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Insect pests trying to avoid detection

While walking through some soybean plots earlier this week, I noticed several things that made me take notice.


The grasshoppers that had been prevalent in the neighboring spring wheat fields had hopped on over to soybean now that their small grains meal ticket has nearly matured.  While it was very nearly impossible to capture a grasshopper photo as they spooked each time I’d get within camera-aiming proximity, here is one that was making its way toward the field edge (Figure 1).
Figure 1. Grasshopper caught feeding on soybeans in Crookston, MN.

Now that most soybeans are in the reproductive growth stages, Extension field crops entomologist Dr. Bob Koch and research assistant Suzanne Wold-Burkness recommend estimating defoliation and pod injury to determine whether treatment thresholds have been reached.

From a webpage devoted to grasshoppers on soybean, Koch and Wold-Burkness state,

As plants get larger, visually inspect plants for defoliation. To estimate defoliation, examine a minimum of 10 plants.

To estimate percent defoliation:
From each plant, select a leaf from the top, middle and bottom thirds of the plant.
  • Use Figure 2 to estimate percent defoliation for each leaf and determine the average percent defoliation across the three leaves from each plant.
  • This average percent defoliation for the field's canopy can be compared to treatment thresholds.
Risk for infestation by grasshoppers is greater in years following long, warm autumns and warm, dry springs. Populations tend to build over multiple years, so high populations observed in one year could indicate higher risk the next year.
Figure 2. Percent defoliation of soybean leaves. Photo: Robert Koch, UMN Extension


For defoliation-based thresholds, the following recommendations apply:
  • For vegetative plants (before flowering), treat if grasshoppers are present and defoliation reaches 30 percent.
  • For reproductive plants (flowering to pod fill stage), treat if grasshoppers are present and defoliation reaches 20 percent.
  • Treat if grasshoppers or other pod feeding insects are present and pod injury reaches 10 percent. Treat aggressively if populations are large and pod clipping is occurring.
Management decisions can also be based on insect counts. To find out more about count-based thresholds, see the NPIPM fact sheet”.
My quick and dirty injury estimate of both defoliation injury and injury to or clipping of developing pods suggests that not all treatment threshold elements are present; while grasshoppers are indeed present, defoliation is nowhere near 20 percent nor has pod injury or clipping yet been found.   The situation in this field does not suggest that a treatment would pay, but the situation may be different in your field – this is why scouting fields beyond field edges is so important for making management decisions.

Should your scouting efforts indicate that treatment is warranted, the NDSU Extension IPM team has put together insecticide efficacy data for pests of each of the major agronomic crops (including soybean) in North Dakota.   While it is always important to be ensure that your preferred product is licensed in Minnesota, NDSU’s North Dakota Field Crop Insect Management Guide is a great starting resource.

Thistle caterpillars.

While grasshoppers can leap long distances to avoid predators (and human scouts) other insects create their own hideout, away from prying eyes.  Soybean leaves in the same plot that the grasshoppers were in were found curled up around insect larvae that had each created an enclosed ‘restaurant' in which to feed undisturbed by predators (Figure 3).
Figure 3. Soybean leaf curled to conceal feeding caterpillar.
A closer look revealed webbing that the caterpillar used to tie leaflet edges together to create its enclosure (Figure 4, yellow arrow), shiny piles of frass (a fancy word for insect waste, white arrow) and the cause of the webbing and frass – a thistle caterpillar (thanks to UMN Extension’s Bruce Potter for id, blue arrow).
Figure 4. Webbing (yellow arrow) and frass (white arrow) produced by a thistle caterpillar (blue arrow).
Leaflet tissue that is curled and being fed upon is not contributing as much to eventual yield as a non-curled, non-fed-upon leaflet.   However treatment thresholds for the thistle caterpillar and other leaf feeding insects depends upon presence of the caterpillar and a growth stage-dependent percentage leaf injury being reached (20 percent throughout the canopy on this reproductive crop).  For additional information about this pest – and the beautiful butterfly it will become – visit the UMN Extension thistle caterpillar on soybean website, or Extension IPM specialist Bruce Potter’s article about painted lady butterflies in the June 20, 2019 issue of his newsletter.


Koch, R. and Wold-Burkness, S. 2015. Grasshoppers on soybean. University of Minnesota Extension.

Koch, R. and Wold-Burkness, S. 2015. Thistle caterpillar on soybean. University of Minnesota Extension.

Potter, B. 2019. Southwest Minnesota IPM Stuff Newsletter. Issue 6.
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