Alfalfa weevil and nutrient management update
I'm Ryan Miller, Crops Extension Educator. Earlier this morning, we recorded an episode of the Strategic Farming Field Notes program. Strategic Farming Field Notes is a weekly program addressing current crop production topics. A live webinar is hosted at 8 AM on Wednesdays throughout the cropping season. During the live webinar, participants can join in the discussion and get questions answered.
Ryan Miller:An audio recording of the live program is released following the webinar via podcast platforms. Thanks. And remember to tune in weekly for a discussion on current cropping and crop management topics.
Claire LaCanne:Welcome to today's strategic farming field notes program from University of Minnesota Extension. These sessions are brought to you by University of Minnesota Extension and generous support from your farm families through the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council. So we're happy to have you join us today for updates on alfalfa weevil and nutrient management. My name is Claire Lacan, University of Minnesota Extension Educator in Crops, and we welcome Anthony Hanson, extension educator for integrated pest management. And then Brad Carlson, extension educator in nutrient management and water quality.
Claire LaCanne:With that, I'll turn it over to Anthony with our alfalfa weevil updates.
Anthony Hanson:Sure. Thanks, Claire. And, yeah, we're kinda flipping the script a little bit here today. So I'll be kind of, talking more about content this time instead of hosting. But, yeah, a lot of times this time of year, we are talking about what's going on in the alfalfa, especially with alfalfa weevil.
Anthony Hanson:So that's one where depending on where you are in the state, that's, gonna matter based on just how far along the crop is. Some folks, depending on what system you're in, they've already done their first cutting. They're looking the second versus here in about central Minnesota. I'd say we're, you know, just starting to start take that first crop a lot of places or just starting to think about cutting pretty soon here. So So when you're out there scouting in those cases, you're gonna find some alfalfa weevil larvae.
Anthony Hanson:But as you head further down south, I know Bruce Potters mentioned that, he's found some larger larvae and definitely some fields that should have been cut a little earlier, and we'll get into that in a little bit and kinda what's working with alfalfa weevil there. So I, in general, don't have a lot of new news about alfalfa weevil in terms of what we can do differently about it, compared to previously years. But it's one of these. It's a really good example of integrated pest management because we kinda need every tool in the toolbox we have for this pest here because insecticide is looking for some challenges there. And there's some other issues with this one too.
Anthony Hanson:It makes it a little bit trickier for pests to deal with there economically. So I think, just to start off, if you haven't scouted for alfalfa before, a lot of times what we'll do is you'll take a sweep net just to see if you have adults out there. You might find find a few larvae in that sweep net, but we usually don't use that for determining actual thresholds. What we'll do instead, and it's a little bit more accurate this way, if you go out into a field to spring a 5 gallon bucket and you wanna be cutting stems and then basically beating those plants in the bucket, it's a little bit easier way to actually capture some of those larvae out there and see what kind of counts you can get. So if you're gonna be looking up any of our online resources through extension for alfalfa weevil thresholds, all you need is your stem height, average number of weevil larvae per stem.
Anthony Hanson:And from that, if you have your insecticide costs in there, that's pretty much all you need to read those tables as well as, what you value your alfalfa at. So there's a few different things that go into play there. But if you're at a point right now where you're thinking about cutting the next few days, don't worry about spraying for alfalfa weevil. Think about more, managing that stubble afterwards because part of it is a lot of our insecticides. They have a limited window, for a pre harvest interval.
Anthony Hanson:So some are just, not legal to apply if you're gonna be cutting pretty soon here. So you gotta keep that in mind a little bit. But one of the big questions too is erythroids. And we're, you know, pretty confident that there's probably resistance out there, but it hasn't been confirmed in Minnesota yet. It's definitely been seen in western states, and that's making things pretty tough for our management strategies right now.
Anthony Hanson:So, if you do suspect pyrethroid resistance in your field, if you have a history of failures with those products, so that gets into ones like Warrior. You get Bifenthrin and a few others. Those products, you know, might be at risk here. Some of the other ones we can use, we have, organophosphates. Chlorpyrifos is back just for this year as far as we know.
