Beetle Alert

Japanese Beetles: Keep Calm and Scout On

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Japanese beetles cluster on a young corn ear. At high populations, the beetles can clip corn silks and disrupt pollination. (DTN photo by Pamela Smith)

ROCKVILLE, Md. (DTN) -- Don't be blinded by the metallic gleam -- Japanese beetles are often more flash than substance when it comes to row crops.

"Beetle damage to cultivated crops is often minimal and defoliation (leaf removal) on soybeans usually looks much worse than it is," cautioned Purdue University entomologist John Obermeyer, in a university pest alert.

Japanese beetles are out in force this year across the Midwest. Entomologists from Indiana, Kansas, Kentucky, Missouri, Nebraska, Michigan and Wisconsin are advising corn and soybean growers to be on the lookout for the brown and green metallic beetles in their field.

Japanese beetles are well established in the eastern U.S., and they can feed on a huge range of plants, especially ornamentals, horticultural crops and fruit trees. In recent years, their populations have been growing in Midwestern states, where they've proven fond of corn silks and soybean leaves.

The beetles do the most damage during flowering and podfill in soybeans and silking in corn, Obermeyer noted. More than half of the nation's cornfields are at or beyond silking, and nearly 60% of beans are entering the bloom stage, according to USDA's most recent crop progress estimates.


Soybeans handle beetle damage far better than one might imagine. Before blooming, a soybean plan can withstand up to 50% defoliation before treatment is necessary, Obermeyer said. After the plant starts flowering, up to 25% defoliation is acceptable, he said.

It's important not to overestimate the damage and spray prematurely, because the pyrethroids labeled for Japanese beetles can invigorate spider mite populations, which are also on the rise in parts of the Midwest.

Here are some visuals from the University of Illinois that show what 6% to 50% defoliation of a leaf actually looks like:…

Beetles tend to congregate on field edges, so be sure to venture deeper into your field on scouting trips to get a more accurate picture of the damage, Obermeyer said.


Japanese beetles will settle for green corn leaf tissue, but they much prefer young corn silks, University of Nebraska entomologist Bob Wright told growers in a university pest update.

Most entomologists agree that insecticidal treatments in corn may be necessary if all three of these conditions exist -- three or more beetles per ear, silks clipped within 1/2 inch of the ear and a field less than 50% pollinated.

Japanese beetles only have one generation during the summer, and their populations usually peak about four to five weeks after you first spot them in your area.

There are a number of look-a-likes around, so be sure the beetles in your field are true Japanese beetles.

You can distinguish them from other beetles like the masked chafer, green June beetle and the false Japanese beetle (or sand chafer) by the six white hair tufts on each side of their abdomen. See pictures of each insect in this Pioneer guide:…

You can find Obermeyer's full comments on Japanese beetles in Indiana this year here:…, and Bob Wright's article here:…

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Emily Unglesbee