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Extension > Agriculture > Crops > Soybean Production > Insects & mites > Bean leaf beetles in Minnesota soybean

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Bean leaf beetles in Minnesota soybean

Robert Koch, Extension entomologist; Walter Rich, Junior scientist; and Suzanne Burkness, Research assistant; University of Minnesota
2015

Name

Bean leaf beetle-Bugwood

Photo: Ward Upham, Kansas State University, Bugwood.org

Bean leaf beetle-Koch

Bottom photo: R. Koch, U of MN

Figure 1. Common color morphs of bean leaf beetles found in Minnesota. All bean leaf beetles have a black triangle at the top of their wing covers.

Bean leaf beetle (Cerotoma trifurcata)

Identification

Adults (Figure 1)

Eggs

Small, orange–colored eggs are laid in the soil.

Larvae

Larvae are white–colored with a brown head, less than ½ inch long, and occur in soil.

Natural history

Adults overwinter under leaf litter near or within soybean fields, and emerge from mid–May to early–June. Adults feed on alfalfa and clover until mid–June, at which time they move to their preferred host, soybean. As adults colonize soybean, they will mate and lay eggs at the base of soybean plants. Developing larvae feed on soybean roots, and within three weeks the 1st generation of adults emerges and feeds on soybean (often in July). In southern Minnesota, a second generation of adults is produced (late August).

Impacts

Injured soybean pods

Photo: Adam Sisson, Iowa State University, Bugwood.org

Figure 2. Injured soybean pods

Adults chew holes in leaves. Though soybean can compensate well for defoliation, heavy infestations can cause yield loss.

Adults will chew holes in pods; occasionally damaging seed (Figure 2).

Adult feeding can also transmit plant diseases, such as Bean pod mottle virus (BPMV), which can cause stunting, seed coat mottling and discoloration, and a decrease in yield. See the Extension fact sheet on Bean pod mottle virus for more information.

Larvae feed on soybean roots and nodules, but impact is assumed negligible.

Scouting

Begin scouting plants at the seedling stage and continue through pod and seed development.

defoliation guide

Figure 3. Percent defoliation of soybean leaves.

Visually inspect at least 10 plants spread throughout the field for adults and defoliation. Pods should also be inspected for injury later in the season.

To estimate percent defoliation, examine a minimum of 10 plants:

  1. From each plant, select a leaf from the top, middle and bottom third of the plant.
  2. Use Figure 3 to estimate percent defoliation for each leaf and calculate the average percent defoliation across the three leaves from each plant and then across the multiple plants (also see Visual guide for estimation of soybean defoliation).
  3. This average percent defoliation for the field's canopy can be compared to treatment thresholds.

Management

Thresholds

Treatment

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