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New Mexico State University

New Mexico State University

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Weevils Threaten to Damage Alfalfa Crop

Las CRUCES--Mild winter temperatures have created a breeding ground for alfalfa weevils in many parts of the state. A large number of insects are infesting crops in Bernalillo, Socorro and Valencia counties.


"In some fields, we're getting as many as 30 larvae per sweep of a net, which is extremely high," said Mike English, entomologist and superintendent of New Mexico State University's Agricultural Research Center at Los Lunas. Reports also indicate that alfalfa weevils are becoming a problem in the eastern part of the state as well."

Alfalfa plants are beginning to show stress from insect infestation. Growers need to scout fields for insects immediately. "It's pretty simple to lose 1,500 to 2,000 pounds of hay due to alfalfa weevils," English said.

Any time temperatures rise above 51 degrees, weevils start laying eggs, he said. The first sign of damage are small holes eaten in leaves, which increase in size as larvae get bigger.'

Weevil infestation is most severe in the first crop or cutting of the season when insect populations are high. Larvae usually attack the first crop, while adult weevils damage regrowth by feeding on developing crown buds.


"This retards growth and prevents fields from 'greening up'," said Shane Ball, agronomist with NMSU's Cooperative Extension Service. "Weevils significantly reduce hay quality and yields by chewing leaves and skeletonizing them. The feeding can be so severe that plants lose their leaves, giving the entire field a grayish-brown cast."

The first cutting usually produces the highest yields, he said. High-quality hay contains 40 percent more leaves by weight. Alfalfa have greater digestibility than stems.

Several registered insecticides can be used to control the weevil, but farmers should contact the county Extension office to see which treatments work best in their areas, English said.

"With the weevil occurring this early in the season, I'm concerned that we may have to spray a second or a third time," he said. "Most insecticides are effective for only about four to five weeks, ad it may be longer than that for some growers before the first cutting."

The cost to treat fields can run from $7 to $13 or more per acre, Ball said. Producers may want to consider cutting their alfalfa early, especially if they have one to two weeks before harvest, to get rid of the first batch of weevils.