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Scouting For Cutworm In Field Peas

Jun 09, 2016
By Adam J. Varenhorst
Assistant Professor & SDSU Extension Field Crop Entomologist
 
Cutworms in South Dakota
 
Recently, cutworms were reported in field peas in North-Central South Dakota. Ruth Beck, SDSU Extension Agronomy Field Specialist, observed some of these fields first hand (Figure 1). There are several species of cutworm that are occasionally pests of field peas in South Dakota. The primary cutworm species observed is the dingy cutworm; however, the pale western cutworm and redbacked cutworm also will feed on pea plants. Cutworm presence in pea fields is often not realized until after leaf feeding or plant cutting have occurred. Field peas and other pulse crops are considered more susceptible to cutworm due to their inability compensate through regrowth and their small diameter stems. Cutworms are nocturnal feeders, and hide beneath the soil surface during the day. This can make sampling and management difficult.
 
 
 
Figure 1. Cutworm damage to field pea in South Dakota. 
 
Dingy Cutworm
 
Dingy cutworm caterpillars have smooth skin, and can be identified by the four black spots (tubercles) present on each of their body segments. These spots are all similar in size. For many crops, dingy cutworm primarily feed on the leaves of plants, and their feeding is rarely associated with cut plants.
 
 
 
Figure 2. Dingy cutworm caterpillar. Photo courtesy of John Capinera.
 
Pale Western Cutworm
 
Pale western cutworm caterpillars also have characteristic black spots on each segment of their bodies, but they differ greatly in color. As their name indicates, these caterpillars are pale (light gray-white in color) with a slightly darker head capsule. Pale western cutworm caterpillars feed under the soil surface and generally cut the plants slightly below or at the soil line.
 
 
 
Figure 3. Pale western cutworm caterpillar. Photo courtesy of John Capinera.
 
Redbacked Cutworm
 
Redbacked cutworm caterpillars are named for the two red to reddish-brown stripes that run the length of their body. The caterpillars have a yellow-brown head capsule. Redbacked cutworm caterpillars generally cause notched defoliation on leaves, but as they mature they will cut plant stems near the soil surface.
 
 
 
Figure 4. Redbacked cutworm caterpillars. 
 
Scouting & Management
 
Scouting for cutworm should begin shortly after crop emergence, and for field peas continue until the end of the season. Often the first signs of cutworm feeding are defoliated leaves or cut stems. However, bare patches in fields may also be a potential indicator of their presence. In general, areas of fields on south facing slopes or with lighter soils tend to be more attractive to moths for egg laying, and caterpillar activity due to increased soil temperatures. To observe the cutworm caterpillars use a small shovel or trowel to dig up the first 1-3 inches of soil around cut plants. The current thresholds for cutworms in field peas are 2-3 cutworm caterpillars per square yard. Younger (and smaller) cutworms indicate the potential for additional plant damage to occur, therefore, caterpillar size must be considered when making management decisions.
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