Anthony Hanson:But there's only a few products out there that we know of that have been approved so far for this year. So Pilot 4 e is one that's come up. Warhawk is another for some trade names. Those are the only 2 I know of that have been registered in Minnesota this year. So if someone's looking at those products, it's, maybe gonna be a little more limited supply possibly.
Anthony Hanson:So it's looking like a pretty narrow toolbox. The only other insecticide we have is, Steward or Endoxcarb. So there's only 3 insecticides we can rotate through in that case. And one of the new recommendations is every 3 years, basically in that cycle only apply one insecticide once per year at most and then rotate to one of the other ones. So if you're having to spray twice in 1 year, you're already limited in the toolbox that's left there just because of these resistance concerns.
Anthony Hanson:So that's kind of the short version of what's going on with alfalfa weevil, with the problems we're facing there. So if you wanna be out there scouting, be sure to be doing that before cutting. Use the threshold tables if you can do that. If it is just after cutting, be sure to look at that stubble, especially if you didn't get those windrows, raked up know, pretty quickly. There can be some feeding underneath there.
Anthony Hanson:You can do what we call stubble scouting. And in that case, it's about 8 larvae per square foot in the ground, especially if they're these big alfalfa weevil larvae, later instars. In those cases, they might be able to survive a little bit more, might cause more damage, and that has caught some people off guard there. So, Claire, I don't know if you have, you know, any thoughts on, kinda what you're seeing in your neck of the woods or hearing about some other folks there on the alfalfa pest side of things.
Claire LaCanne:Yeah. I haven't heard in in kind of the, you know, Rice Dakota County area. I haven't really heard of people having alfalfa weevil issues, but I was just gonna ask Anthony if you could remind us the timing of when we should be scouting and maybe when the most damage might occur too.
Anthony Hanson:Sure. So we do have some, resources online. There'll be some links provided there. We do some, phenology or degree days for alfalfa weevil. And now this gets a little tricky because some of the models, you know, aren't as exact, and this is just for what we call the eastern strain of alfalfa weevil.
Anthony Hanson:There is potentially another strain coming in, but the short version of that is for alfalfa weevil, at least this eastern strain we've had in Minnesota normally. We scout up until larvae are basically developing into pupae. So that's, basically stage right before they turn into adults. If it's pupil stage or adult, they're not feeding on the alfalfa essentially. They might be a little bit of feeding from the adults.
Anthony Hanson:They don't cause economic damage. It's not worth trying to manage those adults. So, basically, once you don't find larvae anymore, you don't have to be scouting. So right now when we look at the degree days, that pupil stage is should be showing up roughly in Northern Iowa right now or at least as of Monday. Please jump ahead to, this coming Sunday.
Anthony Hanson:That should be June 2nd. It should be just on our southern border, Minnesota, just across the border that we should be seeing those larvae stop feeding and develop into a tube either. So we're getting closer hopefully to the end of the season. But the problem in this mix here is that western strain, they tend to develop about a week or 2 later. So I would say we probably have 2 more weeks.
Anthony Hanson:You definitely want to be out there scouting, see if you have anything. We don't know the status of that, western strain throughout the state, but it was on our western borders back in the nineties. So it's been a little while. That's some work that could be done in the future there, but that's one just to keep an eye out for that, we may be looking at these other strains out there causing some of these issues where in past years, I've seen, damage in fields up to about July 1st. So, we could have extended seasons.
Anthony Hanson:We'll see what this warmer weather maybe push development along a little more, and we might see the season end sooner. We don't know yet. So that's why you wanna be out there scouting. I would recommend against any just insurance applications on insecticide. Like I mentioned, we have very limited options left.
Anthony Hanson:It's higher chance you're gonna burn yourself out in terms of, what options you have. Probably not gonna get a return on investment in some of these cases if if they're doing a consent set of applications too early. So just be mindful of that one. We do have parasitoid wasp out there too, I should mention. Those ones do help suppress the populations, but they're susceptible to our broad spectrum insecticides.
