Wayne Ohnesorg says some sources recommend spraying when 30 to 40 aphids are on the plant. Ohnesorg says yield drops won’t be noticeable until the aphid population per plant reaches 600 to 800, so he recommends treatment at 250 aphids per plant.
“If they get to the 250, it’s very likely they will reach the 600 to 800 needed to lose yield,” Ohnesorg says. “The cost of soybeans does not play into that at all, because the biology of the pest and the plant don’t allow for the cost to necessarily influence it as much as they are claiming.”
Ohnesorg says the situation with alfalfa is even more complicated because there are four species of aphids reported in local alfalfa fields and each has a different threshold for treatment.
“Pea aphid, for example, if you’ve got alfalfa that’s been recently cut and baled up, you’re looking at about 40 aphids of more, on average, per stem,” Ohnesorg says. “If you move over to spotted alfalfa aphids, it’s ten aphids per stem. You have to be really careful about knowing which pests you’re dealing with.”
Ohnesorg says determining the economic threshold for treatment depends on the value of the crop, the biology of the crop, the pest present, the biology of the pest and the cost of treatment.
For more information, Ohnesorg recommends the article “Aphids in Alfalfa” from Colorado State University, or the Iowa State University Integrated Management News article that includes color photos to make identification of alfalfa aphids easier.
Ohnesorg is based in Norfolk and covers Madison, Wayne, Stanton and Pierce counties.
By Susan Risinger, WJAG, Norfolk