One of the first insects to feed on soybeans is the bean leaf beetle and now is the time to be on the lookout. Bean leaf beetles overwinter as adults and emerge to give rise to 1st and 2nd generations of adults. Below is the life cycle of the bean leaf beetle.
Source: Purdue University
Early planted soybeans, and especially the first fields to emerge in an area, may exhibit seedling injury due to feeding by overwintering adults as they emerge from the soil in the spring. If seedling injury and early defoliation are severe, and stand loss is evident, a rescue treatment may be necessary to protect the stand. These early emerging adults mate and lay eggs, giving rise to the 1st generation. Feeding injury appears in late June and continues until fall as a succession of first- and second-generation adults emerge and feed. The second-generation adults then burrow into the soil to overwinter.
Emerging overwintering and first generation adults generally don’t do that much damage to soybeans, other than the early emerging fields. The reason is that soybeans produce ample foliage and some defoliation can be tolerated.
Most damage occurs when second-generation beetles feed on the developing pods. This yield loss can occur in several ways. Pods may be clipped from the plants, or fungal pathogens may enter the pod, causing seeds to appear shrunken, discolored and moldy, reducing seed quality. Bean leaf beetles are known to transmit bean pod mottle virus, cowpea mosaic virus and southern bean mosaic virus.
The economic threshold can range from as low as 3 beetles per plant to as high as 6 beetles per plant in the VC stage and 4 to 8 beetles in the V1 stage. The threshold on prebloom soybeans is 40% defoliation before treatment is justified. This rarely happens except in special circumstances.
Seed treatment insecticides offer a degree of control when soybeans are planted, emerge early and are the only crop around. However, the protection is relatively short-lived as the plant grows and the internal dose dilutes down. There are a number of postemergent insecticides available that provide good control.
Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at email@example.com or call him at 402-649-5919.