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Serving: MN

How to curtail pesticide resistance

Paula Mohr sprayer in soybean field
KNOW BEFORE YOU SPRAY: Crop consultants and scientists advise season-long scouting, so you understand field conditions and know weed and insect populations before you apply pesticides. Overapplication leads to resistance.
Scout all season long to know field conditions, and document your field history to avoid using the same products every year.

Scout, rotate chemistries, scout, use sticky traps and scout.

That’s the overall advice from University of Minnesota Extension specialists and an independent crop consultant to farmers who have concerns about building resistance to insecticides, herbicides and fungicides.

During a recent webinar on the topic, entomologists Ian McRae, Ken Ostlie and Bruce Potter, agronomist Tom Peters, plant pathologist Dean Malvick and independent crop consultant Dan Nielson offered their recommendations on what growers could do. Here are their suggestions.

Scout, rotate. “First and foremost, you need to be out there looking for resistance,” Nielson said. Look into the history of that site, the chemistry used. “Try to catch it earlier [rather] than later. Have a plan, and rotate chemistries — that’s huge.” If you use generic products, sometimes you might be using the same product that you applied years prior. Know your product ingredients, he adds.

“Nothing replaces scouting,” Peters said. “Trust your eyes.” He shared an example of plants looking affected, questioning what might have happened and then blaming the sprayer. “I’d rather address weed resistance in Year 2 than Year 3,” he said.

The drought has made it challenging to tell if weeds are stressed or resistant. One way to tell the difference is to look for dead weeds next to live weeds. That’s usually a sign of trouble.

“Generally, weed resistance doesn’t show a pattern in the field,” he said.

Keeping good records and rotation maps is also important.

“One advantage of crop rotation is that it opens up options [for control],” Peters said. “I like three-crop rations, because they add more families of herbicides.”

Specific to insecticides, McRae said that rotating chemistries isn’t as effective in reducing resistance.

“With insecticides, it’s better with rotating single modes of actions over time,” he said.

If you have a mix of alive and dead pests, such as the Colorado potato beetle, you may have a problem with resistance. To check, McCrae suggested using an on-farm dip test provided by the Ontario Ag Ministry.

Use sticky traps. Ostlie said the challenge for growers dealing with corn rootworm this season is to keep an eye on populations and manage them in their landscape.

“This year, all conditions pointed to an increase in corn rootworm injury — early planting, overwintering, not much rain to drown larvae and now drought,” he said. Now in late June, growers are dealing with secondary issues — growth effects, lodging and yield impacts.

“In the end, the two most important things — [checking for] root injury and the numbers of beetles emerging. The best tools to use are to scout and use sticky traps.” He estimated peak injury occurring around and after the July 4 holiday.

Do not overapply. With fungicides, product failure is more related to foliage coverage instead of resistance, Malvick said. He noted concern about overapplication of fungicides in other states that would impact fungal diseases migrating here, such as frogeye leaf spot.

“The more we apply herbicides, insecticides and fungicides when they are not needed, the more they could develop resistance,” Malvick said. “Do not apply them for yield benefit.”

Concluded Potter: “The biggest take-home with resistance is, don’t poke the bear. Don’t apply [chemicals] if the populations are not there doing damage.”

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