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Refresher on scouting for and managing defoliating insects in soybean

by Robert Koch (Extension Entomologist), Tavvs Alves (Grad. Student), Anh Tran (Grad. Student), and Wally Rich (Junior Scientist)

As you begin scouting soybean fields for soybean aphid, you should also be on the lookout for other insect pests and the injury they can cause on the plants. One such group of additional pests is referred to as “defoliators” or “defoliating insects.” Defoliators are insects that eat the leaves of plants. In soybean, we can find a diversity of defoliators, including various beetles, caterpillars and grasshoppers.

This article is not a call to arms. So far this year, we have not seen or heard of any significant infestations of defoliating insects in Minnesota soybean. Here are a few recent observations:

  • Low levels of bean leaf beetle adults and green cloverworm larvae (caterpillars) have been observed in some fields.
  • Japanese beetle adults are beginning to emerge in southeastern Minnesota. This week, Japanese beetles and low levels of defoliation were observed on soybean on the St. Paul Campus of the U of MN. This insect is not yet widely distributed in Minnesota. Larvae feed on plant roots and adults feed on leaves of a long list of plants, including soybean and corn.
  • Interestingly (at least to entomologists), we encountered a soybean field with an abundance of colaspis beetles near Rosemount. Last week, we collected more than 40 beetles per 50 sweeps in the field, but levels of defoliation were very low. We will continue scouting the field. We have not heard of other soybean fields with similar levels of infestation this year.  These tan-colored beetles with dark stripes are about the size of northern corn rootworm adults. They can be found in various crops such as soybean and corn where adults feed on leaves and larvae feed on roots. Colaspis beetles are sporadic and economic loss due to this pest is rare.

Defoliating pests infrequently reach significant levels in Minnesota soybean, but when they do, treatment may be required to protect yield. To determine when to apply insecticides, rely on scouting and thresholds (which we will explain below). Begin scouting plants at the seedling stage and continue through pod and seed development.

To obtain an estimate of the level of defoliation for a field:
  • Select at least ten plants (more for larger fields) spread throughout the field.
  • From each plant, select a leaf from the top, middle and bottom third of the plant.
  • Use the visual guide for estimation of soybean defoliation on the bean leaf beetle or Japanese beetle webpages 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. 
  • 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 (up to R1) and 20% defoliation from bloom (R1) to pod fill (R6) 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 determine what pests are causing the damage and if they are still present. A sweep net can be used to collect insects from the canopy to determine what defoliators are present and likely causing the injury observed. In addition, the shape, size and pattern of holes chewed in the leaves or on the edges of the leaves can also provide some insight as to what pest is causing the injury. For example, bean leaf beetles often leave small rounded holes, Japanese beetles “skeletonize” the leaves leaving a lacy appearance, and grasshoppers and some caterpillars can leave large irregularly-shaped holes. 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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