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Japanese beetles feeding on soybean: What should you do?

by Robert Koch (Extension Entomologist)

In parts of southeastern Minnesota Japanese beetle has been actively feeding on soybean. These large beetles with shiny green- and copper-colored bodies, chew small holes in the leaves leaving a lace-like appearance ( see image 1). Japanese beetles will also feed leaves and flowers of many other plants, including silks of corn. Japanese beetle abundance and activity is most intense through July and August, but some individuals will remain active into fall. Here, I briefly review scouting and thresholds for management of Japanese beetle in soybean.

Image 1: Japanese beetles feeding on soybean leaves.

Treatment thresholds for Japanese beetle in soybean are 30% defoliation prior to flowering and 20% defoliation after flowering. These defoliation estimates are for the entire soybean canopy, not just the top leaf or two of the plants where the beetles tend to aggregate. Soybean is a very resilient plant and tolerant of defoliation. Economically significant yield losses are not expected at these threshold levels. Treatment at these thresholds should prevent an actively feeding infestation from reaching levels that could cause economically significant losses.   To obtain an estimate of the level of defoliation for a field: 
  • Select soybean plants spread throughout the field. 
  • From each plant, select a leaf from the top, middle and bottom third of the plant. 
  • Estimate percent defoliation for each leaf. When assessing defoliation visually, most people tend to think the level of defoliation is much worse than it really is.  The
    Visual guide for estimation of soybean defoliation can help you calibrate your visual assessments of defoliation. Average the percent defoliation across the three leaves from each plant and then across the multiple plants to obtain the average percent defoliation per field. 
  • The average percent defoliation per field can be compared to the treatment thresholds for the decision about pest control. 
  • By using this method you ensure that the estimate of defoliation is representative of the whole canopy. Leaves in the middle of the plant can compensate for defoliation of the uppermost leaves. 

If threshold levels of defoliation are reached and Japanese beetles are still present, most labeled insecticides will effectively suppress this pest (follow instruction on the insecticide label). When multiple defoliating pests occur (e.g., Japanese beetle and green cloverworm), defoliation from both pests should be combined and related to the thresholds mentioned above. After treatment, continued scouting is required because this mobile pest can recolonize previously treated fields. 

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