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New Mexico State University

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Cowpea Aphid Infestation Damages Alfalfa Across New Mexico

LOS LUNAS - Mathew Maez, who grows about 20 acres of alfalfa in Tomé, says cowpea aphids spread through his field like a wildfire in late March before he realized he had an infestation.

"The field was coming up green and looking good when I went out there in mid-March, but when I went back two weeks later the whole field was brown and the alfalfa had been wiped out as if somebody had burned it down," said Maez. "It had been eaten away by the cowpea aphid. I'd never seen that insect before, but this happened so fast it just blew me away."

Although the growing season has barely begun, the cowpea aphid–also known as black legume aphid and groundnut aphid–is damaging fields in central, eastern and southern New Mexico. Farmers in Valencia, DeBaca, Curry, Quay, Eddy and Doña Ana counties have reported infestations, with thousands of acres of alfalfa damaged.

In Valencia County, for example, agent Frank Holguin with New Mexico State University's Cooperative Extension Service estimates that at least one-third of the county's 12,000 acres of alfalfa has been infested.

The bug took most farmers by surprise because the cowpea aphid hadn't been known to attack alfalfa. The aphid–a tiny insect just 1.4 to 2 millimeters long–has been present in western states since the 1900s, but until recently it had only appeared on cotton and beans, said Mike English, entomologist and superintendent of NMSU's Agricultural Science Center at Los Lunas.

In early 1999, what appeared to be a new strain of cowpea aphid began to attack alfalfa fields in California and neighboring states, but this is the first season the bug has infested New Mexico fields, English said.

"We've always had this aphid in New Mexico, but it was never a serious pest on alfalfa," English said. "In the past two years, it has changed its habits and has decided it really likes alfalfa. Now it's become a virulent pest that's taking off with a vengeance around the state."

The aphid spreads extremely fast because it reproduces asexually, English said.

"These insects don't need males to reproduce and the females start giving birth almost right away, within one or two days after birth," he said. "That's why farmers have been so taken by surprise. You get this gigantic, geometric reproduction and before you know it you've got thousands, millions of aphids in the field."

In addition to eating alfalfa, cowpea aphids can transmit nearly 30 different viral diseases, including alfalfa enation, which kills alfalfa crowns and stunts growth. Alfalfa enation has affected fields in Europe, North Africa and Saudi Arabia. Left untreated, the insect can cause farmers to lose their alfalfa stands, English said.

Cowpea aphid is known to multiply much faster in warm weather, meaning the infestation may spread a lot more around the state in the late spring and summer.

As a result, farmers fear substantial losses. Mathew Maez, for example, expects to lose his entire first cutting in July, worth about $6,500, or nearly a third of his average annual income from alfalfa.

Alfalfa is New Mexico's biggest cash crop, earning about $182 million last year. Some 290,000 acres were in production in 2000, producing about 1.5 million tons of hay.

Cowpea aphid has a number of natural predators, such as bigeyed bugs, damsel bugs, lacewings and lady beetles. As those insect populations spread in the summer, it may help reduce the aphid infestation, English said.

The most effective control is pesticides. Several insecticides work well on cowpea aphids, English said. Flood irrigation also seems to substantially reduce aphid numbers.

The most important step, however, is for farmers to monitor their fields throughout the growing season to stop the bugs at the first sign of infestation, before they have a chance to spread, English said.

"All New Mexico growers need to be aware that this thing has hit the state and they have to get out there in the fields and look for them," English said. "Farmers need to get down on their hands and knees and scout for those critters."