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Will a Japanese beetle breakout this year force growers to spray?  Our advice – scout your fields to see if beetles are a threat and if treatment is warranted.

Photo courtesy of Eric Ifft, Bayer CropScience

In mid-July I was scouting some soybean fields in central Illinois. About 30 percent of the fields I walked in had Japanese beetle populations at the treatment levels. The other 70 percent had some beetles in them even though spraying was not justified. Of course I was only in 10 fields so that is hardly a big sample but the fact that each of the 10 fields had beetles present may foretell the need to spray.

I reached out to the ILSoyAdvisor Soy Envoys for their thoughts.

Adam Day, North Central Soy Envoy: “Looking at Japanese beetles in the Route 80 corridor I do not anticipate them reaching an actionable threshold this season. In past years we have sprayed our soybeans specifically for the Japanese beetle and at this point in our season I do not see the need to treat only for this pest.”

Lance Tarochione, Northwest Soy Envoy: “Japanese Beetle pressure in our area has been worse than normal this year but variable and in most cases has not reached a threshold where treatment is required. Infestations are typically much worse on field edges. Visual benefits may be seen from controlling moderate populations but it is not known if that will translate into a yield increase. The visual damage is often worse than the yield damage. Many producers who plan to make a fungicide + insecticide application to soybeans at the R3 growth stage will select an insecticide that offers good Japanese Beetle control.”

Stephanie Porter, West Central Soy Envoy: “There may be "hot pockets", but overall I have not seen Japanese beetle feeding beyond threshold. Some fields may have heavy feeding on edges. As we enter the reproductive growth stages, an insecticide may be warranted on field edges or throughout the field if defoliation exceeds 20 percent on several plants within 5 areas throughout the field.”

Terry Wyciskalla, Southwest Soy Envoy: “Japanese Beetle damage in the southwestern part of Illinois is sporadic. There has been very little damage to corn and soybeans in this region. Beetles emerged about 2-3 weeks ago and any soybean damage was insignificant because most soybeans were very small. Japanese beetles can be in localized hot spots and nonexistent in others and I did not encounter any hot spots or hear of any spraying in the last two weeks in Washington, Jefferson, Perry, Jackson, Randolph, or St. Clair Counties in my travels.”

Mike Wilson, Southeast Soy Envoy: “Here in Southeastern Illinois we are seeing significant Japanese beetle feeding as well as green cloverworm. We plan to use a combination fungicide/Insecticide application to most of these fields with the disease friendly weather we are having.”

When you scout your fields look for the percent defoliation. The University of Illinois recommends insecticidal treatments when defoliation reaches 30 percent before bloom and 20 percent before bloom and pod fill. In other words, the economic threshold for Japanese beetle feeding in soybeans during pod development if you see leaf defoliation at 20 percent in 5 different places in the field.

Photo courtesy of University of Illinois

The University of Illinois has two resources on the Japanese Beetle available online here and here. 

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.