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New Mexico State University

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Cowpea Aphids Poised To Attack Alfalfa Fields

LOS LUNAS - Thanks to a wet, warm winter, alfalfa fields in the mid-Rio Grande region may face a major infestation this spring of cowpea aphid, a tiny but virulent pest that can multiply at astronomical rates.

Mike English, entomologist and superintendent at NMSU's Agricultural Science Center at Los Lunas, scouts for cowpea aphid in an alfalfa field at the center, where researchers found thousands of aphids in early March. A wet, warm winter has allowed alfalfa fields to green up early this year, giving cowpea aphids ample feeding grounds. (NMSU Agricultural Communications photo by Kevin Robinson-Avila)

"During the last cowpea aphid infestation in 2001, the bugs ravaged thousands of acres of alfalfa," said Mike English, entomologist and superintendent at New Mexico State University's Agricultural Science Center at Los Lunas. "We haven't had big aphid problems since then, but that's about to change. It looks like the aphids are ready to explode in the fields."

The aphids eat alfalfa and can transmit nearly 30 different viruses, including alfalfa enation, which kills plant tops and stunts growth. Left untreated, the insect can damage alfalfa stands themselves. Healthy stands can remain productive for up to five years or more, but if stands are damaged, growers must replant their fields.

Cowpea aphid is not difficult to control, English said. At least 10 different pesticides are very effective. Flood irrigation also seems to substantially reduce aphid numbers, but growers need to act early, before the insects massively infest fields.

"Growers need to get out there and look around, because once the cowpea aphid starts to appear, it will spread through alfalfa fields like wildfire," English said. "These insects reproduce asexually. They multiply at geometric rates that lead to thousands, even millions of aphids in the field before you know it."

Drought probably helped keep aphid populations down in recent years, but above-average moisture and mild temperatures this winter have allowed alfalfa to start growing earlier, creating ideal conditions for cowpea aphids.

"Favorable weather and the fact that growers are planting more varieties of alfalfa with shorter dormancy rates means fields are greening up earlier," English said. "That gives hungry aphids something to feed on."

During the first week of March, English and other NMSU researchers found thousands of aphids crawling through the science center's alfalfa fields. Few growers have reported major infestations, but that's probably because most farmers are not yet scouting their fields, English said.

Tom Dean, agricultural agent with the Socorro County Cooperative Extension Service office, said he already spotted aphids in fields around Socorro.

"It's started," Dean said. "We're in danger of big infestations because growers aren't aware of it. They need to get out there now."

The cowpea 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. In 1999, what appeared to be a new strain of aphid began to attack alfalfa in neighboring states.

It infested New Mexico fields in 2001, taking growers by surprise. That year, the aphids damaged fields in central, eastern and southern New Mexico, causing substantial losses.

Alfalfa is New Mexico's biggest cash crop, earning about $145 million last year. Some 240,000 acres were harvested in 2004, producing nearly 1.2 million tons of hay.

English said growers should also check much earlier this season for alfalfa weevil and blue alfalfa aphid. Like the cowpea aphid, weevils have already begun to appear in fields in Los Lunas and Las Cruces.