Japanese Beetle Scouting and Thresholds for Corn and Soybean

Japanese Beetle Scouting and Thresholds for Corn and Soybean

Eileen Cullen, Extension Entomologist

Japanese beetles have been reported this week feeding on corn silks (and in one report, kernels) in southeastern Wisconsin. Japanese beetle scouting and thresholds for corn and soybean are covered in this article.

Japanese beetles overwinter as late stage grubs within 5 inches of the soil surface in areas surrounding soybean or cornfields, typically in grassy and turf areas. However, some grubs can overwinter in soybean fields.

In early summer as temperatures increase, larvae move closer to the soil surface, where pupation occurs, followed by adult emergence in June. There is one generation of Japanese beetle per year with adults active in July and August through to early September.

Photo: Roger Schmidt, University of Wisconsin-Madison


Adults feed on soybean leaf tissue between leaf veins leaving a lace-like, skeletonized appearance.

  • The treatment threshold for Japanese beetle in soybean is based on percent leaf defoliation and not number of beetles per plant. Treatment should be considered at 20% leaf defoliation for reproductive growth stage soybean. (30% for pre-bloom soybean).

Scout for soybean leaf defoliation throughout the field rather than just along field edges where clusters of Japanese beetles aggregate and feeding can sometimes be more concentrated. Field edge/border treatments may be sufficient if damage is confined to this area, but interior field scouting is recommended.

In addition to percent defoliation estimates based on observation of field areas with beetles and obvious defoliation damage, a systematic field sampling method is to select 10 plants throughout the field choosing a trifoliate from the upper, middle and lower canopy on each plant.  This will give you a 30-leaf sample to assess whether feeding is progressing through the canopy or only at the upper canopy.

It can be easy to overestimate defoliation percentage. The following images provide a guide to determining the percentage insect defoliation on soybean leaves from Japanese beetle and other leaf defoliating insects such as grasshopper, green cloverworm, second generation bean leaf beetle. (Images courtesy of University of Wisconsin-Madison Integrated Pest and Crop Management IPM Toolkit App for iPad and iPhone).

Do not treat too early for Japanese beetle below the defoliation threshold of 20%. This is particularly important if you have twospotted spider mite populations in the field. Pyrethroid insecticides are labeled for Japanese beetle control in soybean, but some pyrethroids provide mite suppression only and may flare, or increase, mite densities. For information on twospotted spider mites in soybean, please see the WCM article Dry Weather Increases Risk of Twospotted Spider Mite.


Consider a foliar insecticide treatment during tasseling and silking if there are

  • 3 or more beetles per ear
  • Silks have been clipped to ½-inch


  • Pollination is less than 50 percent complete

As field corn pollination period draws to a close, this threshold can still be applied to later planting dates and sweet corn at sensitive tasseling and silking stages.

I have heard of at least one report this week from the Janesville, WI area in southeastern Wisconsin of Japanese Beetle feeding on kernels within the ear. If this is the case, it’s likely that beetles are very heavy in the field.  The main concern with Japanese beetle feeding is to protect silks for pollination, and the economic threshold above is based on this goal. Beetles must be on the outside of the ear, which is normally the case, to be contacted by insecticide treatment.

As with soybean, obtain a representative field sample in corn because Japanese beetle may aggregate or clump near cornfield edges. Edge/border treatments may be sufficient if damage is confined to this area, but scout inner field locations to confirm.

Additional Resources:

Field Scouting Video: Japanese beetle in soybean and corn, Integrated Pest Management. University of Wisconsin-Madison Integrated Pest and Crop Management Program. Full selection of IPM videos available here.

Cullen, E., V. Davis, P. Esker, B. Jensen and M. Renz. 2012. Pest Management in Wisconsin Field Crops. Publication A3646. University of Wisconsin-Extension Cooperative Extension Publishing, Madison, WI. 258 pp.

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