–by Dr. Jeff Whitworth
Bean leaf beetles (fig. 6) overwinter in Kansas as adults. They are usually first found in late winter/early spring in alfalfa fields because as the temperatures start to warm these adults become more active and actually feed a little on alfalfa foliage. However, there are not enough to cause any problems in these alfalfa fields. However, every year they are impressive because of their super ability to detect the very first germinating soybean plants, whether volunteer or planted. They can apparently detect these early plants from many miles away. As these first germinating plants are found by these adult beetles from many different overwintering sites, they can do some quite noticeable leaf feeding damage (fig. 7). These plants are usually mainly on border rows as the adults fly to and start to feed on the first plants they find from distant overwintering sites. These oval/oblong holes can cause considerable concern, especially if only border rows are examined and growing conditions are stressful. Please remember these young soybean plants are very resilient at absorbing this early season defoliation without any subsequent impact on the plants or yield, as long as the defoliation is less than about 50% in the vegetative stages and good growing conditions return. These adult beetles then lay eggs in the soil and around the base of these plants where they hatch and the larvae feed on the roots/root hairs. These adults then emerge in mid-summer and start feeding on new leaves and/or, more problematic, sometimes on the pods.
Figure 6. Adult Bean Leaf Beetle (red color phase)
Figure 7. Damaged Soybean Seedlings (Cody Wyckoff