Anthony Hanson:So that's the catch 22 with some of these. You know, there's a lot of opportunities to do something. There are insecticides that may cause problems further down the line. And that's just for alfalfa weevil. I should mention down in Iowa, they've been running a little warmer than us in Minnesota.
Anthony Hanson:They had a more continuously warm spring. They had some pretty serious issues with alfalfaebo a couple weeks ago, but also PAFID is another one I wanna keep an eye out for. I haven't heard reports in Minnesota yet, but considering how thick they were in some fields, down in Iowa, definitely something to be out there scouting for. Because it may be spotty in certain fields, but it's definitely possible we're gonna see some that reach some economic thresholds in the state too there.
Claire LaCanne:Yeah. And sometimes we see flare ups of pea aphid and alfalfa weevil after insecticide applications like you're saying, Anthony, because then we end up unintentionally, but still killing off the parasitoids. Or are there other natural enemies of alfalfa evil?
Anthony Hanson:There there could be some generalists that would be eating on them too, especially if they fall to the ground. But primarily, USDA actually released some Paris toid wasps that were very specific to alfalfa weevil, back in the seventies eighties, and they did pretty well. Like, for me, growing up on our farm, actually, I didn't have to deal with alfalfa weevil very much until, you know, the last 5 years or so. And we always wonder, is that maybe the parasitoids helping keep populations down as possible, or are we having issues with those parasitoids if populations are dropping for those ones? So there's a lot of unknowns with what's causing all these alfalfa weevil flare ups.
Anthony Hanson:There's a lot of potential, options that, you know, haven't been explored too well yet. Others, you know, look pretty sound at least for ideas. So we're keeping an eye on what may be happening with alfalfa weevolt across the state. And hopefully in the next year or 2, we'll, have a little more capacity to do some testing on these pyrethroid resistance questions. We're gonna see what goes on with that one there.
Anthony Hanson:One thing I do wanna mention too is we do or that we have some resources for alfalfa growers out here too. So the alfalfa scissor cut program, if you haven't heard of that, that's one where you can get an idea what's going on across the state, especially central Minnesota, some southern counties, just to see what they're reporting for height, maturity, and so on. So I know at least as of May 23rd around Stearns County at Freeport, we're looking at heights about 23 inches tall, bud stage for alfalfa. But you get further south, a lot of folks have been, cutting already too. So if anyone's interested in that, they can look that one up.
Anthony Hanson:I think we'll have links provided in the description for that one too there. So that's definitely the resource we wanna make sure alfalfa growers have. We're talking about a lot of our other crops sometimes, but definitely this time of year, as a lot of folks know, it's the time we're really thinking about alfalfa if you're growing it.
Claire LaCanne:Thanks, Anthony. Before we hand it over to Brad, we have a question that's in the bug world, the kinda entomology related, but, off the alfalfa weevil topic. So, if you want to answer live here, you can. Otherwise, you can type in the q and a. But do we expect that we might see more pressure from western corn rootworm and or northern corn rootworm in corn this year?
Anthony Hanson:Yeah. That's possible. And I talk a lot about insect forecasting a lot of times. And the angle I'll talk about is, you know, we had a warm winter. And we really even though we had a bare, soil basically most of the winter, it wasn't a case where we really plunged down in temperature.
Anthony Hanson:So a lot of those insects that were in the soil, they're pretty either protected or they really weren't stressed too much by the winter. So if you are in a field where you know you had high populations last year, winter wouldn't have helped you out necessarily. So if you know if you're in a high risk situation this year, it's probably gonna be the same. For folks that, you know, have some low level populations, it's possible. But, yeah, that's one where you just wanna be out there keeping an eye each year, especially later in the season, trying to see what emergency you get.
Anthony Hanson:And I know Bruce mentioned here too that, you know, we might actually see larvae drowning. We always like to talk about that. If we get a lot of rain, that's very stressful on these, soil or, insects here. So that's one where you get pathogens in there or just enough water that it actually makes it enough to actually get enough oxygen in there too. So we'll see what happens with that one there.
