The damage started in the Southwest and quickly moved north and east. Now cowpea aphids are a potential threat to alfalfa growers from Texas to the Canadian border.
The losses are similar to those caused by other aphids that invade alfalfa — lower yield and quality from plant yellowing and stunting. Aphids also produce honeydew that harbors a sooty mold and makes the alfalfa sticky, causing problems at harvest.
Small numbers of cowpea aphids have been found in alfalfa for a long time, mostly in the South. But they weren't troublesome until 1998, when damaging populations were discovered in California's Imperial Valley. During the next two years, they infested alfalfa fields throughout that state, as well as in Arizona, New Mexico and parts of Texas.
By 2003, fields with heavy cowpea aphid numbers had been identified in several states farther north, including Kansas, Oklahoma, Nebraska, Wisconsin and Iowa.
Iowa State University extension entomologist Marlin Rice says fields should be closely monitored. He fears the aphids might become a serious problem in Iowa, especially in years when the crop is stressed by drought.
“Aphids seem to be more damaging when the alfalfa's under stress from inadequate moisture,” says Rice. “It's sort of a double whammy.”
But Tom Hunt and Phil Sloderbeck, extension entomologists at the University of Nebraska and Kansas State University, respectively, seem less concerned. Both identified cowpea aphids in alfalfa fields in 1999. Since then, they've seen scattered infestations, but nothing widespread.
“There probably are other pests that will be more of a problem more frequently,” says Hunt. “But it's a new one that we'll have to keep our eyes open for.”
“We've got enough history to say it doesn't look like it's rapidly developing into a problem,” Sloderbeck adds. “But there are occasional fields where it at least reaches levels that get people's attention.”
Cowpea aphids are easily identified by their dark color: The adults are shiny black and the nymphs are gray. They tend to congregate at the plants' growing points, but also can be found on the leaves and stems. Infestations are most often patchy within fields, but whole-field infestations have been reported, too.
In California, they've been most troublesome in spring and fall, disappearing when daytime temps regularly hit 85°. But they're considered a warm-weather threat in some other states. Sloderbeck says they can cause problems almost anytime during the growing season.
“I've had calls from mid-spring almost to freeze-down in fall,” he says.
These juice-sucking pests can be controlled by any of several insecticides cleared for use on alfalfa. But no economic thresholds have been developed specifically for cowpea aphids. Entomologists use the same ones as for other aphids, but recommendations differ. Californians use blue alfalfa aphid thresholds — 10-12 per stem in new regrowth and more than 60 per stem when the alfalfa is 12" tall or taller.
In Oklahoma, however, cowpea aphid thresholds are similar to pea aphid recommendations - 50 per stem in alfalfa less than 10" tall; 100 per stem when the crop is taller.
If the alfalfa is almost ready to harvest, cutting it will probably solve the problem.
“Heavy rain can help you out, too,” says Hunt. “It seems like a nice, heavy rain will clean them off and they can't get a foothold after that.”