Prepare for the Onslaught of Japanese Beetles

That whirring noise you might be hearing isn’t a plane in the distance, instead it’s the approach of one hungry pest: The Japanese beetle. You’re likely seeing damage in your gardens, soybeans or corn, it’s time to scout to see exactly how bad the damage is.

The pest first emerges in late May or early June, reaching its peak emergence just over one month later, according to Wyffels Hybrids research. The average beetle adult lives 30 to 60 days and lays eggs frequently—up to four eggs every three days. Larvae overwinter, emerging when soil temperature exceeds 50 degrees.

Corn Management

If you see Japanese beetles in your corn fields it could mean poor pollination is imminent. The pest loves to snack on corn’s delicate silks—and if they clip them to ½” or less the crop might not pollinate.

Consider a foliar insecticide treatment during tasseling and silking if there are three or more beetles per ear, silks are clipped to ½” and pollination is less than 50% complete, according to Eileen Cullen, University of Wisconsin Extension entomologist.

“[If applying an insecticide] beetles must be on the outside of the ear, which is normally the case,” Cullen says “The main concern with Japanese beetle feeding is to protect silks for pollination.”


Scouting tips in corn and soybeans, source: University of Wisconsin
Soybean Management

Japanese beetles have colossal appetites, and soybean leaves make for a satisfying snack. If they eat too much it might mean you need to consider insecticide treatment.

According to Purdue Extension, Japanese beetles skeletonize soybeans leaves, striping tissue between larger leaf veins but eating smaller veins. Sample fields to determine if you’ve met threshold. When sampling, take 20 sweeps with a sweep net in five areas of the field and count the number of beetles. Then, determine defoliation for the field as a whole.

Consider an insecticide treatment if beetles are actively feeding and there is 30% defoliation prior to bloom or 20% defoliation during flowering or pod fill.

Soybean defoliation

Check out what farmers and agronomists are seeing across the Midwest:

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