Anthony Hanson:It's gonna be pretty variable. I know some parts of the state, we got, you know, some pretty heavy rain. Might get some help there. Others, if it's more moderate, I wouldn't bet too much on that wiping out the corn rootworm issues there, but keep an eye on those, especially as adults begin to start, emerging there, and you might see them in other fields too.
Claire LaCanne:Anthony will hand things over to Brad for our updates around nutrient management.
Brad Carlson:Alright. Thanks, Claire. Been getting a lot of calls since it's been so wet the last, week or or 2 now about whether we've lost nitrogen. Probably the short answer is yes. Although I don't know how much it's been in a lot of cases.
Brad Carlson:I think, everybody's aware that we've, had 3 very dry years in a row. Last year, we had actually kind of similar conditions to what we're experiencing now in that it got extremely wet early in the year. Although last year was a little different because it came pre plant and so it was slightly earlier, the soils were cooler, and it delayed planting. A lot of the rainfall this year, of course, happened during the planting time, and there were there were adequate windows to get most of the crop planted, but the timing of that was a little bit different. Either way, in both cases, we've reached field capacity both years.
Brad Carlson:Most of our soils, if they're they're heavier textured soils, our clay loam soils, our glacial soils will hold about 10 to 12 inches of water. And so anything excess of that, it's either going to, percolate through the soil into the drain tile or it's gonna run off across the surface. And and so kinda gives us a starting point, as far as, where we expect the the, moisture or the water, so water situation to be, in the profile. That's significant because the loss processes of nitrogen are water based. And so either nitrogen, when it turns to nitrate, well, first of all, it has to turn to nitrate for either of these processes to happen.
Brad Carlson:The nitrate molecule being negatively charged does not adhere to the soil. So as water percolates through the soil, it will move with the water downward. How much? Well, it's obviously, it's going to just depend on a lot of factors, but kind of a rule of thumb that we use is that nitrate will move down about 6 inches in the soil profile for every inch of of drainage that we've had. So, you can kinda look at the, the the where you're at as far as field capacity and how much rain fell.
Brad Carlson:It's hard to know how much ran off and how much is going through the drain tile, but you can figure that some of that nitrogen probably did move. It's pretty safe to say that almost all of the nitrogen that was applied, this year is in the nitrate form now. There's, it's been warm enough and enough time has passed that, that that probably is the case. Obviously, anybody that's left, you know, split split split applying and still has part of their nitrogen to put on, that'll be a different story. But if you put the, full amount on, pre plant, it's probably all in the nitrate form.
Brad Carlson:And so the other process in addition to leaching, is denitrification, and that's a biological process that happens when the soil is completely saturated. As with anything that's biological in nature, it takes time to get going. It doesn't start immediately. The the, microbes that that that do this conversion, take some time to, get functioning and to multiply in the soil. Typically speaking, the first 24 or 48 hours, we don't see a lot going on.
Brad Carlson:And as with anything that's biological, it also is temperature dependent. And so the warmer the soils are, the greater the amount of loss we would see through denitrification. Denitrification is a process where that nitrate, turns into nitrogen gas and then just goes off into the atmosphere, and is lost. So usually, because, again, it's biological and it doesn't, doesn't, a lot of it doesn't happen. The soils are cold.
Brad Carlson:We don't look at that as being a significant loss process prior to planting because the soils typically are cold at that point in time. The average date we hit 50 degrees soil temperature at the Southern Research and Outreach Center in Waseca is May 1st, and most microbes don't do much when it's below 50 degrees. But now we are in a situation where the soil temperatures are much warmer. If you wanna figure that, a lot of our nitrogen has moved down to maybe 8 inches deep, or 10 inches deep, We see that the 8 inch soil temperature at Waseca is probably in the vicinity of 65 degrees right now. And so that is warm enough to see some significant completely saturated at 4 days, we could see that the loss of of nitrogen is about 6% at 60 degrees, but it doubles to 12% when it gets to 70 degrees.
