As we get into the R5-R6 growth stage of soybean, now is the time to look out for pod and seed feeding insects, especially bean leaf beetles, grasshoppers, and stink bugs. Last week’s article focused on stink bugs, which pierce the pod and suck out seed matter. Bean leaf beetle and grasshopper chewing damage is more obvious. Grasshopper damage is usually focused on field edges, but bean leaf beetles will be spread more evenly through the field. We have not had any reports of unusual levels of pod feeding activity, but growers are still advised to monitor their fields for these two insects. This concern is especially important with fields that stay green in September because of late planting, including organic, double-cropped, or intercropped soybeans. These juicy green islands will be particularly attractive to second-generation bean leaf beetles stocking up for the winter, and they may move into green fields from other fields that are starting to yellow, resulting in unpleasant surprises for the unwary.
Treatment to prevent pod damage from bean leaf beetle is based on the level of insect injury observed on the pods. Select 10 plants at random and inspect all of the pods on each. Count the total number of pods and the number of pods exhibiting pod scar injury, and use this to determine the percent of pod injury. It is important to estimate percent pod injury on inspection of the entire plant. Treatment is justified if the percent pod injury is approaching 10-15%, and bean leaf beetle adults are still present in the field. When pods dry down the beetles will leave of their own accord. The same general guidelines can be used for grasshopper damage, though keep in mind that this damage is often restricted to field edges and might not be a widespread problem in the field.
At this time of year producers should also be mindful of the pre-harvest intervals (PHI) of any insecticides they use, many of which are as high as 45 days. More information on bean leaf beetles can be found at: http://ohioline.osu.edu/factsheet/ENT-23