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Alternative defenses against increasing husk fly threat

This should be another good season for the walnut crop in Colusa County, Calif., says University of California Cooperative Extension Farm Advisor John Edstrom.

“It’s not what we had with last year’s record crop, but the walnuts are doing very well,” he says. “Production of Chandlers looks more normal, the quality of Howards is looking very good, and we haven’t seen sunburn to any drastic extent.”

But, he says, concern about walnut husk fly is increasing. Mid- to late-season varieties such as Chandler, Eureka, Franquette, Hartley, Howard, Mayette and Tulare are very susceptible to damage caused by the mid-to late-season pest. Maggots feeding inside the husk can cause staining of the nut shell, rendering the nut unsuitable for in-shell sales. Infestations can also lead to shriveled and darkened kernels, increased mold growth and lower yields.

“It seems to be coming on earlier and stronger, Edstrom says. “It’s a trend that we’re watching.”

Bob Van Steenwyk, UC Berkeley Cooperative Extension entomologist, isn’t surprised. “For some reason, the husk fly is becoming a bigger pest,” he says. “It’s moving in earlier and producing higher populations — and it’s spreading. A few years ago, the husk fly was primarily a problem along the coast; now, it’s spreading into areas where it hasn’t traditionally been found. That’s disconcerting.”

Along with this growing concern is increased use of sprays to control the pest. Organophosphate insecticides, such as malathion, Lorsban, or Imidan plus NuLure bait provide effective control of the husk fly at a reasonable cost, Van Steenwyk notes. “They do a wonderful job. They’ve been used a long time and there’s no indication of the husk fly developing resistance to them.”

But, he notes, the possibility of future EPA restrictions raises doubts about the long-term registration of these products. Pyrethroid insecticides are less effective than organophosphates for controlling the pest and can cause flare-ups of secondary pests, like spider mites. That’s why Van Steenwyk is encouraged by the introduction in the past few years of several new materials that have proven effective for controlling husk fly.

They include three formulations of spinosad — Entrust for organic growers and Success for conventional growers, both of which are combined with NuLure bait — and GF-120 fruit fly bait, for both conventional and organic growers.

Consider GF-120, which was developed by Van Steenwyk and other UC researchers and farm advisors. This pre-mixed spinosad and bait material is applied at the rate of up to 20 ounces acre. That compares to 3 pints of malathion plus 3 pints of NuLure per acre, or 4 pints of Lorsban plus 3 pints of NuLure per acre. GF-120 can be applied with a modified weed sprayer mounted on an ATV.

“You can use it to treat an orchard very quickly,” he says. “The orchard can be a little wet and you can leave the irrigation pipes in place because the ATV rolls right over them.”

GF-120 works best with low populations of GF-120 and is applied once a week when flies are present. Higher populations require two sprays a week.

Also, unlike organophosphates which are applied as the husk fly population begins to increase, GF-120 is sprayed as soon as the first fly is trapped.

“If you have a big carryover population from the previous year, GF-120 may not work,” Van Steenwyk says. “In that case, it’s better to spray an organophosphate for a year or two to reduce the population, then switch to GF-120.

Other alternatives to organophosphates for controlling adult husk fly in walnuts, he notes, are Delegate, a spinetoram which is similar to Success, and two neonicotinoid compounds — Assail, which also is effective against coddling moth, and Provado, which also controls aphids. Both Assail and Provado require adding NuLure as a feeding stimulant, Van Steenwyk notes.

TAGS: Management
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