By Scout Nelson
Bean leaf beetles remain active in South Dakota soybean fields even though their winter survival is expected to be lower this year. Some beetles survive by finding protected places during winter. Adult beetles feed on young soybean plants in spring and also feed on alfalfa before soybeans emerge. South Dakota usually has two generations of bean leaf beetles each year, with the next generation expected during mid- to late July.
Adam Varenhorst, Associate Professor and SDSU Extension Field Crop Entomologist along with Philip Rozeboom, Patrick Wagner, Connie Strunk, and Madalyn Shires, highlighted the importance of regular scouting for bean leaf beetles and bean pod mottle virus throughout the growing season.
Adult bean leaf beetles can be brown, yellow, orange, or red. They are identified by a black triangle behind the head and four black spots on their hardened wings. Because these insects quickly move or hide when disturbed, they can be difficult to find. During early soybean growth, growers should look for beetles and small feeding holes in leaves. Later in the season, sweep nets provide a more effective scouting method.
To evaluate leaf damage, growers should inspect 10 plants at five different locations across the field for a total of 50 plants. The average percentage of leaf area removed should then be calculated. During the vegetative stage, treatment may be needed when defoliation reaches 30%, which can result in 3% to 7% yield loss. After flowering, the treatment threshold is 20% average defoliation across the field. Foliar insecticides may be used if economic thresholds are reached.
Bean leaf beetles also spread Bean Pod Mottle Virus (BPMV) by feeding on infected soybean plants and then moving to healthy plants. Infected plants may develop yellow and green mottling, wrinkled or distorted leaves, stunted growth, delayed maturity, green stem disorder, fewer pods, smaller seeds, and seed coat discoloration. The virus may also increase seed infection by other diseases, reducing grain quality.
There are no in-season treatments once soybean plants become infected with BPMV. Preventive practices remain the best defense. Farmers are encouraged to plant tolerant soybean varieties, use insecticide seed treatments where overwintering beetle populations survive well, and continue regular field scouting. While foliar insecticides can reduce bean leaf beetle populations, they do not eliminate the virus after infection has occurred, making early detection and prevention essential for protecting soybean production.
Photo Credit: istock-urpspoteko
Categories: South Dakota, Crops, Soybeans