KTIC Radio Extension Corner: What You Need to Know About Japanese Beetles

Figure 1. Japanese beetle feeding on corn silk northwest of Fremont during in July 2016. This picture represents low pressure well below economic thresholds for treating. 

Listen to this week’s KTIC Radio Extension Corner that discusses what you need to know about Japanese beetles:

“This is Dr. Nathan Mueller, your local agronomist with Nebraska Extension for Dodge and Washington counties. Japanese beetles have been considered infested in Dodge and Washington counties since at least 2013 (Figure 3 below). Japanese beetles feed on trees and shrubs in town, but they also can be found in corn and soybeans fields. I have seen Japanese beetles feeding in corn and soybean fields for the last three years, but at very low levels.  So let’s answers some important questions for you.

  1. What does a Japanese beetle look like?
  2. How many generations do they have per year?
  3. When and where do they start to feed?
  4. What are the thresholds that warrant an insecticide application in corn and soybeans.

First, Japanese beetles are about ½ inch long and have a metallic green head and thorax and their back or abdomen is coppery brown with a series of white tufts of hairs on the side of the abdomen. There is another beetle called the sand chafer or false Japanese beetle that looks similar, but without the features I already mentioned. Japanese beetles fortunately only have one generation per year and they typically start to appear in June into July. They are often found at the highest populations on field borders. So it is important you walk beyond the end rows and scout your corn and soybean field before jumping to any conclusions.

The treatment thresholds that warrant an insecticide application are different for corn and soybeans. In soybeans, we look at the total defoliation of the entire canopy from defoliating insects. Now that most soybeans are flowering, a soybean insecticide treatment is recommended when defoliation is expected to exceed 20% and insects are still present. In past years when grasshopper reached thresholds on field borders, some growers have elected to spray. If this is the case this year, look at an insecticides and rates that would control both grasshoppers and Japanese beetle if both are present.

In corn, we are concerned about Japanese beetles feeding on corn silks during pollination similar to corn rootworm beetles. An insecticide treatment should be considered during pollination if you answer yes to the following three criteria: There are 3 or more Japanese beetles per ear, silks have been clipped to less than ½ inch, and pollination is less than 50% complete.

As stated before, I have only seen Japanese beetles at low populations within corn and soybean fields in previous years in Dodge and Washington counties that would not warrant any treatment (Figure 1 above). So if you find Japanese beetles, determine the pressure within your field, and use these economic thresholds to make decisions. To listen to this radio message again and to get more information, visit our local website at croptechcafe.org or give me a call at 727-2775. Know your crop, know your tech, know your bottom line. This is Dr. Nathan Mueller, your local agronomist for Nebraska Extension on KTIC radio.”

This KTIC Radio Extension Corner was based on Bob Wright’s CropWatch article on Japanese beetles emerging in corn and soybean The article includes a distribution map and pictures.

Market Journal Video with Bob Wright discussing both grasshoppers and Japanese beetles on field borders:

Figure 2. Japanese Beetle Survey. Source: Nebraska CropWatch

Figure 3. 2013 Japanese Beetle Survey. Source: CropWatch and Nebraska Department of Agriculture


KTIC Radio Extension Corner: What You Need to Know About Japanese Beetles
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