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Alfalfa Weevils May Be Active This Year
Thursday, April 16, 2015 10:20AM CDT

By Cheryl Anderson
DTN Staff Reporter

OMAHA (DTN) -- Livestock might not be the only thing munching on those lush, green alfalfa fields this spring. Due to a milder winter in some parts of the country, alfalfa weevils could take a bite out of the crop this year.

Although it is still a bit early in the growing season to know the extent of alfalfa weevil levels, some state experts are already predicting they may be a challenge this year.

Warnings have already gone out in Missouri, Iowa and other states that alfalfa weevils may be problematic this year, due largely to areas with milder-than-usual winter temperatures that allow adult weevils and eggs to overwinter.

Alfalfa weevils can be found in many alfalfa-growing states, Wayne Bailey, associate professor and University of Missouri Extension entomologist, told DTN. Bailey said Missouri consistently has issues with alfalfa weevils every year. He said he believes that is simply because Missouri is the second-largest state for cow-calf production, so a lot of alfalfa is grown to supply those operations.

"Weevils are a problem in Missouri due to the sheer number of alfalfa fields we have," he said.

Adult weevils move to alfalfa fields in late summer as temperatures begin to cool off and lay eggs during fall, winter or spring -- whenever the temperature rises above 60 degrees Fahrenheit for at least three days. Each female can lay up to 500 eggs, each of which will produce a small larva. Typically, eggs are laid inside plant stems and begin to hatch in the spring, usually in May and June.

Weevils have several larvae stages, Bailey said. After the egg hatches, the growing larvae will crawl to the top of the plant, even when the alfalfa is only 6-8 inches tall. The larva initially hides in and eats the growing tips of emerging alfalfa, but as the larvae grow, they will feed on foliage and defoliate leaves. The amount of damage varies.

"In heavy populations, weevils will leave only the steam on the plant," Bailey said.

SCOUTING

Bailey advised that alfalfa growers should "scout early and scout often."

Alfalfa growers in southern Missouri should have started scouting fields in early April and should be continuing through the first harvest, Bailey said. Other growers in northern Missouri should be scouting for weevils by mid-April.

Weevil eggs are cream colored, but turn an olive brown color before hatching. Weevil larvae are a grayish color when small, but turn green when fully grown (about 1/4 of an inch), with a white stripe across the top.

Bailey recommended beginning to scout for larvae first on southern slopes, as infestations occur first in such areas which are generally warmer. He recommends growers randomly collect 50 alfalfa steams -- 10 steams from five different field locations -- then tap them into a white bucket so the larvae are visible. He cautioned those scouting to handle the samples gently, as the larvae can easily be dislodged from the growing tip of the plant. He recommends cupping the top of the alfalfa steam in one hand while cutting with a knife near the base of the steam.

The threshold for some kind of control measure, such as spraying, is an average of one or more larvae per stem. Bailey added that each state has different thresholds levels.

Examining leaves is another way to tell if some kind of spraying is needed. Bailey said if 30% of more of leaves show damage, growers need to employ some kind of control strategy.

A video of Bailey demonstrating techniques for scouting alfalfa is available at http://bit.ly/….

CONTROL

Alfalfa growers can best control weevils by spraying or cutting hay early. Research done at the University of Missouri Extension had proven that spraying reduces weevil larvae numbers by about 95%. Grazing alfalfa by cattle can cut numbers by about 90%.

Bailey recommends spraying fields before harvest and said that some fields in southern areas may require two applications to successfully control weevils.

Recommendations on weevil management and insecticides for alfalfa weevil larvae for 2015 can be found at the MU Integrated Crop and Pest Management web page at http://bit.ly/….

OTHER STATE WARNINGS

-- Pennsylvania State Extension reports that alfalfa weevils are just becoming active in southern counties; other counties should still be in the egg stage.

-- University of Nebraska Extension has had no mention of weevils yet, but army cutworm activity is increasing in western portion of the state.

-- Oklahoma State University Extension said that compared to previous years, lower numbers of eggs were detected in February, but as temperatures warm up, scouting is recommended. Warm weather could increase egg populations.

-- University of Idaho Extension reported concern over a different weevil: The clover root curculio feeds below ground on the roots of alfalfa. Growers are advised to wash roots in order to see feeding scars; however, no insecticides are registered in Utah for them.

-- Iowa State University Extension predicts that weevils may be active throughout the state this year, based on accumulated temperatures since January. Extension advises growers to watch the accumulated growing degree days and begin scouting fields in southern Iowa at about 200 degree days, and northern areas at about 250 degree days.

-- University of Kentucky Extension said that weevils typically occur throughout the state and said tip feeding should show up soon in established fields. Some weevils may be killed by strong winds or rain which knocks them to the ground. A useful control measure in 2015 may be early harvest in the late bud stage or just as first flowers open, as many larvae die during harvest and curing due to exposure to the elements, such as sun, low humidity and a lack of fresh plant tissue.

-- The Kansas Hay Market Report by the Kansas Department of Agriculture reported on April 14 that in southwest and south-central Kansas: "The fight continues to control aphids and alfalfa weevil; some treatment has worked well, others have had poor results."

Cheryl Anderson can be reached at cheryl.anderson@dtn.com

(SK/AG)


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