Corn and Soybean Growers Encouraged to Scout for Japanese Beetles Producers shouldn't spray until a certain threshold is reached. Published on: Jul 7, 2010 Tweet Post to Your Wall. Email Story RSS Permalink Print Pioneer experts are urging corn and soybean growers to scout now for Japanese beetles. They say a number of trappings and large swarms of the pest are surfacing in central portions of the Corn Belt. Marlin Rice, Pioneer Senior Research Scientist, says growers need to do a complete evaluation of fields before deciding to treat. They should scout multiple areas in a field to determine infestation levels. The beetles are mobile, and once feeding begins they emit feeding or aggregation pheromones that attract other beetles to the same location. Japanese beetles defoliate soybean plants and clip corn silks. According to the University of Missouri growers should look for a lace-like pattern on soybean foliage - starting at the top of the plant moving downward. Adult Japanese beetle feeding can cause severe damage to tassels and corn silks. The silk feeding can disrupt corn pollination and result in substantial yield losses. The foliage feeding is less damaging to soybeans, though newly planted double-crop soybeans may sustain yield loss. Rice says just seeing the pest is not enough to warrant an insecticide application. For soybeans he says control may be necessary when plants reach 30 to 40% defoliation prior to bloom, 15 to 20% defoliation from bloom to pod fill, or 25% defoliation from pod fill to maturity. He says to consider spot-spraying areas of the field with heavy infestation. For corn, the University of Missouri recommends insecticidal treatment during the silking period when growers see an average of three or more beetles per ear, when silks have been clipped to a half-inch or less in length, and when pollination is less than 50% complete.