Scientific name: Cerotoma Trifurcata
Order: Coleoptera
Family: Chrysomelidae

Info

Biological Description

• Adults: Beetles are about ¼ inch long and can be a variety of colors including tan, red, green, or yellow. Often there is a distinct black band around the outer edge of the wing covers and two black spots on each wing cover. The only consistent marking is the black triangle immediately behind the head on the wing covers.
• Confused with: Corn rootworm
• Larvae: Larvae are white and distinctly segmented, with brown heads and a brown hardened area at the posterior end of the body.
• Pupae: White.
• Eggs: Orange and spindle-shaped.

Economic Importance

• Bean leaf beetle is a common pest of soybeans.
• Susceptible crops:
  • Soybeans
  • Snap beans
  • Peas
  • Dry beans

Life Cycle

• Bean leaf beetle overwinters in Wisconsin as an adult under plant debris in woodlots and fence rows, and near infested soybean fields.
• Adults become active in the spring in the beginning of April when temperatures reach 50-55°F. Early in the season they feed on cultivated and wild legumes, but once soybeans emerge beetles migrate to soybean fields. Here they feed on soybean foliage and lay eggs. Eggs are laid in May.
• Larvae feed on soybean roots and nodules.
• Generations:
  • There are two generations per season
  • First generation takes about 50 days to complete, with adults emerging in late July and early August.
  • Second generation emerges during late August and early September.

Damage/Symptoms

• Primary damage is a result of adult feeding. Colonizing spring adults feed on cotyledons and open unifoliate leaves of emerging soybeans. Adults of the first generation feed on leaves, rarely exceeding economic thresholds. Adults of the second generation feed on leaves but also pods when leaves begin to yellow and dry.
• Adult leaf feeding looks like round holes between the major leaflet veins. This type of damage is in contrast to ragged, irregular holes that are indicators of caterpillar and grasshopper feeding.
• Pod feeding does little direct economic injury, but can allow fungal growth. Pod feeding can lead to pod scarring, pod clipping, and decreased seed quality. Pathogens can enter damaged areas and seeds may become discolored, shrunken, or moldy as a result.
• Larvae feed on root hairs and nodules, however it is not known if this feeding affects soybean yield.
• Bean pod mottle virus (BPMV) can be transmitted by adult beetles during feeding. While BPMV transmission can occur at any stage, early infection in the VC to V2 stages poses the greatest risk to yield loss.

Scouting Procedure and Economic Threshold

• The University of Nebraska recommends considering treatment when the beetle density reaches 16 per foot of row in the early seedling stage. By the V2 stage, 39 beetles per row foot is the threshold at which economic injury is likely to occur. Examine 5 feet of row in each of five locations throughout the field. Divide the total number of beetles found by 25 to estimate the number per foot of row.
• Alternatively, you can calculate the average number of beetles/plant from the five sampled areas in the field. ISU entomologist Marlin Rice has developed a table of dynamic early-season bean leaf beetle economic thresholds in soybean that incorporate crop value and treatment cost. Between VC and V2 growth stages, thresholds range between 2.0 and 10.7 beetles/plant depending on crop growth stage, crop value and insecticide treatment cost. Consult UWEX Publication A3646 Pest Management in Wisconsin Field Crops to match your scouting information with numbers in the ISU table.
• A defoliation threshold can also be used. When soybeans are between pod fill and maturity, economic threshold is approximately 25% defoliation.

Integrated Control

• Cultural control: Fields planted earliest in an area are at higher risk of high beetle populations, as these will be the fields first colonized by the overwintered population in the spring.
• Chemical Control: For current Wisconsin recommendations, consult University of Wisconsin-Extension Bulletin #A3646, Pest Management in Wisconsin Field Crops.

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Site Links

Insect Images

• Soybean pest images

Websites

• UW Soybean Plant Health: Bean Leaf Beetle

Extension-Based Conference Proceedings

• Cullen, E.M., Lambrecht, K. and Phibbs, A. 2005. Bean leaf beetle and bean pod mottle virus in Wisconsin - where do we stand? pp. 83-85 In Proceedings 2005 Wisconsin Fertilizer, Aglime and Pest Management Conference. University of Wisconsin, Madison, WI. (PDF)
• Cullen, E., Gaska, J., Jensen, B. 2004. Bean leaf beetle control with seed treatments. pp. 103-111. In Proceedings 2004 Wisconsin Fertilizer, Aglime, and Pest Management Conference. University of Wisconsin, Madison, WI. (PDF)

Wisconsin Crop Manager Articles

• Bean leaf beetles are active, scout emerging soybeans. Wisconsin Crop Manager, Vol.14 No.12, May 24, 2007. Cullen, E
• Prepare for 2nd Generation Bean Leaf Beetle in Soybeans. Wisconsin Crop Manager Newsletter, Vol.13 No.21, Aug. 3, 2006. Cullen, E.
• What kind of bean leaf beetle activity are we expecting as soybean seedlings emerge? Wisconsin Crop Manager, Vol.12 No.11: 78-80. May 19, 2005. Cullen, E.
• 2005. Bean pod mottle virus - a no-show in Wisconsin soybeans in 2004. Wisconsin Crop Manager, Vol.12 No.1: 6-7. Jan. 17, 2005. Cullen, E., K, Lambrecht, A. Phibbs.

PowerPoint Presentations

• 2007 Pest Management Updates, slide 52

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