What is that bug?

If you have been walking soybean fields recently, you may have noticed leaves with holes or a "skeletonized" appearance. One possible culprit is the Japanese beetle, a pest that has become more common in Nebraska over the past few years.Japanese beetles are probably more familiar to many homeowners because of the damage they cause to ornamental plants, especially linden trees. In large numbers, they can completely defoliate a tree. While healthy trees usually recover from this damage, repeated defoliation year after year can weaken them. These same beetles can also move into soybean fields and feed on soybean leaves.The good news is that Japanese beetles often cause more concern than actual yield loss. Soybeans can tolerate a fair amount of leaf feeding, and in many cases, fields recover without an insecticide application. The important thing is to scout fields and make decisions based on the amount of damage, not just how the field looks from the edge of the road.For soybeans in the reproductive stages, the recommended treatment threshold is 20% average canopy defoliation. This means that, on average, 20% of the leaf area across the plant canopy has been eaten away. Depending on crop conditions, soybean growth stage, insect pressure, and soybean prices, the threshold may range from about 15% to 25% defoliation. One of the biggest challenges is that soybean defoliation can be difficult to estimate. A few heavily damaged leaves can make a field appear worse than it actually is. When scouting, look at the whole plant canopy rather than focusing only on the most damaged leaves. To get a good estimate, check several areas throughout the field. Collect soybean trifoliate from the top, middle, and bottom of plants in multiple locations. Compare the amount of feeding damage to soybean defoliation reference pictures to determine the average level of injury across the field.Japanese beetles often show up first along field borders, especially near grassy areas, pastures, lawns, and roadside ditches where adults emerge. In some situations, beetles may be concentrated only around field edges, and a border treatment may be enough instead of treating the entire field. Keep in mind that Japanese beetles can continue emerging from grassy areas for several weeks, sometimes up to 10 weeks. Even after an insecticide application, new beetles can move back into the field. Also, unnecessary insecticide applications can sometimes create additional problems by reducing beneficial insects or increasing the risk of secondary pests such as spider mites.Nebraska Extension has a soybean defoliation guide with pictures that can help with scouting decisions. The guide can be saved on your phone or printed and kept in your scouting notebook for quick reference. Taking a few minutes to accurately assess damage can help prevent unnecessary insecticide applications and protect your crop investment.