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Less is more when spraying for walnut husk fly

Although using a handgun to spray an insecticide-bait combination at the rate of 50 gallons per acre or less reduces coverage of a tree, it can be just as effective in controlling walnut husk fly (WHF) as a conventional higher-volume, full coverage application made with an air blast sprayer.

Further, reports Bill Coates, University of California Cooperative Extension Farm Advisor for San Benito, Santa Cruz, Santa Clara and Monterey counties,

stained shells and hulls sticking to the shell aren’t the only reasons to limit infestations of this insect.

“It can have serious impact not only on the shell of walnuts, but also on the quality of walnut kernels,” Che says. “The walnut husk fly actually damages the portion of the crop you sell.”

Some growers using air blast sprayers are applying as much as 50 to 400 gallons per acre of solution containing an insecticide or insecticide and bait, he notes. But, the amount of solution applied by growers using low-volume insecticide and bait sprays ranges from 50 gallons to as little as 5 gallons per acre.

Such a low-volume treatment for controlling walnut husk fly can also reduce chemical costs. But the big savings comes in lower equipment costs, he notes. Unlike the bigger tractors required with an air blast sprayer, these low volumes can be achieved at faster ground speeds, using a smaller tractor pulling a tank equipped with a hand gun or a small hand-operated weed sprayer mounted on an ATV.

Coates has been researching the benefits of low-volume for about a decade, dating back to his first trials with GF-120, where he controlled WHF using about a gallon of total spray solution (20 oz. of bait-containing GF120 + 30 oz. to 80 oz. water) per acre.

He decided this might also work with conventional insecticide and bait mixtures and tried a small scale trial with malathion and Nu-Lure bait at 5 gallons per acre, which worked well.

Some of the growers he’s worked with on low-volume WHF sprays have used a weed sprayer with one nozzle on either side of the tank directed upwards at a 45 degree angle. Others use up to three nozzles on each side directed into the tree at different angles.

Growers using hand guns have directed sprays into the trees at a 45-degree angle or created patterns such as an up-and-down motion in a “W” or “M” pattern. Growers with very large trees may need additional sprays directed to the tops of trees.

These methods don’t cover the tree with the type of uniform spray that a high-volume spraying does — but, that’s not necessary, Coates explains. High volume sprays result in finer drops of the chemical solution, than the larger drops produced by low-volume bait spraying.

These larger drops may be more effective — because they stay wet longer, they can remain effective longer. Plus, they also contain a more concentrated amount of bait, which works better to attract WHF, as well as having more of the insect-killing pesticide.

Coates suggests growers use low-volume sprays on a trial basis to see if works in their particular situation.

“Not everyone will necessarily be successful with low-volume sprays,” he says. “More replicated testing must be done to verify the effectiveness of various rates of low volume sprays. Sprays should be applied about every three weeks, beginning with a sudden rise in trap counts, which usually occurs in late June or July, depending upon the particular orchard.”

New pesticides are also being tested by Coates and his colleague, Bob Van Steenwyk from UC Berkeley. Last year, for example, in a test of new, reduced-risk chemical treatments of WHF, they achieved excellent seasonal control with three applications of two neonicotinoid insecticides applied full coverage at 250 gallons spray solution per acre.

The walnut husk fly causes more than cosmetic problems that disrupt harvesting, hulling and shelling activities. “Over five years of sampling WHF damage in Chandler, Hartley, Payne and Serr walnuts, almost all the mold I’ve found was associated with this insect,” Coates says. “If you run 100 nuts infested with WHF through a quality analysis and compare them to 100 nuts which are free of WHF, you’ll find that the infested nuts were worth about 30 percent less.

Although Coates has seen trees in which almost 100 percent of the nuts were infested with WHF, infestations of 10 percent to 25 percent are much more common.

“If you have a 25 percent infestation of WHF and it’s reducing the value of the nuts by 30 percent,” he says, “you could be losing 7.5 percent of your whole crop value to the damage caused by this one insect.”

TAGS: Tree Nuts
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