Bean Leaf Beetle
Cerotoma trifurcata Forster
Appearance and Life History
The bean leaf beetle can be a destructive pest of soybean in the Midwest. This beetle is native to North America and was first described as a pest of edible beans when the continent was first colonized. As soybean production increased, the bean leaf beetle adapted to feeding on soybean foliage and pods.
Photo by J. Obermeyer
Photo by J. Obermeyer
Photo by J. Obermeyer
Photo by B. Christine
Bean leaf beetle adults are found in soybean throughout most of the season causing damage to soybean foliage and/or developing pods. They are about 1/4 inch (6 mm) long and usually yellow, tan, or red in color. The beetle is distinguished by a distinct black triangular mark on its wing covers directly behind the thorax (the "neck" area of the insect). Often two black spots are present on each wing cover and a distinct black band extends around the outer edges. Sometimes the wing covers have no markings, except for the black triangle. When disturbed, the bean leaf beetle typically folds its legs and falls to the ground where it may remain motionless for some time.
The bean leaf beetle larva is white and distinctly segmented, with a brown head and a brown hardened area at the posterior end of its body. To the naked eye, it looks almost identical to the corn rootworm larva. It feeds on soybean nitrogen fixing nodules and, to a lesser degree, soybean roots.
A heavy bean leaf beetle infestation may cause serious leaf and pod damage. Overwintering adults colonize early emerging soybean, feeding on the cotyledons, stems, unifoliate leaves, and emerging trifoliolates. Stand losses are possible in soybean at this stage of growth. First and second generation beetles chew small round holes in soybean leaves during mid and late season. Once the maturing foliage becomes less attractive to beetles, they feed on the green tissue of pods, leaving a thin membrane over the seed. During pod maturation, this membrane often cracks leaving an entry hole for air borne plant pathogens. These pathogens may cause discolored, moldy, shriveled, and/or diseased beans.
As noted above, the larvae feed on root nodules and girdle roots. The amount of damage that normally occurs has not been shown to significantly reduce yields. Therefore, no specific sampling technique has been developed to determine larval population levels.
There are 3 important times to evaluate fields for bean leaf beetle activity:
- Early in the season it is important to determine the amount of cotyledon, stem, and foliage feeding damage caused by the overwintering adults. Since plants are small at this time, it's easy to determine the presence of beetles and their damage. While scouting, determine the degree of cotyledon or stem damage and/or the average percentage defoliation level for plants in each of 5 areas of the field and note the number of beetles per plant. Also, note whether the growing point is being severely damaged or killed on any of the plants. Watch for areas where replanting may be necessary due to seriously damaged or dead plants.
- Mid and late season defoliation from first and second generation beetles must be assessed to determine whether a control application is necessary. Determine the percentage defoliation level for individual plants in 5 separate areas of the field and estimate the average percentage defoliation level for the field as a whole. Assess the beetle population, and other defoliators, in these same areas to determine their density and activity. It is important to approach the sampling area with caution since the beetles will drop from the foliage to the ground and hide with the least bit of plant movement. Mid-morning (after the dew has dried from the foliage) and early evening sampling will give an accurate picture of the population since beetles seek cooler areas such as cracks in the ground during the heat of the day.
- Late in the season observe whether beetles are feeding on the pods. This damage results in a white paper thin membrane which covers the seed. Seldom does feeding damage extend into the seed. Do not confuse this with grasshopper damage which usually extends through the pod to the seeds. Randomly select 2 plants in each of 5 areas of the field and count the number of pods per plant and the number that show insect damage (10 total plants). Figure the percentage of damaged pods per plant for the field as a whole. Note if the pods are green, beginning to turn yellow, or are yellow/brown. Also determine the number of beetles per sweep using an insect sweep net. Take 5 sets of 20 sweeps in the field. Determine the number of bean leaf beetles per sweep. Additionally, note whether beetles are still actively feeding while surveying the field.
Soybean Insect Control Recommendations: E-series 77-W (PDF)
- For VE - VC stage soybean, refer to the following thresholds:
- For defoliation in V1 - R6 stage soybean, consult the following table; also take into consideration whether the number of beetles present is increasing, decreasing, or appears to be constant.
- For pod feeding in R5 - R7 soybean, see the following table:
If control is necessary, contact your state Cooperative Extension Service or click here for control materials and rates.