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New Mexico State University

New Mexico State University

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Mild Winter Produces Surge in Damaging Alfalfa Weevil Populations

LAS CRUCES - The unseasonably warm weather that gripped much of New Mexico this past winter is starting to bug Carol Sutherland. The entomologist with New Mexico State University's Cooperative Extension Service says the high temperatures have swelled the ranks of the alfalfa weevil, a notoriously damaging pest that loves to chomp on the state's newly emerging No. 1 cash crop.

Using a traditional sweep net, New Mexico State University entomologist Jane Pierce hunts for alfalfa weevils in a newly emerging alfalfa field at NMSU's Agricultural Science Center at Artesia. Populations of the damaging bugs are up because of New Mexico's unseasonably warm winter. (04/23/2002) NMSU agricultural communications by Norman Martin

Millions of dollars are at stake because alfalfa weevils are especially destructive to the season's first cutting. Research has indicated that weevils remove leaf tissue, beginning with new leaves in the top of the plant, and motor their way down the stem to other leaves. Their voracious dining habits reduce both forage quality and quantity.

"They're in hog heaven, living in all that new, succulent tissue," said Sutherland, who also serves as the New Mexico Department of Agriculture's state entomologist. "And while they're having a good old time, the plant is getting clobbered. A lot of that early spring growth that the plant is trying so hard to produce is eaten by these little critters."

Valued at $161 million, alfalfa is New Mexico's leading cash crop, largely because of its ability to grow in just about any of the state's widely different climate zones. More than half of the state's 270,000 acres of alfalfa are planted in just four counties - Chaves, Dona Ana, Eddy and San Juan. Still, farms from the sweltering south to the high, cool mountain regions of the north contribute to the total.

While New Mexicans are breaking out their barbecue grills and slathering on sunscreen as a summerlike swelter settles over the state, alfalfa weevils are equally active, said Jane Pierce, an entomologist with NMSU's Agricultural Science Center at Artesia. She explains that alfalfa weevils overwinter as adults in plant debris in alfalfa fields and sometimes along fence rows and ditch banks.

The milder the winter, the more weevils survive to lay their eggs, Pierce said. As temperatures drop in the fall, adults emerge from outside the fields and begin to migrate to alfalfa fields, laying eggs in dead plant stems. Egg laying continues as temperatures permit, especially in lower elevations. Peak larval damage usually has occurred by March or April in southern New Mexico and by late April to early May in the north.

"It's not hard to demonstrate the loss of 1,500 or more pounds of hay per acre under heavy infestations," said Mike English, superintendent of NMSU's Agricultural Science Center at Los Lunas. Heavy infestations can shred leaves or destroy all but the leaf veins and stems,
leaving a skeleton. Damaged fields often take on a ragged, grayish or white appearance.

Chemical control measures are available, and a number of insecticides are registered for alfalfa weevil larval control. Sutherland advises producers to closely monitor alfalfa weevil activity. "Keep an eye on it," she said. "It may be possible to cut the crop early, which will kill or starve many of these weevil larvae."

Do a chemical treatment if necessary, she said. By timing an appropriate treatment early in the production cycle, growers can control these pests before they do significant damage to the first cutting. "If you can afford to cut early, you might save yourself the cost of an insecticide application, plus save some bees in the process," Sutherland said. Bees are crucial in pollinating many crops in New Mexico, and they're especially active in the early spring.

Rich in protein, vitamins and minerals, alfalfa is prized as a primary component in dairy cattle rations and is an important feed for horses, beef cattle, sheep and goats. It can be used as a pasture, hay or silage crop, in addition to being cut and dehydrated to make protein-rich livestock meal and pellets.