March 2016
On Guard in the PNW

Introduction by Bill Schaefer

There are a lot of tools available to protect your crop — fungicides, herbicides and insecticides, to name three. All three of these inputs require growers to practice management plans to avoid developing resistance to the diseases, weeds and pests from adversely impacting your crop.

Alan Schreiber, president of the Agriculture Development Group, presented a seminar at the 2016 Washington/Oregon potato conference in Kennewick, Washington on resistance management when applying insecticides in the Pacific Northwest (PNW).

Never bet against the insects. They’re always going to win in the long run,” Schreiber told the audience. “They’ve been around a long time. We will never control them, we will only manage them.”

Schreiber said that 586 different kinds of insets have developed resistance to 325 insecticides.

“Insects are really out in front in developing resistance to insecticides as more insects have developed resistance to insecticides, diseases and weeds have developed resistance to fungicides and herbicides,” he said. “Susceptibility is a natural resource we need to cultivate and conserve. It’s something we want to preserve, keep and build.”


Potato psyllid, adult Frankliniella and bird cherry-oat aphi.

Here are Schreiber’s seven rules for resistance management.

#1: Rotate modes of action (MoA)

First thing, you have to rotate your modes of action. MoA are listed on the first page of every insecticide label in a specially marked box. Rotation is the single most important thing you can do to avoid resistance is rotate, rotate, rotate. A new insecticide is a gift to you that you should not squander. When an insecticide comes out on the market for whatever insect pest — Colorado potato beetle, aphid or psyllid — they are susceptible to that insecticide.

How you manage that susceptibility determines whether you can keep a new insecticide or you can squander it. If you use up that resource, you lose the ability to use that product.

Generally, growers select the cheapest, most effective product. What they need to include in that calculation is rotating mode of action as well as considering price and efficacy.

Generally, what growers do is they select the cheapest, most effective product. What they need to include in that calculation of insecticide selection is rotating mode of action as one of the key factors. The Insecticide Resistance Action Committee classifies insecticides by their MoA by group. At this time there are 28 different group classifications. Each insecticide has a group classification label denoting its MoA. Don’t use the same classification repeatedly.

For more information on different MoAs consult the Insecticide Resistance Action Committee website at IRAC-online.org.

#2: Each crop/pest combination needs to have at least three modes of action and they need to be used

Some pests like wireworm rarely develop resistance but several potato insect pests have developed resistance and that’s why it’s important to develop a three-MoA plan. You have to rotate wisely. No insect pest of potatoes has ever developed resistance when you’re using three different MoAs because they genetically cannot develop resistance to three MoAs simultaneously.

#3: Never make more than two sequential applications of the same MoA

Do not apply two modes of actions against the same generation of an insect pest. For example, do not apply pyrethroid insecticides against first generation Colorado beetle more than twice. In some cases you should make only one application against a single generation of pests. In the case of a neonicotinoid, only use it once and then rotate. Never apply a neonicotinoid twice in succession – ever! In the case of some neonicotinoids there are situations where you want to use it once in a year and never use it again. If you use a neonicotinoid at planting, don’t use it again in that season.

#4: Do not make two applications, with the same MoA, against two generations of a pest

If you’re going to use a pyrethroid or a neonicotinoid insecticide against first generation Colorado potato beetle, don’t use them on the second generation. You do not want to use the same active ingredient against multiple generations of a pest. You’ve got to rotate MoAs through generations. If you only have one generation of a pest in a growing season, you have to rotate chemistries between growing seasons. Not only do genetics have a long memory, they have a permanent memory. Once there’s a resistant gene, that gene will always be in that population. If you develop resistance to a neonicotinoid (or other insecticide) and you stop using that neonicotinoid, then come back and use it 10 years later, the resistant gene will still be in that population. Once you lose susceptibility, it’s gone forever.

#5: Applying a low rate of chemistry is a risk factor for developing resistance

Using a low rate of chemistry is a risk factor for developing insecticide resistance. It has to do with insect genetics. It’s easier for them to mutate and develop tolerance to a low rate than it is for them to develop resistance to a higher rate of a pesticide.

Never use a below labeled rate. It’s very tempting, particularly when using products that are very expensive or very effective.

We’ve seen this with pyrethroids. Colorado potato beetle (CPB) has developed resistance to certain pyrethroids in certain places in southern Idaho.

#6: Tank mixing insecticides is less desirable than rotating MoAs

Tank mixing insecticides is less desirable than rotating insecticide MoAs. Using two modes of action at the same time repeatedly is a resistance management tactic but studies have shown that it is harder for insects to develop resistance by rotating modes of action than to tank mixing. It’s better to rotate than to tank mix. Also, if you’re going to tank mix insecticides, then don’t tank mix products with the same MoA.

#7: Try to create refuges

No one wants to have a refuge for insect pests on their farm but a refuge is a place where they’re not subjected to the lethality from the MoA you’re using against that generation of insect. It can mean leaving part of a crop untreated. More likely you create a refuge by not treating the whole farm with the same insecticide.

An example of an insecticide refuge would be using a pyrethroid on 80 percent of the farm and a neonicotinoid on 20 percent. A statement in the PNW insect and mite management guidelines is to never apply a neonicotinoid to the whole farm. Apply a neonicotinoid to 80 percent of the farm and apply a different MoA to the remaining 20 percent.


Potato aphid giving birth and an adult Colorado potato beetle.





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