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Alfalfa weevil and pea aphid alert

Richard Snell
Barton County Extension Agent

Beware of the weevil! If you are an alfalfa grower, you had better keep that in mind. Alfalfa weevil is the most destructive pest we have on alfalfa in Kansas. I want to alert you farmers that this is the time of year to be scouting your fields for alfalfa weevil in the Golden Belt.

They can rob you of a ton per acre if left unchecked. Then if you still don’t control them, the adult weevils can totally wipe out your stand. Some of you have been raising alfalfa for years and could probably teach me a thing or two about weevils and alfalfa. However, I know that every year we have some new growers that don’t know much about the finer points and even for you old-timers this might be a good refresher.

In some areas and in some years, you might get by without treating for weevils, but anymore, it is rare with the number of acres we have, that you can get by without spraying. You see that is the good thing; there is something we can do about it. Sometimes we have pests that when I asked what to do, there is no simple answer. In this case the cost of spraying will usually pay big dividends.



Sometimes I get asked, what about other methods besides chemical treatments? Removal of stem material (which destroys fall-laid eggs) by grazing, burning with a match or flaming with a propane torch may reduce weevil populations. Kansas State has done several studies on this. Timing is important and the possible long range effects on alfalfa plant survival and productivity varies. Tiny parasitic wasps and fungus diseases that affect the weevil have also been tried and do help limit the damage.

There are some alfalfa varieties that tolerate low infestation levels of alfalfa weevil larvae. However tolerant varieties do not possess adequate resistance when we have moderate to high levels of insects. Some of these varieties have erect, glandular hairs or sticky hairs that have been bred into them that help keep the weevil from moving as rapidly and doing as much feeding on the plant.



The two main things that I want to talk about are chemical treatments and how to scout for weevils. Timing of treatments is important so that you don’t have to spray twice and as I mentioned earlier, not every field merits a treatment. I don’t want you spending money that you don’t need to.

The easiest method of determining whether you need to spray is the stem count method. Go to each corner of your field and one or two places out in the middle so that you can sample in 5 or 6 different places. Then cut off 5-10 stems in each of those areas at the ground level. Then shake them into a bucket and count all the weevils and divide by the number of stems. Oh yeah, one other thing, you need to determine the height of the average plant or at least establish a height range in inches.

The larvae (worms) that you are looking for are yellowish-green to dark green, black-headed and legless. They have a distinctive white stripe down the middle of their back. Even when mature, they are only about 1/4 of an inch long. They may be seen in our area as early as April 1 some years, but in some years they don’t show up until the first of May.

The larvae eat the leaves and foliar damage is not obvious at first. Young larvae create scattered pin holes in the top leaves and developing buds. As the hatching weevil larvae increase in size and number, leaf damage increases. Leaf loss results in a reduction of quantity and quality of hay. Most damage occurs on the first cutting unless they come in late.

However, carryover losses can occur when there are high infestations. In an Indiana study, almost one ton was lost on the second cutting, .6 tons on the third cutting and .25 ton on the fourth cutting, even though no direct damage occurred after the first cutting. This was a replicated study as compared with where they sprayed adjoining hay with an insecticide.

The adults usually aren’t seen until the middle of May and they normally don’t show up at all until October unless you don’t spray. If not treated, they will do damage on the second cutting.

The price of hay has something to say about the economics of spraying. Right now, with good hay prices, you probably should spray 5″-8″ tall alfalfa when you have an average of 2 worms per stem. When you get into 11″-14″ hay, which is probably where it will be when you read this, it takes 2-1/2 weevils per stem before you spray. If you are at 2 per stem at that size, resample it in 3-5 days. As you get to taller hay, 17″-20″ tall hay should be treated when there are 3 weevils per stem. If it gets past 20″ tall and you haven’t had to spray, then I might just consider cutting it early and watching close to see if you need to spray the stubble later.

Scout your fields now! However, don’t wait too long to pull the trigger on spraying. Also be on the alert for pea aphids. Many predators, such as lady beetles, were killed by the spring freeze so pea aphids are plentiful.

For more information, pick up a free copy of the K-State bulletin, Alfalfa Insect Management 2009 at your local extension office or get it online at http://www.oznet.ksu.edu/library/ENTML2/MF809.pdf. It has a summary of the treatment thresholds and the labeled insecticides.


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