By Anitha Chirumamilla
The hot topic of discussion at the IPM field school last week in Beresford was grasshoppers becoming an issue in soybeans. According to Dr. Sarah Zukoff, K-State Extension Entomologist, severe grasshopper outbreaks were being reported from south western Kansas. Grasshoppers are a constant problem in South Dakota and producers are often forced to spray to reduce their damage.
Grasshoppers
Irrespective of the species causing damage, grasshoppers are known to be voracious feeders removing large chunks of leaf tissue. As the hoppers increase in size, their feeding damage increases, and the efficacy of the insecticides to suppress them goes down. It is easier to control small hopper nymphs than the larger adults.
Caterpillars
Figure 1. Green cloverworm.
While hoppers are one of the major defoliators in soybeans, there are a few caterpillars that are often seen feeding on the leaf tissues. Caterpillar species noticed this year and last year in soybean fields include green cloverworms (Figure 1), woolly bear caterpillars (Figure 2), and thistle caterpillars (Figure 3). However, these caterpillars rarely become a serious concern in South Dakota.
Figure 2. Woolly bear caterpillars feeding on soybean and sunflower leaves.
Figure 3. Thistle caterpillar with webbing and frass pellets.
Sampling and Management
When it comes to defoliators, SDSU Extension Soybean Entomologist Kelley Tilmon recommends using percent defoliation and thresholds to make treatment decisions, irrespective of the insects. In general, chemical treatments are recommended if the crop suffers 25-30% defoliation at vegetative stages and 20% defoliation in reproductive stages (see Figure 4 for the defoliation estimation chart). Soybeans are capable of sustaining any defoliation less than the above stated percentages without yield loss.
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