know over how long of a period of time that those adult flies are going to be active. And we need to know all of that to be able to figure out when to start spraying, how many times we would need to spray or how frequently we would need to spray to be able to protect the plants. So that's where all this research that we've got going on through some checkoff funded multi state grants that we've been able to secure. We're looking at the seasonal dynamics. When are the flies emerging?
Bob Koch:When are the larvae showing up infesting the stems? So just a lot of that foundational biological information around which we can develop pest management recommendations.
Anthony Hanson:Are there any other insects that could be coming up in the summer that would be something to scout for in the field such as potentially different defoliators which we generally don't see too much but can occasionally be a problem?
Bob Koch:Yeah, this year we've already been seeing a pretty high abundance of some of the defoliating caterpillars throughout a lot of the state in soybeans. So some of these caterpillars are going to be a thistle caterpillar, green clover worm. So I think since we're already seeing a fairly high abundance of those, it's something we're going to want to keep our eyes on through the growing season. And we want to be scouting the fields, estimating defoliation of the soybean plant, so what percentage of the leaf area has been removed by the pest. And when we do this we want to make sure we get a good estimate of defoliation for the whole soybean canopy.
Bob Koch:So that means selecting soybean plants from throughout the field and selecting soybean leaves from the top, middle and bottoms of those plants so we get the whole field covered and kind of the whole height of the canopy covered as well. And then average those estimates of percent defoliation across those plants and those leaves and the thresholds that we're recommending for the different defoliating insects combined. So that could be these defoliating caterpillars that I mentioned or bean leaf beetle or grasshoppers. If the defoliation from any or all of these insects reaches 30% prior to the reproductive soybean growth stages, that's where we would recommend an insecticide application. And in the reproductive growth stages of soybean, it would be 20% defoliation.
Bob Koch:And again, soybean is a very resilient crop so I think these are pretty conservative thresholds to be using. So it should allow some time once you hit that 30% or 20%, allow some time to be able to line up that insecticide application and knock those populations down before they get to damaging levels. However, you want to make sure that if you're seeing high levels of defoliation out there, you want to make sure that those caterpillars or whatever the defoliating insect is still there. So maybe using a sweep net to go through the field or looking closely at those plants. Now you hear about Japanese beetles sometimes.
Bob Koch:Has that been a problem in Minnesota yet? Or is it more out east and it's starting to spread out this way more? Yeah. So Japanese beetle has been in Minnesota for decades, especially in the urban areas, Twin Cities, Rochester. But over the last several years, populations have been growing and it's been spreading out into some of the surrounding rural agricultural areas.
Bob Koch:So we are seeing, at least the last couple of years, we've seen pretty significant defoliation from Japanese beetle. Some fields were pushing treatable levels based on those thresholds that I mentioned before. This year, the Japanese beetles started emerging earlier this month and we're seeing some defoliation already but not a whole lot yet. But it's something we should definitely keep our eye on and as we're managing the defoliators, factor in defoliation from Japanese beet along with the caterpillars or the other insects kind of altogether in that decision making.
Anthony Hanson:Thanks again Doctor. Cook for coming to talk to us today here. And this will conclude our fourth episode of 2019 IBM Podcast.