Brad Carlson:If it saturate for 10 days, the loss is either 12% at, at, 60 degrees or it more than doubles at 26% at 70 degrees. And so really it's kind of important to kinda keep track of that. Fortunately, the total saturation part of this is, you know, primarily low in the landscape. It's not field wide. In a lot of cases, unfortunately, we've received enough water that the places we're going to see severe denitrification are also the places that drown out.
Brad Carlson:So you probably don't have a crop there to worry about whether it needs nitrogen or not. The the place where this is going to get real severe, looking forward is is if the soil temperatures, the soil temperatures will continue to warm, but if it stays saturated as we get into June, when the soil temperature approaches 80, degrees, you can lose almost half your nitrogen if the soil stays saturated for 10 days. Now, 10 days is a long time. We don't see that happen very often, but it it can happen. Now the the other side of this is because of how dry it's been the last few years, there has been a fairly significant in a lot of cases, accumulation of soil nitrate that would normally be lost on an annual basis.
Brad Carlson:We know from analyzing soil samples that were sent into Minnesota Valley testing labs last year, that a large portion of them had residual nitrates kind of above what we normally see. There's usually a kind of a background level that's up to about 6 parts per million that we don't really worry about because it's kind of always there. After a drought, because the loss processes of nitrogen are water based, when there's no water, we will see more nitrogen accumulate. And so if you took a pre plant soil test and you credited that amount of nitrogen, you reduced your application rate, you're probably cutting it close, and I would be, watching very closely what the soil conditions are to see if if this is might be a problem for you. However, if you didn't credit that, there's a good chance that what you've been losing so far is some of that accumulated nitrate leftover from last year, that probably was not part of your fertility program in the first place, and so it's probably not of a lot of consequence.
Brad Carlson:I guess really it's gonna kinda depend on on where we go moving forward. Kind of a couple of rules of thumb this time of year, We use as kind of a standard value for evapotranspiration. That's the amount of water we evaporate from the cell surface or used by the plant at about a tenth of an inch a day. When the, corn plants are this small, bean plants are this small, they don't take up a lot of water. So most of that's just kind of what we evaporate on a daily basis.
Brad Carlson:So it takes about 10 days, to lose about an inch of water, just through that method. It goes up to about doubles when we get to v 6, but of course, we're not we're not quite there yet in most places. So so you can keep kinda keep track of the moisture status in that way. Now I think the other thing of this is I I heard, somebody from the state climatology office this morning talking about the potential for a high pressure ridge setting up, which I think most of us that have paid attention know that has the, tendency to push rain on away from, us here. And so it is possible it's gonna just simply dry out now over the next couple of weeks and, probably not have a lot of consequence at that point if if that does happen.
Brad Carlson:However, if it does continue to rain, we are gonna have to keep closer track of this.
Claire LaCanne:Thanks, Brad. Yeah. Lots to think about on the the moisture front and, kind of changing gears a little bit. But, yeah, how should we think about our timing for split applications? You know, what are your thoughts or research on that?
Claire LaCanne:Does it have to be at v 4 like we
Brad Carlson:hear sometimes? Or No. There's there's there's kind of a a misnomer out there that that you want to spoon feed the crop. There's really no such thing as spoon feeding, nitrogen in the soil, knows no freshness. The plant does not tell the difference between whether you put it on yesterday or it was just simply, there from a fall application.
Brad Carlson:It's either there or it's not there. And so from that standpoint, if you're going to be putting on split applied nitrogen once we get to this time of year, just simply do it when you can get out and the field conditions are adequate because, you're only talking the difference of a week or 10 days timing. The odds of losing nitrogen in that period of time are very small. So, just simply get out and get the side dress on as soon as you can. Of course, it's it's too wet for that in in most places right now.
Brad Carlson:But I would guess, based on kind of what I'm seeing as far as field conditions in most places, I would bet we're gonna start seeing that in about a week.
Claire LaCanne:Thanks. Yeah. Should we be thinking at all about, like, collecting tissue samples to help diagnose potential nutrient deficiency later on, or what are some things to keep in mind if we do think we should collect samples?
