Once thought to become a real threat as early as July, the arrival of walnut husk fly (WHF) in the Sacramento Valley has accelerated in recent years with June 1 being a more recommended date to monitor for the arrival of the pest.

But over the last couple of years there have been reports that the troublesome and damaging fly has made its presence known as early as mid-to-late May, and researchers are still trying to determine why.

While a few theories have been noted, the prevailing thought may be that that soil temperatures could be a contributor to the problem. Overwintering WHF pupae in the soil develop and generally emerge as adults in late spring or early summer as a rule. But in recent years, dry, hot weather arrived a little earlier than usual and helped to support extreme drought conditions by the last week of May, or by the first week of June in the northern Central Valley region.

Researchers are telling growers that if they failed to hang WHF traps by the end of May this year, they might still be OK. That’s because of cooler spring temperatures and soil temps that may not have warmed as quickly thanks to substantial rain events.

But the effect of soil temperatures on the timing of pupae emergence is still only a theory, and early arrival of the flies in June is a real possibility, the researchers caution. The only way to know for certain is to have your traps in place as early as possible. Bottom line, if your traps are not hanging yet, best guess is you are too late to properly monitor WHF. That being said, it’s better late than never, especially if neighboring orchards have not reported many flies in their traps yet.

After emergence, it takes a female about two weeks to mate, develop eggs and start laying them into walnut husks in the orchard, says Emily Symmes, a University of California Cooperative Extension integrated pest management advisor in Butte County. The most effective spray timing is the period when the female is developing eggs before egg laying occurs.

The favorite target variety for the fly seems to be Tulare, Hartley, and Franquette, but all walnut varieties can be infested.

Saving grace

This two-week buffer time before egg lay may be the saving grace for walnut producers who have been slow to hang and monitor their traps earlier this month or in May. But failing to begin spray treatments before egg lay could be problematic and too late to prevent some degree of nut damage this season.

As most walnut growers know, the best way to monitor WHF is by using yellow sticky traps charged with ammonium carbonate bait. Even if you haven’t spotted WHF in your traps, keep in mind the pests are not likely to stay confined to a single orchard if there are other walnut orchards nearby. Traps should be placed in areas that may have been subject to WHF in recent years, but more traps than fewer traps are never a bad idea.

Traps should be hung high in the canopy, three to four traps per ten acres minimum. Watch for first female fly carrying eggs (resembling small grains of rice) that become evident when the abdomen is gently squeezed. But the presence of any female, usually detectable by the yellow coloring on the front legs, is a good signal it is time to start treatment.

In a recent Growing the Valley podcast hosted by Luke Milliron, Area Sustainable Orchard Systems Advisor in Butte County, Dr. Bob Van Steenwyk, research scientist and professor emeritus from University of California, Berkeley, said the peak of WHF season is around mid-July, but the best time to start treatment is about a week after the first female egg bearers are found in traps.

“Females can continue to lay their eggs and can-do damage all the way until a week or two before hull split, so it’s a long time to control this pest. The main control for adult mortality is by using a pesticide…usually spraying about every two-to-three weeks depending on population density [and other factors]. As the numbers come under control, you can continue to monitor traps and plan additional treatments accordingly,” Steenwyk said.

He noted that most growers put on about two to five applications across the season depending on population density.

“If you get heavy infestations in early June and July, you could get damage including mold, shriveled nut meat and blackened shell that can reduce your value up to about one-half of the value of the crop (for shelled cultivars) and will completely destroy inshell nuts…they must be shelled to get any value at all,” he added.

Steenwyk said conventional airblast sprayers can be used on every other row.

“But there is an organically approved treatment called GF-120 that is sprayed from the back of an ATV that blasts a stream about 20-feet in the air that contains a bait that attracts the flies…effective especially in smaller trees.”

Steenwyk says many commercial pesticides also work well in controlling walnut husk flies.

For more news on tree nuts as reported by growers and farm advisors, subscribe to the Tree Nut Farm Press e-newsletter.