Learning how to get the bugs out of alfalfa ought to begin with a primer on pest identification.
Parts of the Southwest could see a lot of newcomers to forage production as large dairies move into the High Plains to take advantage of economical land, improved water availability, and fewer waste disposal problems.
Row crop farmers may find new markets for corn and sorghum silage and other forage crops, including alfalfa. They also may discover new insect pests.
“The alfalfa weevil is the No. 1 pest for alfalfa,” says Greg Cronholm, integrated pest management specialist for Hale County, Texas. Cronholm discussed pest management in alfalfa during a dairy forage crop seminar recently in Plainview.
He says the pest resembles a boll weevil, about the same size but with a shorter snout. It lays eggs on alfalfa stems in late fall. Larvae hatch in the spring and display a white coloration at first and then turn yellow. By the third in-star, it turns green and with a dark brown or black head.
When full grown, the alfalfa weevil measures about one-fourth inch.
Cronholm says early treatment should begin when producers find one weevil per stem. The threshold increases to two per stem as the plant gets to 14 inches and close to bloom.
Injury symptoms include skeletized foliage with a grayish-silver cast. “With heavy infestations, producers can lose the stand,” Cronholm says. “Damage is more severe during drought.”
Parasitic wasps help control alfalfa weevils. Insecticide options include Malathion, Furadan, Pounce, and Lorsban. “Furadan has a long residual, 28 days,” Cronholm says. “Pounce and Lorsban have 14 day residual activity and Malathion has three days or less.”
Caterpillars also may cause injury to alfalfa. Cronholm says growers should look for green caterpillars or worms with yellow stripes. “A number of caterpillars, including armyworms and cutworms, will injure alfalfa,” he says.
He recommends scouts use a sweep net to determine infestation levels. “Populations may be high and may be a mixture of caterpillar types.”
Cronholm says alfalfa blister beetles are rare in Texas but have caused problems in Oklahoma and New Mexico. “We've found only a few,” he says, “but we need to be alert because the pest is toxic, extremely so to horses. Some species are more toxic than others.” The beetle will cause blisters on human skin.
“It's about one-half inch long and feeds on a wide range of host plants.”
Alfalfa producers should spray for the pests and wait a few days before baling to avoid mashing the pests into the hay. “Allow the adults a few days to leave after a pesticide application,” Cronholm says.
“The pea aphid is the most common aphid species in alfalfa. The blue aphid is not a major pest in Texas but does cause problems in other areas.
“The species dictates treatment threshold levels. Growers should check up and down the stems and watch for a yellow cast to the foliage. Alfalfa is an easy crop to sample.”
He says lady beetles help control pea aphids, “if they move in early enough.”
He also notes that some alfalfa varieties show resistance to some aphids. “The first year after establishment, resistance may be low and growers need to be prepared to spray.”
Other potential alfalfa pests include three-cornered alfalfa hoppers, which may cause stem damage. “The stem then breaks off as it reaches maturity.” Cronholm says the pests are rare in the Texas High Plains.
Several species of Lygus also may infest alfalfa. “They often move into cotton after alfalfa is cut,” Cronholm says.