Alfalfa Weevils

(Jim) Good morning folks, welcome to That’s My Farm. I’m Jim Shroyer, your host and we’re in luck because we’re standing in an alfalfa field in Riley County. And we also have Dr. Jeff Whitworth with us, our Extension Entomologist and he’s going to talk to us about alfalfa weevils and what to expect from it this coming year. So, stay tuned, we’ll be right back after these words from our sponsors.Closed Captioning Brought to you by Ag Promo Source. Together we grow. Learn more at

(Jim) Good morning folks, welcome to That’s My Farm. I’m Jim Shroyer, your host. And we’re in luck because we’re in an alfalfa field in Riley County and we also have Dr. Jeff Whitworth with us. He is our Extension Entomologist and we also have Dr. Holly Schwarting with us as well. She’s going to be out in the field with us. But we’re going to be talking with Jeff, even though he says, Jeff says he has more of a radio face than a TV face. So, Jeff, with the temperatures that we’ve had in January and February and into March here that have been well above normal for the most part, I’m thinking of some pest problems in alfalfa and alfalfa weevil being the number one problem. So, kind of take it from there. (Jeff) It is Jim, it’s the number one problem in the state of Kansas throughout the state in alfalfa, every year. The only nice thing about the alfalfa weevil is it only comes once a year, but it comes before the first cutting. So, this year, you know I heard it’s like the sixth or seventh warmest winter we’ve had on record. And that affects insects because temperature allows insects to completely control insect development. So, I’ve seen alfalfa weevil eggs start hatching. And the eggs right now are in the stems. of the alfalfa plants, whether the stem is horizontal or vertical it doesn’t matter, the alfalfa weevil female lays the egg in the stem. OK? So, I’ve seen the eggs hatch anywhere from the last week of February through mid-May. But that’s about it. We consider the alfalfa weevil a cool weather insect. By that I mean they do best from 45 to about 80 degrees, while it’s cool. That’s why they’re out there in the first cutting. (Jim) Right. (Jeff) And they don’t affect the second or third cutting whatsoever. OK? So, anywhere from the last of February to the mid part of May, you’ll have the eggs hatch or in some years we have them hatch throughout that whole period of time. That’s what makes the alfalfa weevil such a difficult pest to try to control. They’re voracious. The eggs hatch. They hatch into little tiny green, yellowish larvae. This year we’ve found them the first of March already. The first week in March they’re starting to hatch so if we don’t get cool weather or cold weather, I think we’re going to have an early hatch. (Jim) So you’re saying though, even though it’s been warm, if it cools back down that slows their development as well? (Jeff) It does. The temperature range for the alfalfa weevil, remember I said it’s a cool weather insect. They develop between 45 and 80 degrees; 45-48 degrees Fahrenheit is the cutoff. So anytime the temperature is below that, they’re not developing. We actually have a thermal unit accumulation system or a growing degree day system we developed that you can tell exactly where the different stages of the alfalfa weevil is, based upon thermal units. You can accumulate it right out in your own pasture or your own field. (Jim) That’s why it’s so important when it’s so warm in January, February and early March, that keeps ’em going, kicks ’em into gear and we may have to treat earlier. (Jeff) Yes, that’s exactly right. The alfalfa weevil starts laying eggs in the fall. It lays eggs in the stems like I said, whether they’re horizontal or vertical, it doesn’t matter. They’ll lay eggs anytime the temperature is over 45 degrees starting in October. So, October, November, December, January, anytime the temperature is over 45 degrees the adult’s active. They feed a little bit, but not much. But they’re laying eggs in the stems. So, when those eggs are in the stems, those eggs are also developing a little bit inside the stem anytime the temperature is over 45 degrees. It takes 180 to 300 thermal units or growing degree days before these eggs will hatch. Starting whenever they were laid, October, November, December, whatever. (Jim) Hold that thought, we gotta take a break. We’ll be right back after these words from our sponsors.

