Lower spray volumes improve walnut husk fly control

Lower spray volumes improve walnut husk fly control

Walnut husk fly is becoming more of a problem for California’s walnut growers. Spraying faster and reducing spray volume leads to successful control of the walnut husk fly.

Spraying faster and reducing spray volume leads to successful control of the walnut husk fly. At least that’s what University of California Cooperative Extension entomologist Bob Van Steenwyk is finding in his trials. That comes as the house fly-size pest is becoming more of a problem for California’s walnut growers.

Walnut husk fly has only one generation per year, with overwintering larvae emerging from June until early September. The maggots damage walnuts indirectly by feeding on the inside of the husks turning it soft and black. This causes staining of the nutshell and makes it unsuitable for in-shell sale. One study found that walnut husk fly can reduce nut value by as much as one-third.

This pest once was a concern mostly for walnut growers in the Stockton area of the northern San Joaquin Valley and along the coast from Hollister in San Benito County north to Mendocino County.

“It wasn’t a big problem and growers there were able to control the pest with just one or two sprays during the season,” Van Steenwyk says. “Then, five years ago, the problem began spreading. Now, walnut husk fly is a problem in the southern San Joaquin Valley and as far north as Tehama County in the Sacramento Valley. Some growers are having to spray as many as five times to control it.”

Last year the pressure on trees from walnut husk fly threat was particularly high.

“If it was in your area, it was bad,” he says. “If you did everything right in your spray program and you sprayed at least three times, you probably got through the season without much of a problem. Otherwise, you could have had a big problem.

And, if that’s the case, you’ll face a similar challenge this year in controlling the pest once flies start to emerge from the over-wintering pupae in late June or early July.


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“If you had a big population of walnut husk fly in your orchards last year and the weather favors the insect this year, you’re set up to have a big problem.” he says.

Several reduced risk pesticide products, applied with baits, are available for managing walnut husk fly.

In addition to evaluating the effectiveness of various traps for monitoring infestation levels and baits, Van Steenwyk has been experimenting with low-volume pesticide applications.

Instead of spraying every row of trees at the rate of 100 gallons per acre at a ground speed of two miles per hour, he’s treated test plots by applying just 10 gallons per acre while traveling at 10 miles per hour. In doing so, he sprayed every other row with two oscillating streams on either side of the drive row in a W pattern, hitting the tops of the trees.

“A grower would have to modify a spray rig to reach the tree tops,” Van Steenwyk says. “But, this approach has proved to be effective.”

Unlike spraying for codling moth, where the nut must be covered with the insecticide so that the larvae will feed on it, the walnut husk fly materials target the adults. They fly all around the tree where they are attracted by the bait. That’s why this reduced application rate technique works, as long as the spray covers the entire tree, including the top, he explains.

“This approach may not reduce the amount of pesticide needed per acre,” he says. “But, it’s more cost effective, because you can treat more trees in less time.”

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