Brad Carlson:Well, the the problem with tissue testing is is a lot of the the, experience we've had with that shows that there's some pretty big differences simply based on the genetic background of of a corn plant. And so unless you've got some numbers perhaps supplied to you by the company that you bought seed from, it's very hard to be very precise. In a lot of cases, it'll tell you if if we just simply don't have enough or if we've got adequate, but then in the middle, if you wanna know, well, do I need some, you know, rate more, we're not calibrated to do that. You know, similarly, an in season soil test has become more popular lately. Technically, University of Minnesota, doesn't have that as a, a recommended practice.
Brad Carlson:However, Dan and Fabian have been doing quite a bit of research on that the last couple of years. What Dan has said is, he is fairly confident if you take a 1 foot soil test, this time of year, in this with a standing crop, if the, if you get back a result that is 26 parts per million soil nitrate or higher, you have adequate fertility to grow the crop. You do not need to supplement it. And that's, again, when we get below that, ex exactly trying to calibrate the you know, at what soil test level you need, what rate, that gets more difficult. However, it is a tool you can use if you feel like you put the full rate of nitrogen on and you're worried that you, that you don't have enough left, you can, when it's, the conditions get better, you can get out and take a 1 foot test.
Brad Carlson:And, again, if it's above 26, you can feel fairly comfortable that you probably have enough nitrogen out there to grow the crop.
Claire LaCanne:So the soil test more so than
Anthony Hanson:Correct.
Claire LaCanne:Tissue test.
Brad Carlson:Correct. Yes. Yep.
Claire LaCanne:Yeah. Any questions for Brad at all? We're coming up on our time here. Or, Brad, if you have any other final thoughts or, I don't know. We could
Brad Carlson:I I the only, I guess, the only thing other thing I would say, Claire, is is you can kinda keep an eye on your drainage systems if you want to. We usually recommend drainage design for Southern Minnesota being at a half inch drainage coefficient. However, a lot of drain tile went in a long time ago at a 100 feet apart, which is not a half inch, and then farmers seeing that that was not adequate, just simply went in and split the lines. Half inch drainage coefficient in most of our glacial soils, like a a a Webster soil or a Glencoe soil, a LaSuea soil, something like that is about 60 feet apart. And if you if you shorten that up to 50 feet, which might have been what happened if you split 200 foot lines, that actually goes up to 3 quarters of an inch.
Brad Carlson:That's the amount of water it'll take away in 24 hours. And so you can kind of look at, how much rain falls from the sky and how well the tile is flowing and kind of get a feel for how much water is coming out of your field. That's one more, little tool you can use, for for trying to estimate whether you might have lost some nitrogen.
Claire LaCanne:Thanks. Yep. No. That's a good good thought there. And, I have a question, in the chat here of if there are any worries where water has been standing in fields for nitrogen loss.
Claire LaCanne:You touched on that a little bit, but, you know, do people need to think about putting more down?
Brad Carlson:Well, again, I think a lot of what I've seen in the places where I'm worried about that we've had a lot of nitrogen loss, we've also got drawn out crops. And so I guess the key there is, does it dry out fast enough you're gonna replant? I mean, at this point, we're heading into June. Are you gonna get corn back out there? I don't know.
Brad Carlson:I mean, if you do, you probably do need to think about putting some nitrogen on with that. If you got the ability to put a high rate of starter on with the planter, that probably will supplement, what was lost in those areas. I guess as far as if it wasn't wet enough to kill the crop, I I personally, unless you really shaved it tight on on rates, and I know there's there's a crowd of people out there who, really like to play it tight with that. You know, our our university recommendations have kinda crept up towards about £150 an acre for corn following soybeans. You know, if you're still putting on a 110, 120, you you might be kinda tight, you know, but if you were at 150, which is kinda where our recs are, I don't I don't think you're probably gonna be in a lot of trouble yet.
Claire LaCanne:Thanks. Thanks for that thought, and we'll wrap up here. And I just wanna thank everyone for attending our University of Minnesota Extension field notes program and thank our sponsors, the soybean research and or research and promotion council and the corn research and promotion council.
