Caption: Soybean pod-feeding (photo by Ron Hammond); examples of resulting seed damage (photo by NCSU Cooperative Extension); bean leaf beetle adult
As the season winds down, soybean growers need to continue scouting their fields, especially later-planted fields that will remain green well into September. As other fields in the area begin maturing and yellowing, some insects will migrate to soybeans that are still green and continue their feeding there. Two of those insects are second generation bean leaf beetles and the stink bug complex consisting of several species. These insects feed on the pods and seeds of the plant, causing direct damage to the harvestable part of the soybean.
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, spread around the field, and examine all the pods on each plant. Count the number of total pods and the number of pods exhibiting pod scar injury, and then determine the percent pod injury based on the 10 plants inspected. 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 adults are still present and still active. Beetles will start to leave the field as beans mature, so it is important to verify they are still there. A sweep net is an efficient way to sample for the presence of adults. Take 10-sweep sets in several locations in the field to determine presence or absence.
Caption: Stink bug life stages, and damage to beans.
Stink bug damage to soybean pods is not apparent from the outside since they don’t feed on the pod surface. Instead, they pierce directly into the seed with a straw-like mouthpart. Scouting for stink bugs is based on numbers of adults and nymphs (immatures). Scout by by walking into the field at least 100 ft from the field’s edge (numbers tend to be higher on the edges and are not representative of the whole field). Use a sweep net to take sets of 10 sweeps at 3 to 5 locations in a field. Both adults and nymphs should be counted together. Experience suggests that the brown marmorated stink bug is difficult to sample using sweep nets, so you might need to walk slowly through the soybeans and attempt to count the bugs directly on the plants. Insecticide treatments should be considered when an average of 4 or more adults or nymphs of all species are collected per 10 sweeps in regular soybeans. When grown for seed or are food grade soybeans, we suggest lowering the threshold to only 2 adult or nymphs per sample. For brown marmorated stink bug, control is suggested if you see 1-2 per row ft through at least the R4 stage.
When the decision to make a rescue treatment is made to prevent pod and seed injury to later maturing soybeans, there are numerous foliar insecticides to use for bean leaf beetle and stink bug control. Growers should also be aware of pre-harvest intervals for the insecticides, which range from 14 days to 60 days. The time period left before anticipated harvesting of a field might dictate the insecticide chosen.