Watch for Alfalfa Weevils as Weather Warms

OMAHA (DTN) -- Midwest alfalfa producers need to be scouting now for alfalfa weevils. Weekly scouting might not be enough as the weather warms and the pests may be beginning their destructive feeding. Every few days might be a better scouting period, according to experts.

There are some different options with insecticides if treatment is warranted. There have been some reports of products seeing some resistance issues.

MIDWEST ALFALFA IN GOOD SHAPE

Jeff Jackson, an alfalfa and forage specialist for CROPLAN by Winfield United located in northwest Missouri, told DTN current conditions for alfalfa appear to be decent in his territory. He covers the Dakotas and southern Minnesota down into Kansas and Missouri.

Alfalfa stands made it through the winter in generally good shape, with perhaps some older stands suffering some winterkill. There were some questions about how much of this type of plant injury would be seen, but it hasn't been too widespread, he said.

With the warmer days, producers really need to be turning their attention to scouting for alfalfa weevils and treatment if necessary. An accumulation of 275 to 300 growing degree units is about the time the pests will begin to feed on the crop, he said.

"Kansas and Missouri are probably there now, whereas Nebraska and Iowa might not quite be there yet," Jackson said.

March was very warm just about everywhere in the country, according to DTN Ag Meteorologist John Baranick. There weres a couple of colder spells, but they were short, and we have had some good stretches of record warmth in there too, he said.

Baranick said early April has been seesawing back-and-forth between warm and cold temperatures so far.

"The Plains have been warmer than the Midwest, but we saw some frosts early this month behind a strong storm system and cold fronts earlier this week," Baranick said.

The DTN forecast suggests temperatures should trend above normal for the rest of April in the Plains, but near to even cooler than normal in the Midwest, he said.

Jackson said the economic threshold for weevils is when 30% of the plants show some feeding on the leaves and is the time to consider a treatment option. Different producers have different thresholds as those who sell hay might spray at lower levels than perhaps someone with beef cattle.

Not treating the crop affected by the pests will come with some lost yield and lessen quality levels, he said. Generally, he said about 35% of the yield could be lost as well as 30 points in forage quality.

"The less tonnage really adds up quickly," he said.

SCOUTING ALFALFA WEEVILS KEY

Jackson recommended alfalfa growers scout their alfalfa this time of year for weevils. Traditional scouting time was maybe once a week, but weevils can appear very quickly, especially with warmer weather in the spring.

He would like to see producers look at their alfalfa acres every few days as the weather warms up. This means getting out of the pickup and walking out into the fields to look at plants.

"You really need to get out there and get the boots on the ground for this," Jackson said.

If the stage of the plant is right, it also might benefit the alfalfa plants to see a fungicide application if an insecticide is going to be applied anyway. This would help alfalfa plants get off to a good start at the beginning of the growing season, he said.

A recent article titled "On the Lookout for Alfalfa Weevil Emergence and Scouting" from the University of Nebraska-Lincoln Extension (https://cropwatch.unl.edu/…), takes a closer look at scouting and treating of the pest.

Damage from alfalfa weevil is typically small holes on the newest leaflets near the stem tips, according to the report. The larvae are small and pale yellowish-green and a darker green when larger.

UNL suggests if weevil larvae are present, randomly select at least five sampling sites from across the entire field. At each site, pick or cut six stems at ground level.

Shake the larvae off the stems by beating the stems into a deep-sided bucket. Count the larvae and determine the average number of larvae per plant.

"Make sure to check for small larvae that may be enclosed in new, folded leaflets at the tips of the stems," the UNL report said. "Measure stem lengths and determine the average stem height."

WHEN TO TREAT FOR ALFALFA WEEVILS

Jackson said when to treat alfalfa weevils depends on a grower's location.

Alfalfa growing in Kansas and Missouri tends to be treated sometimes multiple times before the first cutting is taken off. Some producers in southern Kansas into Oklahoma could spray three times before the first cutting, he said.

In Nebraska and Iowa, generally weevils are treated once before the first cutting is taken. Some areas of South Dakota and Minnesota will do the same practice, but further north locations might cut first and then spray after the first cutting, he said.

Jackson said most locations in his territory will come back after the first cutting and spray between the first and second cuttings to assure the second cutting is not damaged by weevils.

David Hallauer, a Kansas State University Extension district agent -- crops and soils, located in northeastern Kansas, told DTN that some alfalfa acres in his district were treated for weevils earlier this week.

The spraying was mainly done in the southern area of his Meadowlark district (Jackson, Jefferson and Nemaha counties) as more weevil feeding has been seen there. Northern areas of his district closer to the Kansas/Nebraska border have not seen as much weevil activity yet.

The general trend in recent years seems to be damage from alfalfa weevils happens earlier in the spring compared to 10 years ago, he said.

Hallauer said this might be because producers in his area are more aware of the pest and are scouting their acres more than in the past. The warmer fall seen last year also might have allowed more eggs to be laid, and thus better development was seen this spring.

"Whatever the case, it seems that the alfalfa weevil feeding is trending earlier here," Hallauer said.

DIFFERENT OPTIONS FOR INSECTICIDES

As for what insecticides to spray to effectively control alfalfa weevils, there are some different choices for growers, Jackson said.

Insecticides labeled for alfalfa weevil include pyrethroids, organophosphates and oxadiazine. Pyrethroids include brand name insecticide Warrior, while oxadiazine includes Steward.

Jackson said there have been some reports of the pest becoming resistant to pyrethroid products. If producers see what appears to be an insecticide failure, do not use the same product again.

Steward is an effective product on weevils but is also an expensive insecticide, he said.

Later season alfalfa weevil control is a bit unknown. Jackson said he has a colleague who has done some research into treating adult beetles in the fall. They lay their eggs in the field edges in the fall, mainly in October and November.

A treatment application in the fall to limit egg laying might be something which is done in the future depending on what researchers discover, Jackson said.

Iowa State University Extension and Outreach also had a recent article about the 2025 alfalfa weevil egg hatch. It can be found here: https://crops.extension.iastate.edu/….

Russ Quinn can be reached at Russ.Quinn@dtn.com

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