Minnesota Soybeans: Keep Scouting for Defoliating Pests

Thistle caterpillar in soybeans. Photo by Erin Hodgson, Iowa State University

While soybean aphid numbers have been generally low this year, defoliating insects (especially green cloverworm and thistle caterpillar) have been abundant in soybean fields across much of Minnesota. I have also received some scattered reports increasing defoliation from grasshoppers in northwest Minnesota and Japanese beetles in southeast Minnesota.

To determine when to apply insecticides, rely on scouting and thresholds. Continue scouting through pod and seed development. Below is a summary of scouting and thresholds for defoliating insects from an earlier Crop News article.

Thistle caterpillar on soybean. (Photo credit: Adam Sisson, Iowa State Univ., Bugwood.org)

Green cloverworm on soybean. (Photo credit: Adam Sisson, Iowa State Univ., Bugwood.org)

To obtain an estimate of the level of defoliation for a field:

  1. Select at least ten plants (more for larger fields) spread throughout the field.
  2. From each plant, select a leaf from the top, middle and bottom third of the plant.
  3. Use the reference below (Visual Guide for Estimation of Soybean Defoliation) to estimate percent defoliation for each leaf. 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.
  4. The average percent defoliation per field can be compared to 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. Furthermore, this should help overcome the tendency of many people to overestimate percent defoliation for a canopy.

Treatment thresholds for defoliation from any combination of defoliating insects are 30% defoliation in pre-bloom growth stages and 20% defoliation from bloom to pod fill growth stages. When thresholds are exceeded, labeled rates of foliar insecticides can be used to protect soybean yield from losses due to defoliating insects. Follow directions on the product label. However, prior to treating a field, it is important to ensure the pests are still present. If the pests are no longer present (or no longer in feeding life stages), it makes little sense to treat the field.

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