(Jim) Welcome back to That’s My Farm. I’m Jim Shroyer and with us we have Dr. Jeff Whitworth, our Extension Entomologist and we’re talking about alfalfa weevils. We’ve got the eggs laid, we’ve got ’em hatching, how do you know there’s any damage? What do you look for, how do you do it? (Jeff) Exactly. Every year it varies, based upon temperature. I just said temperature completely, 100 percent, drives insect development as it does plants really. (Jim) Sure. (Jeff) So as soon as the growers start finding a little bit of shot holing or pinholing or damage on the terminal of the alfalfa as it breaks dormancy. They’re not going to be out there til it breaks dormancy. You’ve got to wait til you start seeing a little green out there. But that’s the reason we developed the Thermal Unit Accumulation System or the Growing Degree Day System because that’s available on the KSU Extension Entomology website. You can download that. You can use it in your area. And starting January 1st you can start accumulating thermal units. Once you get to 150 to 180 thermal units that means those eggs are going to start hatching in your area. OK? (Jim) Right. (Jeff) So, that’s when you go out and start looking. You don’t need to worry about it until you get that many growing degree days accumulated. So, once they do that, start going out and looking in your alfalfa. Probably 10 years ago we used to have a hard time finding alfalfa weevils that would infest fields at that 50-60 percent level. But in the last three or four years, they’re 100 percent in every field. I don’t know why that is, it just is. OK? So now they’re easy to find. So, now it’s not a matter of if I need to treat my field, but when I need to treat my field. (Jim) Right, right. (Jeff) So that’s why it’s very important to get out, once you’ve accumulated those thermal units or those growing degree days, get out and start looking for those little pin prick holes in that new alfalfa, the green parts of the alfalfa. Once you find that, you can open them up and you’ll find the little worms. Those worms are really small, OK? They’re kind of a yellowish, greenish color. But they’re really small. I can’t over emphasize that. They are really small. OK? So, some of us older guys, you may need to use the reading glasses to see ’em. (Jim) Magnifying glass. (Jeff) Yes. But that damage will start showing up and those worms, as soon as they hatch, they feed 24/7. Anytime the temperature is over 45 degrees, those worms are feeding 24/7. A lot of times the growers in Kansas they’re not as busy during the early part of March or April, so what they’re doing is looking at their wheat and they’re looking at their alfalfa because that’s what’s out there growing right now. They’re very interested in if anything is affecting the alfalfa or the wheat. So, as soon as they start seeing the pin prick holes in the alfalfa, a lot of them call up their local applicator and they want to get on the list to get that alfalfa treated. OK? (Jim) Pronto. (Jeff) Pronto, right away. As soon as they start seeing those holes because they think, I want to get on it early. (Jim) Stop it in its tracks. (Jeff) Nip that infestation in the bud. (Jim) But is that a good thing? (Jeff) It doesn’t exactly work that way. (Jim) OK. (Jeff) OK? Because remember I said those eggs are starting to be laid in October, November, December and they develop a little bit anytime the temperature is over 45 degrees all winter. But they’re also laid in January, February, and March. The adults are still out there. They’re still laying eggs. (Jim) So what you’re saying is if you treat too early, the insecticide effectiveness, efficacy will diminish and then there will be more that are coming out? (Jeff) That’s pretty good for an agronomist. (Jim) Oh thank you sir, thank you. (Jeff) That’s exactly right. If the insecticides that we have that are labeled for us in alfalfa, they’re going to last and that means the residual activity is going to be there for two to three weeks. Maximum. (Jim) OK. (Jeff) So, if you start having those eggs hatch first week in March, they’re just starting to hatch the first week in March, they’re probably going to continue to hatch for another at least three or four weeks, maybe even two or three months, depending upon the weather. (Jim) We gotta take a break. Hold on to that thought. Stay with us folks, we’ll be right back after these words from our sponsors.

