Bean leaf beetles in Minnesota soybean
Photo: Ward Upham, Kansas State University, Bugwood.org
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)
Adults (Figure 1)
- Oval–shaped body, about ¼ inch long
- Often greenish–yellow in color, but can also be red, brown or orange
- Generally with four black spots on wing covers, but spots can be absent
- Regardless of color, always have a black triangle at the top of wing covers
Small, orange–colored eggs are laid in the soil.
Larvae are white–colored with a brown head, less than ½ inch long, and occur in soil.
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).
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.
Begin scouting plants at the seedling stage and continue through pod and seed development.
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:
- From each plant, select a leaf from the top, middle and bottom third of the plant.
- 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).
- This average percent defoliation for the field's canopy can be compared to treatment thresholds.
- For vegetative plants (before flowering), treat if beetles are present and defoliation reaches 30 percent.
- For reproductive plants (flowering to pod fill stage), treat if beetles are present and defoliation reaches 20 percent.
- Treat if bean leaf beetles or other pod feeding insects are present and pod injury reaches 10%. Treat aggressively if populations are large and pod clipping is occurring.
- Labeled rates of foliar insecticides can be used to manage defoliation and pod injury (follow directions on the product label).
- Insecticidal seed treatments may be considered for fields with histories of significant early infestation or increased vulnerability to this pest and diseases it can transmit (e.g., fields grown for seed production).
- Fields should be scouted after treatment for re–colonization.