While walnut husk fly isn’t a new problem, it’s a persistent one worth examining a bit more closely as it evolves. Bob Van Steenwyk, a University of California-Berkeley entomologist, has been studying the pest for years and says it’s actually becoming a problem in areas it didn’t used to surface.
“It used to be more centrally located in the eastern, cooler climates,” he says. “Now it seems to be spreading everywhere, and it tends to be coming earlier in the season.”
Previously a late-season pest, Van Steenwyk says the husk fly is creeping in sooner, and it’s now necessary to put out traps in June instead of July.
“Whether that’s global warming or not, who knows, but it could just be population sizes that are a little bit bigger than what they were, so they start a little earlier.”
Van Steenwyk says earlier infestations while the nuts are still developing can cause more damage to the kernel itself, while later damage hurts the shell, ruining it for in-shell sale, but not really affecting the quality of the kernel inside. Earlier infestation also results in more mold and shrivel, as well as color distortion.
“Late season, occurring in August and September — that type of infestation — will cause the shells to stain, which you can’t bleach out well, so that downgrades it for in-shell sale,” he says.
Fighting The Fly
Van Steenwyk was instrumental in the development of Dow AgroScience’s spinosad product, which is available in the form of GF-120 for organic growers and the Success formulation for conventional growers. He says the products are as effective as other commonly used materials and don’t have some of the negative side effects, such as flaring mites or aphids. With some of the older, conventional materials, you’d catch flies first, and then as their eggs started to develop after two or three weeks, you’d apply the insecticide. After about three or four weeks, if flies persisted, you’d apply more.
Husk Fly’s Biology
Walnut husk fly is a mid- to later-summer pest, and one female can generate a large number of larvae. They lay their eggs underneath the husk, which protects them, so effective control means killing the adults. “If you mess up your timing, or your sprayers don’t get to the top of your canopy and you allow some of the infestation to continue on, it doesn’t take too many flies to perpetuate your population,” says University of California-Berkeley entomologist Bob Van Steenwyk.
The females need to feed on a sugar and a nitrogen when they first come out, which they can get from anything from aphids to bird feces. In addition, the walnut husk has to be the right softness and thickness for the fly to oviposit. “It’s a big pest, and it’s becoming a bigger pest,” Van Steenwyk says.
“Those materials did quite well, but then when they started moving to these more reduced risk materials, they have to be consumed,” Van Steenwyk says. “They’re not effective topically and they take a little longer to kill, so it’s not so instantaneous. You have to apply them a little earlier than you would the conventional materials.”
He points out, too, that GF-120, which is a premixed bait and spinosad material, is not necessarily the best option for controlling large husk fly populations, because they can consume all the bait.
“You only put out about 50 ounces or 100 ounces per acre — a very small quantity, and it’s very effective, but if you have a lot of flies, they just eat it all, and the rest of the flies are not killed. It takes multiple applications, especially in these large population orchards,” Van Steenwyk says. “If you’re going to use that material, it’s almost every week, but once you get the population really low, then you can back off.”
The best bet with a large population, he says, is to start with a registered conventional material, get the population low, then try the GF-120 on the lower population.The next phase of Van Steenwyk’s research, which he hopes to begin with San Benito County farm adviser Bill Coates sometime next year, involves husk fly susceptibility.
“Something like a Payne or a Hartley is much more susceptible,” Van Steenwyk says. “There are definitely different susceptibilities in the various cultivars, so that’s important for growers to know.”