(Jim) Welcome back folks to That’s My Farm. With us we have Jeff Whitworth and Jeff tell us,continue the line of thought here. (Jeff) Well, the growers in Kansas this time of year, early spring, late winter, whatever you want to call it, they’re looking at their wheat and their alfalfa. So the first thing they see usually are little pin prick holes in the alfalfa, the new alfalfa from the alfalfa weevil. And a lot of time they call up their local applicator because they want to get on the list early, they want to get ahead of the game, spray the weevils, kill the weevils, so they don’t have to worry about it for the first cutting. The alfalfa weevil will actually decimate the first cutting if it’s not treated. At least it has in year’s past. (Jim) And it has a lingering effect too. (Jeff) It does, yes. It’s a snowball effect on the alfalfa since it’s a perennial. But anyway, what they want to do is get on the list early; get it sprayed early. Plus you have the vagaries of the weather, right? (Jim) Right. (Jeff) In the spring we can have wind, it’s Kansas, or we can have rain, sleet, snow, hail for three or four days. That’s going to stop the application of the insecticide. (Jim) But not the bugs. (Jeff) But the bugs are going to continue. Anytime the temperature is over 45 degrees, those insects are feeding 24/7. (Jim) I’ve heard that. (Jeff) And they can decimate those plants. They will defoliate completely alfalfa plants in three days. We have some really good pictures of our plots where the control that we didn’t treat were decimated completely in three days. Three days! I am still amazed that these little tiny worms can actually completely decimate that. (Jim) So, what about some thresholds? How do you know when to get your biggest bang for your buck? (Jeff) So, the thing is, when you first start seeing the damage, the little pinprick holes, go away. Go to the Gulf for a week, take Spring break or something, don’t go back and look in the field. OK? Wait a week or two weeks, especially if it cools down. If it’s below 45 degrees, our average temperature, 50 degrees those insects are going to continue to hatch, they’re going to continue to feed, but not quickly. So, anytime it’s over 60-70 degrees is when they do the most damage. So, just relax. When you start seeing those pin prick holes, you know they’re there, you know those eggs are starting to hatch, but don’t get too excited yet. My treatment threshold, alfalfa is a little bit different for a crop, it’s not like corn or soybeans. Alfalfa is grown locally. A lot of growers use it for their own use. They grow some, feed their own cattle, or they sell it to their neighbor, so treatment thresholds are a little bit different, but our treatment threshold that we use when we put out our insecticide trials is 50 percent. One larvae infesting or one larvae per stem. OK? A 50 percent infestation. (Jim) OK. (Jeff) Or sometimes you might want one larvae per three stems. We found out that works really well. You’ll still have more eggs to hatch, but you put out that insecticide at that time 50 percent infested, one larvae per two stems, that seems to work pretty well to kill the larvae and the eggs that were going to hatch over the next two or three weeks, which most of them that will take care of those. What you need to remember and what a lot of guys are confused about is all these insecticides are contact insecticides. They’re not systemic. They’re not moved in the plant. (Jim) Jeff, hold that thought. We’ve got to take a break. Folks, we’ll be right back with these words from our sponsors.

(Jim) Welcome back to That’s My Farm. I’m Jim Shroyer and Jeff Whitworth didn’t run off during that last break. Glad you didn’t either. Jeff, let’s get back to that 50 percent threshold, one weevil larvae per two stems. How do you know you got that? (Jeff) That’s a good question. It’s really easy making the determination on when to treat alfalfa weevils, I think is one of the easier commodity groups that we have because the stems are sticking up there. You just go along with a little white bucket, one gallon bucket and you collect so many stems at random, shake them in the bucket, the larvae fall out. Remember how many stems you had, count the number of larvae and even an agronomist can figure out… (Jim) No probably not, have to have a calculator. (Jeff) But OK, if you do 20 stems, you’ve got ten larvae, that’s your 50 percent infestation. (Jim) Right. (Jeff) Do that in several spots in the field. It’s time to treat. Like I said, the alfalfa weevil is one of the two pests that we have in the state, in my opinion, that you really don’t need to figure out if you’re going to need to treat it, it’s when. (Jim) OK. (Jeff) And the problem with the alfalfa weevil is the timing. It comes in early like we talked about. You’ve got weather changes because we’re switching over from winter to spring at that time. You’ve got all kinds of problems there. So, 50 percent, get out and check. A lot of the consultants will use sweep nets. You can go out and sweep sample alfalfa, you can collect a lot of larvae that way, but we’ve never been able to correlate that to an infestation level. So, some places there might be a whole bunch of larvae. You pick those up, some other places there’s not. We always recommend the stem shake in a bucket method. I don’t know what you call it. But anyway, that’s the most reliable method. So, once you decide you need to treat, you need to get on it. Fifty percent, that’s the start. Like I said, some people will use 30 percent, one per three stems, because that way they call the applicator and he gets out there as soon as possible. (Jim) Because in a few days it will be. (Jeff) Yes, before it hits the 100 percent. The problem with the alfalfa weevil, like we talked about, they will feed anytime the temperature is over 45 degrees. And they will completely defoliate plants in three days. And like you said, it snowballs for the rest of the year. That plant is not going to come back… (Jim) As quickly. (Jeff) Don’t donate your first cutting. The alfalfa weevil, the only nice thing about the alfalfa weevil is they’re only out once a year. So, as soon as the temperatures get 80 to 85 degrees, they will leave the alfalfa fields. Now, I have seen them in Kansas…it’s been cool into June so they’ll stay out there. But they don’t produce any more larvae. (Jim) Right. (Jeff) They’re only going to have adults. The adults will feed a little bit, but not as voraciously as the larvae do. But they can cause what they call barking. (Jim) I was just going to mention that. And a number of times you get called out, I cut my alfalfa and it’s not coming back. You want to look for adults. (Jeff) Right. Sometimes when you swath your alfalfa, even if you treated your field, even if you got 90-95 percent control on the larvae, as you accumulate that wind row, you’re still putting whatever larvae and adults that are still left in that field in a much more concentrated or small area. So, they will continue to feed underneath that wind row because even if it’s 90 degrees it’s shaded underneath that wind row. So, they will feed on that. That’s where you get the striping of the field. (Jim) Right. (Jeff) So, when you go back and you bale the hay, you have that yellow or that shorter alfalfa underneath there. That’s the reason. Guys will say, well the insecticide didn’t work, or the weevils were resistant to that insecticide. No, it’s just there‚Äôs, even 90-95 percent control is acceptable, but if there’s a lot of them, you’re not going to get as good a control as you would like. (Jim) OK. Thanks Jeff. Folks, we’ll be right back after these words from our sponsors.

