Resistance Management Strategies in Hazelnuts

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A Willamette Valley hazelnut orchard in early August. Incorporating resistance management strategies in pest control programs is key to preserving the efficacy of a shrinking number of pest control options in hazelnuts, according to OSU Extension Orchard Specialist Nik Wiman (all photos by M. Lies.)

In a sweeping presentation on hazelnut pest management at the Nut Growers Society Summer Tour in July to August, Oregon State University Extension Orchard Specialist Nik Wiman urged growers to use pesticides judiciously and to rotate modes of action.

“There are 36 different modes of action that have been identified in insecticides, so it is good to rotate them wherever you can,” Wiman said.

Rotating modes of action is key to slowing development of resistance and preserving the efficacy of a decreasing number of pesticides available for pest management in hazelnuts, Wiman said.

Using full rates when applying insecticides, especially premixes, is also important, Wiman said.

“Premix products are a real trend in the pesticide market, and a lot of these have reduced rates in them,” Wiman said. “So, you have to watch the actual level of active ingredient you are applying with these applications because if you are applying a reduced rate, you are maybe getting more survival of the pest and more likelihood of development of resistance.”
Also, use of selective insecticides such as Intrepid can help slow resistance development: Growers should consider using them despite their higher cost, Wiman said. And, he said, when using Intrepid, growers need to be patient and not expect immediate results.

“It’s a very good selective material. It is going to kill eggs, it is going to kill larvae, but it’s not going to kill adults,” Wiman said. “But the thing to think about here is that residue is very long-lived and it’s still out there working for you even though you’re still capturing moths in your traps.”

Wiman also pointed out that neonicotinoids, while shown to be harmful to pollinators, are selective insecticides and should also be considered as another option for slowing resistance development.

“Neonicotinoids are under fire from regulators because of their nontarget effects on pollinators, particularly,” he said. “But in hazelnuts, as long as we’re not exposing pollinators, I think it is a very selective use.”

Steps to prevent exposing pollinators when using neonicotinoids include avoiding insecticide drift. “Also, make sure that when we’re watering it in, it is not going past the root zone, that it is staying in the tree and not getting into water that can run off into other surface water and cause problems for bees and other organisms,” Wiman said.

Biological, botanical and behavioral insect controls, while also more expensive than broad spectrum insecticides, also should be considered, Wiman said. “For example, for filbert worm, we have excellent mating disruption products,” he said.

But ultimately, Wiman said, the most important step growers can take to slow resistance development, is to use them judiciously. “The most important thing a grower can do is to treat only when needed,” he said.

Three Pathways
Insects develop resistance in three ways, Wiman said. “There is either a target-site resistance where the insect actually is able to modify a receptor so that it no longer receives chemical. There is also behavioral adaptation where maybe the insects will leave an orchard when you spray and then come back when it is safe,” he said. “The brown marmorated stink bug will do that.

“And by far the most common mechanism for resistance is metabolic resistance where the insect upregulates certain molecules that help detoxify the insecticide in the body of the insect,” Wiman said.

A pest like aphid is particularly susceptible to resistance, Wiman said, because of its season-long presence in an orchard, which exposes it to several different pesticides, including those targeting other pests.

“For example, we’re using imidacloprid for a few different pests,” he said. “It is very commonly applied and it’s relatively inexpensive. So, there is frequent exposure of the (aphid) populations and a lot of times you are getting a low dose, which is even more risky.”

Also, Wiman said, because the filbert aphid, one of two prominent aphid species in hazelnuts, was previously resistant to insecticides before the emergence of the samurai wasp in the 1990s, the chance it will become resistant again is high.

“There were some studies that show that filbert aphid was very resistant to a lot of the chemistries of the time,” Wiman said. “They were spraying carbaryl and not getting control. They were spraying diazinon and not getting control of aphids. And since they already have developed resistance once, that makes it more likely they can do it again.”

Wiman added that multiple growers are now telling him they are no longer getting control of aphids with imidacloprid.

“So, again, there’s always the risk of resistance, especially with the aphid population because they are out there all season and getting exposed to everything that we throw at any insect in an orchard,” he said.

The filbert worm, on the other hand, is less likely to develop resistance, he said.

“There is probably only one generation per year. There is a wild population that is in oaks and other sites, which keeps the gene pool relatively diverse,” he said. “So, with filbert worm, developing resistance would be very unlikely.”

Same with the brown marmorated stink bug. “It is moving around in the landscape, so it has a very low probability of developing resistance,” Wiman said.

Few New Insecticides
Among issues that are aligning to increase the importance to preserve efficacy of existing pesticides are that chemical companies are showing a reluctance to sink millions of dollars into the development of new pesticides and the current regulatory climate surrounding use of pesticides is not particularly favorable to growers, Wiman said.

“Basically, we are going to keep losing insecticides,” Wiman said in closing his hour-long presentation. “If you look at the regulatory environment, it is not very kind to the grower.
“Neonicotinoids are on the chopping block. And we’ve had initiatives to limit our ability to air-blast spray. And there are just so many regulatory obstacles. And you talk to these chemical companies, and they don’t have a lot coming down the pipeline,” he said.

“It’s gotten so expensive to register new chemicals that a lot of them are getting out of the business. What we’re seeing is continued repackaging and release of old materials,” Wiman said.

“So, we need to conserve the tools that we have now,” he said. “Use them as judiciously as possible.”

Information Tools
OSU has developed several information tools to help growers use insecticides judiciously, Wiman said, including apps and pest management guides that inform on sampling techniques and action thresholds.

For example, the mobile friendly Hazelnut Pest Management Guide is available for growers to access via an app or through a smartphone.

“They keep coming out with new features,” he said. “So, I think it is getting better and better. And for those who like to be really organized, it’s quite nice to see all the active ingredients and have all those labels in one place. It also has a lot of images of the different pests.”

Growers can access the app on the home page of OSU Extension’s Hazelnut Pest Management Guide for the Willamette Valley.