Bean Leaf Beetle

Ceratoma trifurcata Forstere


Adult bean leaf beetles are approximately ¼ inch long and are green, yellow, tan, or red, often with a distinct black band around the outer edge of the wing covers. A black triangle is always found behind the head on the wing covers. Usually there are two black spots on each wing cover, but they are not always present. Males are distinguished from females by the black coloration between their eyes.

Larvae are white segmented grubs, with brown heads and a brown hardened area at the posterior end of the body. They may reach 1/3 inch long. Pupae are white and approximately 3/16 inch long. The lemon-shaped eggs are red-orange in color and measure 3/100 inch.

Adult bean leaf beetles overwinter beneath plant debris at the edge of woodlots or fence rows near bean fields. They become active in the spring when temperatures reach 50 F, and migrate to alfalfa, clover, and occasionally to various weeds. Bean leaf beetles are strong fliers and move to soybean fields when the plants emerge. Females lay an average of 40 clusters of 10-30 eggs per cluster. The eggs are laid over a period of 3 to 4 weeks in the top 2 inches of soil under bean plants. Temperature impacts bean leaf beetle development. Eggs hatch in 5 to 7 days at 75 F, but may take 3 weeks to hatch at lower temperatures. The time from egg hatch to pupation is 15 to 16 days at 75 F, and 50 days at 65 F. Larvae remain in the soil, first feeding on root hairs, then nodules, as they progress through three instars. Larval mortality can be attributed to inadequate soil moisture, poor nodule development, and natural enemies. Pupation takes place in earthen cells below the soil surface.


Bean leaf beetle damage of economic importance is usually caused by the adult, although the larvae feed on bean roots. The bean leaf beetle has two generations in Illinois and the adults of both generations feed on soybean leaves. Early in the summer the overwintering adults may be found on the newly emerging soybean plants, chewing holes in the leaves. In the summer their progeny will again be found in the plants, but usually by this time there is sufficient foliage that little damage is caused. Occasionally they feed on blossoms or pods late in the season.

Scouting Procedures

Several methods can be used to assess damage by the bean leaf beetle. Early in the season it is easiest to examine the plants by hand and count the number of beetles. Examine five feet of row in each of five locations and divide the number of beetles found by 25 to arrive at the number per foot of row. In addition to counting the number of beetles, estimate the percent of defoliation. After plants become larger it will be easier and more accurate to use a beat cloth. Use the beat cloth in five locations and divide by the total number of feet examined to arrive at a per foot of row average. Remember with the beat cloth you are examining rows on both sides of the cloth.

Threshold Guide

Seedlings: Treat if 20 percent of the plants are cut and the stand has gaps of 1 foot or more; or if at least 1 seedling per foot of row is destroyed.

Before Bloom: Treat when defoliation reaches 30 percent and there are 5 or more beetles per foot of row.

Bloom to Pod Fill: Treat when defoliation reaches 20 percent and there are 16 or more beetles per foot row.

Seed Maturation: Treat when 5 to 10 percent of the pods are damaged, the leaves are green, and there are 10 or more beetles per foot of row.