Leaf feeding by the bean leaf beetle can be identified by small round holes between the veins. Although leaf feeding injures the plants, soybeans can withstand a surprising amount of defoliation without incurring economic losses. In the vegetative stages, soybeans can usually sustain 50% leaf area loss without economic yield reductions.
Bean leaf beetles do not feed directly on soybean seeds, but they reduce soybean seed yield and quality by feeding on the pods. Occasionally, entire pods may be clipped when feeding occurs at the base of the pod. During a drought year, beetles were observed to clip pods at a rate of 0.125 pods per beetle per day (Smelser and Pedigo, 1992). Beetles frequently consume the outside layer of pod tissue, leaving a thin layer still covering the seed. Moisture and diseases can enter the pod through this lesion. Secondary infection by fungal pathogens such as Alternaria results in shrunken, discolored and moldy seeds.
The Virus Connection
The bean leaf beetle is also a vector of several soybean viruses, including yellow cowpea mosaic, cowpea chlorotic mottle, southern bean mosaic, and bean pod mottle virus. Bean pod mottle has been identified at increasingly high levels in Illinois, Iowa and other major soybean-producing states. This virus can reduce yields 10-15%, and by much more in combination with other viruses.
Bean pod mottle virus causes mottling and distortion of the upper soybean leaves. The crinkled leaves and stunted plants can resemble injury from herbicide drift or soybean mosaic virus. Death of new terminal leaf growth may also occur. The virus also gives rise to “green stem” symptoms in some soybean plants. Affected plants do not mature normally, and stems remain green throughout the harvest period. (However, factors other than virus are implicated in green stem syndrome as well.)
Bean pod mottle virus may also affect the seed, causing a light purplish discoloration of the seed coat. Seed mottling may also occur, resulting from pigments diffusing from the hilum of the seed. Yield reductions of 3–52% may occur depending on the soybean variety and the time of infection (see Hadi et al., 2012 for a detailed discussion).