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Watch Out for Soybean Aphid ‘Insurance’ Insecticide Applications

Soybean aphid insecticide applications below economic threshold levels can set the stage for resistance

The next time someone suggests adding a soybean aphid insecticide to a fungicide or postemergence herbicide application on soybeans, watch out.

“You will hear the phrase, ‘let’s just throw in an insecticide with a fungicide or Roundup,’ ” says Ken Ostlie, University of Minnesota (U of M) Extension entomologist. The logic is just a couple dollars per acre of insecticide can “clean up” any aphids or other insect pests lurking in your soybean field.

If soybean aphid populations are at the economic threshold level of 250 aphids per plant and increasing, insecticide use is justified. If not, applications not only waste money and insecticide, but also set the stage for resistance.

“Soybean aphids are resisting more insecticides,” said Ostlie told those attending BASF’s Science Behind Insect Control program held before this week’s Commodity Classic in Anaheim, California.

In any given pest population, there’s a certain amount that can resist a pesticide application. Each time a chemical is applied, survivors live and eventually reproduce. Over time, resistance reaches a point that renders a chemical ineffective. Unnecessary applications – such as insurance applications – hasten this process.

Key Pest

When Ostlie first started working for the U of M in 1984, soybean insect outbreaks were incidental. “There used to be scattered outbreaks (of insects), but there were no key pests,” he says.

That changed in 2000 in Minnesota with the arrival of soybean aphids. When applied at economic threshold levels, insecticides gave good control. Drawbacks exist, though.

Broad-spectrum organophosphate (chlorpyrifos [Lorsban and generics]) and pyrethroid insecticides (bifenthrin [e.g., Brigade, Tundra , Hero and others] and lambda-cyhalothrin [e.g., Warrior and others]) don’t spare beneficial insects that are natural aphid predators. Neonicotinoid insecticides (Belay, Admire Pro, Alias) are another option, but concerns exist about their impact on pollinator populations, says Ostlie.

In 2015, soybean aphids that resisted pyrethroid insecticides also surfaced in Minnesota. Pyrethroid insecticide failure then occurred in 2016, although it affected different areas in Minnesota. “In 2017, they blew up the geography and intensity, centering in the Red River Valley of Minnesota and North Dakota,” he says. Some counties in central and southern Minnesota were also impacted, as well as some areas in eastern South Dakota.

What to Do?

New chemistry will help farmers rotate chemistry to forestall resistance. At the BASF event, company officials announced BASF will be launching Inscalis insecticide. Its active ingredient is afidopyropen. It’s a mode of action Group 9D insecticide that helps control piercing and sucking insects like soybean aphids, says John Descary, BASF product manager of insecticide strategy. It’s slated to debut during the fourth quarter of 2018, pending regulatory approval, say BASF officials.

Ostlie says this will give soybean farmers more options. “It has a narrower spectrum and a good environmental profile,” he says. Rotating it with other insecticides can help forestall resistance. Besides unnecessary applications, another factor enhancing resistance is the continual use of one to two modes of action.

The exact mechanism causing pyrethroid resistance isn’t known, says Ostlie. The cause likely was already present in the aphid immigrant population that came from China and infested soybeans in 2000, says Ostlie.

“What a lot of these resistance situations have in common is overuse of same technology,” says Ostlie. Below-label application rates also contribute to resistance, he adds.

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