What’s Eating My Beans?
3 Pod-Feeding Insects That Impact Midwest Soybean Yields
Insect infestations can quickly turn a great soybean field into one that struggles to yield. Some insects target not only the leaves, but also damage the pods themselves, directly impacting the yield and quality of soybeans in your fields.
Below are three of the primary insects that feed on pods as well as leaves that you should watch for throughout the growing season.
Three Soybean Pests
Bean Leaf Beetles
Bean leaf beetles can damage soybeans throughout the growing cycle, feeding on both leaves and pods. While soybeans can often compensation well for defoliation of up to 30 percent early in the season, or 20 percent during flowering and pod fill stages, damage beyond those thresholds warrants management.
Feeding by adult bean leaf beetles can also transmit plant diseases, including bean pod mottle virus or contribute to the secondary invasion of fungi into the pods.
When scouting foliage for damage, examine at least 10 plants. Select a leaf from the top, middle and bottom third of each plant. Use a figure such as this one from the University of Minnesota to estimate the percent of defoliation for each leaf and the average from each plant, then across multiple plants from throughout the field.
In addition to the thresholds listed above, once soybeans begin to set on pods, examine pods for damage. The threshold for treatment then is 10 percent pod injury if bean leaf beetles or other pod feeding insects are present. If populations are large and pod clipping is occurring, treat aggressively.
Utilize labeled rates of foliar insecticides.
As with bean leaf beetles, grasshoppers can feed on both the leaves and pods of soybeans plants. With several species impacting fields in the upper Midwest, the damage can begin as early as May with the nymph stage of the twostriped grasshopper and extend to the end of the season by the time grasshoppers complete their lifecycle. Both nymphs and adults feed on leaves. While the adults can also feed on the pods and occasionally clip pods from plants.
Grasshopper nymphs can consumer entire soybean seedlings, according to Soybean Research Info, which is a primary reason to begin scouting soon after plants emerge, focusing on insect counts and paying special attention to areas adjacent to undisturbed weedy sites, alfalfa and small grain fields that are especially susceptible to grasshopper infestation.
Sampling should include counting the number of adults and nymphs in a one square foot area. Repeat to include a total of 20 samples. To determine the number of grasshoppers/square yard, multiply the average number of grasshoppers per square foot by nine. This is the measurement used for defining thresholds.
A consensus threshold of 30-45 nymphs or 8-14 adults per square yard within the soybean field justifies treatment control. If the concentration of grasshopper infestation is focused entirely at the fields margins, you may consider treating only the margins.
As the season progresses, transitioning scouting from insect counts to calculations of defoliation and pod damage. Follow the same thresholds as given for bean leaf beetle damage to determine when management control is needed.
While stink bugs only infrequently reach troublesome levels in the upper Midwest states of Minnesota, Iowa and Wisconsin, it is still worthwhile to scout for them. They are an increasingly common late-season pest in soybean fields. When high populations occur, they can cause loss in both yield and quality of soybeans.
The most commonly found in soybean fields in the Midwest being the green stink bug. Other species of stink bugs are the brown stink bug and the brown marmorated stink bug.
Stink bugs prefer to feed on pods and developing seeds, which causes abortion, deformation and discoloration of the seed. It can also cause delayed plant maturity, causing plants to remain green rather than dry down.
University of Minnesota Extension recommends scouting for stink bugs as pods begin to develop on soybean plants and continuing through seed development. Stink bugs typically colonize field edges first, so scouting should include both edge and interior parts of the field. You can use a sweep net or beat cloth for sampling, with thresholds for treatment varying depending on whether the soybeans are being grown for grain or for seed.
For typical grain fields, reasonable thresholds for treatment are 10 bugs in 25 sweeps (counting both nymphs and adults), or 1 per linear foot of row, with seed production thresholds at 5 per 25 sweeps or .5 per foot of row.
Most insecticides labeled for use on soybeans will be effective on stink bugs. Follow all label instructions carefully, including being aware of preharvest intervals as stink bug control may take place later in the growing season.
Additional sources for identifying soybean pests:
Our district sales managers can be strong resources to help you scout fields and make decisions regarding your pest control system, in addition to selecting soybean seed varieties that fit your fields and the agronomic characteristics of your farm.