(Jim) Welcome back to That’s My Farm. Jeff Whitworth is with us. Jeff, what happens after you spray? (Jeff) After you spray the weevils, there’s more pests to follow. (Jim) Well, that’s good news. (Jeff) Yea, it’s always great. It’s called job security for entomologists. (Jim) There you go. (Jeff) The thing is you sprayed the weevils. Everybody will spray the weevils. Otherwise you donate that first cutting. If you spray the weevils, that’s going to kill all the other insects that are out there. (Jim) Good or bad. (Jeff) Good or bad. Doesn’t matter. These insecticides are not discriminatory for the most part. They’re all contact insecticides, like I said. So, whatever comes in contact with them, is gonna die. The thing is there’s a lot of lady beetles. Lady beetles overwinter as adults. You can find them out in the alfalfa field right now. We’re finding pea aphids. Because pea aphids come in early, late winter, early spring. So, the lady beetles will feed on pea aphids. The problem is when you spray the alfalfa for the weevils, you’re killing all of the lady beetles, you’re killing the pea aphids, that’s good. But they aren’t going to come back very quickly, because those adults you killed, those are the ones that are the next generation. But those aphids, they’re going to keep migrating in, and all of the aphids are females. And all of those females produce females. They don’t have to waste time looking for a mate. They don’t have to waste time looking for a place to lay eggs. So, sometimes those aphid populations can just explode and then that… (Jim) Especially after a cool period? (Jeff) Yes, yes. The pea aphid is a cool, again, another cool weather insect. So, you have the pea aphids, no beneficials to help control. Beneficials, lady beetles, green lacewings, little wasps, they’re great at helping to control these aphids on all of our crops. OK? But then the pea aphids early on, later on, you might have some cow pea aphids, you might have some spotted alfalfa aphids, no lady beetles, no beneficials, because you sprayed them to protect your crop for the alfalfa weevil. So the alfalfa weevil is a problem, but that continues the problem also. If you don’t spray it, like you said, it really hurts the next cutting and the next cutting in the alfalfa because there’s no leaf area there. (Jim) But if you do spray, you’ve got these other problems that are going to come up. (Jeff) If you do spray, it exacerbates the problem with the other aphids. And then there’s some other problems. Like we have some different worms some army worms, army cutworms. The army cutworms may be near there now. The army cutworm is a really neat insect, in my opinion as an entomologist, because those eggs are laid in the fall in alfalfa fields, or wheat fields. Those worms again develop any time it’s over 45-48 degrees. And they will be out there feeding in the winter also. (Jim) And you can see, a lot of times you can see crows or black birds out there in alfalfa fields. (Jeff) Seagulls or yes…anytime you see a flock of birds in an alfalfa or wheat field, they’re probably feeding on army cutworms, early on in the late winter or early spring. (Jim) Jeff, I really appreciate you taking time. And say thank you to Holly to do the sweep net there as well. (Jeff) I will. (Jim) So thanks. Folks, thanks for being with us on That’s My Farm. And don’t forget, next Friday, about this same time, we’re going to have another issue of That’s My Farm. See you then.

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