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INTRODUCTION The bean leaf beetle (Ceratoma trifurcata) is a common pest of Midwest soybean. It is a small beetle, smaller than most lady beetles. The red form shown here is just one of the numerous color forms known for this species. Other common colors include yellow, green-yellow, salmon, and orange. The configuration of spots on the back also varies among individuals. The six-spotted form shown here is most common. Regardless of spot configuration, there always is a black, triangular mark on the anterior margin of the elytra (wing covers). |
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LIFE HISTORY Adults lay eggs in soil near leguminous plants. BLB larvae feed on soybean roots and are not considered serious pests. After pupation, adult beetles emerge and begin crawling on the foliage. These newly emerged beetles, such as the one shown to the right, are pale and soft. Such insects are called "teneral" adults. The presence of many teneral beetles in the field is a good indicator of increasing adult populations and feeding pressure. Most areas of the Midwest have two summer generations of BLB, with the second generation overwintering into the following spring. Beetles found on emerging soybean in May and early June are from this overwintered generation. |
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DEFOLIATION Adults chew on foliage, leaving distinctive round holes on the leaves. Although the defoliation can at times look severe, soybean has a remarkable ability to tolerate and compensate for injury. |
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FEEDING INJURY ON GARDEN BEANS BLBs also can cause severe damage to other beans, particularly green bean, snap beans, or garden beans. The picture to the right shows beans from my garden shortly before they were entirely consumed by beetles in 2002. Many gardeners in Janesville, Wisconsin, reported total loss of their garden beans to this pest. |
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POD FEEDING Late in the summer, beetles feed on developing and filling pods, causing pod scarring and decreased seed quality. This pod feeding can cause significant yield loss, necessitating insecticide treatments in some cases. |
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TRENDS IN BLB POPULATIONS In recent years, BLB populations in the Midwest have expanded both in magnitude and in range. The beetle population in Rock County, Wisconsin, was greater in 2002 than in any previous year, continuing a trend in increased abundance. Many growers in the area treated sprayed fields in August or early September to control beetles. Possibly, the great increase in soybean acreage in Wisconsin has contributed to this increase. But the mild winters in recent years likely have affected BLB populations even more. |
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BEAN POD MOTTLE VIRUS - A NEW PROBLEM Insects are not the only pests that attack soybean. A variety of viruses, fungi, and bacteria cause soybean diseases that also can reduce yield. One important soybean disease is bean pod mottle virus (BPMV). This pathogen causes abnormally formed leaves in some cultivars of soybean under some conditions, but more commonly results in mottled (stained) seed and yield loss. BPMV is transmitted from plant to plant by BLB and several other insects, mostly beetles. As BLB populations have increased, so have our yield losses to BPMV. |
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IMAGE LIBRARY The following are images of BLBs and BLB injury. These are the originals, so download time may be slow. Smaller versions of some of these images have already have been used on this page. Bean leaf beetle (red
phase)
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ONGOING RESEARCH Since 2000, we have been working collaboratively with soybean plant pathologists, entomologists, and agronomists at the University of Wisconsin - Madison as part of a 13-state team of researchers to investigate emerging problems with BPMV and other insect-transmitted plant pathogens. We have learned much about the biology and behavior of the BLB and have found that it is a highly efficient vector of BPMV. We are searching and screening for BPMV-resistant cultivars that could in the future reduce our reliance on pesticides. We are evaluating management practices for the BLB, hoping to find ways to limit spread of virus from infected plants. |
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