Alfalfa Weevils Best Controlled with Chemicals

Alfalfa Weevils Best Controlled with Chemicals

Newly hatched larvae can cause losses in first cutting; defoliation can also affect subsequent production.

The alfalfa weevils are out there.

If you haven't scouted your fields already, you should do it soon, says Kansas State University agronomist Jim Shroyer.

Shroyer spotted a field of alfalfa that he wanted to take a closer look out while touring wheat fields in Sumner County. Upon closer inspection, he found a high population of alfalfa weevils in the field.

Weevils are a major cause of defoliation of alfalfa early in the season. The eggs are often deposited in the fall and hatch when temperatures warm in the spring.

The newly hatched weevils start feeding immediately. White or silvery looking patches in a field indicate there may be a weevil problem.

EASY TO SPOT: Alfalfa weevils quickly drop off plants when disturbed by brushing the leaves of the plants.

As temperatures warm up, the adult weevils leave the alfalfa field and over-summer in shady areas or under leaf piles. They return to the alfalfa field in the fall to deposit eggs in the stems of the plants. They will continue laying eggs until temperatures fall below 48 degrees, then overwinter as both eggs and adults.

Hatching begins as soon as temperatures warm back up to 48 degrees.

The adult females also begin deposited eggs in the spring, which may create a second wave of weevils.

Kansas State University's entomology department recommends insecticides as the best method of control. Other methods, including fall grazing by livestock, flaming and burning or using a roller to squash eggs have limited effectiveness. Chemical control has proved very effective.

WEEVIL ID: These larvae were spotted in a Sumner County field recently. The feed for three to four weeks before pupation and over-summer under leaf debris or in shady areas near the field.

Actual feeding and defoliation in Kansas affects only the first cutting of alfalfa, but excessive loss of foliage can reduce the production of subsequent cuttings. If regrowth is delayed by drought or unseasonably cold weather, both of which have occurred this spring in much of Kansas, there may be additional damage.

According to K-State's publication on alfalfa weevils, under those conditions adult weevils may strip the epidermis from the stems of plants, resulting in patches of dead plants in the field.

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