California entomologists wonder whether growers are using alfalfa varieties bred for resistance to blue alfalfa aphid as the pest’s populations grow, stunt alfalfa growth and reduce yields in that state and in Arizona.

They also want to know whether broad-spectrum insecticides used to control other alfalfa pests hurt blue alfalfa aphid management, as they can adversely affect natural enemy populations that manage pests

“Or it could be that it’s an aphid year,” says Vonny Barlow, University of California Extension entomologist/Integrated Pest Management (IPM) advisor. In their search for answers, he and colleagues have surveyed growers attending three aphid-management meetings held across the state.

Barlow’s growers say they’re losing half a ton per acre in yield from insect feeding on alfalfa that then grows slow, with twisted or deformed growing tips and some yellowing. “I’ve had one PCA (pest control advisor) tell me he’s seen fields where they’re losing ¾ ton/acre.”

PCAs have recommended an 8 oz/acre insecticide application to help control the aphid, Barlow says. “But the aphid populations resurge in a very short time.” So some are now resorting to “cocktail” combinations of insecticides at higher rates of ¾ pint to a full pint. Even that, at times, doesn’t work, he says.

The efficacy of the products was evaluated and not found wanting in a study by Eric Natwick, UC Extension entomology advisor in the Imperial Valley.

To try to determine the cause of the outbreaks, entomologists have held three meetings across the state and are surveying participants on their management practices.

“We have reports that blue alfalfa aphid is an issue in Arizona. We have reports that it’s in northern California. But it’s spotty. It’s not in entire areas in every field. It’s in a field here and there. That suggests, and it’s why we’re dong the survey, that there’s some kind of correlation going on … in those particular fields. If blue alfalfa aphid populations were just large, you would expect them in all fields everywhere. But that’s not what we’re seeing.”

Growers hit with the pest should harvest alfalfa early, he says.

“You’re going to have reduced yields, of course, and you’re going to be killing the aphid because you’re removing the host plants during harvest. The alfalfa should start to regrow, and you may have (blue alfalfa aphid) carryover into the next cutting – or you may not.” Growers should then again evaluate aphid populations, Barlow adds.

For information on insect thresholds and treatments, read UC Pest Management Guidelines. For updates, check out the UC Extension blog, called Alfalfa & Forage News. An April blog on the aphids, by Larry Godfrey, UC Extension entomologist, can be found here.

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