Scouting Smarter: September Insect Update
By Hayden Schug, Agriculture Agent, University of Maryland Extension, Charles County
As Maryland’s corn, soybeans, and sorghum move toward maturity, insect pressure begins to shift. September is a key month for scouting late-season pests, particularly in double-crop soybeans as well as those fields that remain in pod-fill stages and sorghum.
Corn earworm remains active in soybeans, though risk declines once fields reach R6 and seeds begin hardening. Corn earworms cannot eat through the pod at this point and treatments are not necessary. Late-planted soybeans are still vulnerable, and thresholds should be closely monitored. Right now, they are at about 3 larvae per 15 sweeps. They can also be a pest of concern in sorghum; use this calculator from Texas A&M to guide decisions: https://extensionentomology.tamu.edu/sorghum-headworm-calculator/. Insecticide resistance to pyrethroids continues to be an issue, so rotating to other modes of action is recommended when treatment is necessary. Products containing chlorantraniliprole have been shown to have good efficacy.
Sorghum aphids have been showing up in Delaware, and I have seen them in Southern Maryland as well. Fields should be scouted to monitor populations and determine when thresholds are reached. Kansas State Extension has a really good infographic about scouting sorghum fields and thresholds: https://go.umd.edu/sorghuminfo. Effective sprays consist of Sefina and Sivanto if thresholds are reached.
Defoliators such as bean leaf beetles, grasshoppers, Mexican bean beetle, and green cloverworms may still cause injury in September. Soybeans can withstand up to 15 percent defoliation during pod-fill, but scouting should include multiple areas across the field to avoid overestimating pressure based on hot spots.
Stink bugs often peak during September. Both green and brown marmorated stink bugs feed directly on pods and developing seeds, reducing seed quality and yield potential. Thresholds are five stink bugs per 15 sweeps during pod and seed-fill stages. Particular attention should be paid to food-grade or seed soybeans, where injury can be more costly.
In corn, insect activity is winding down as fields near harvest. Earworm and western bean cutworm damage may be evident at harvest, and secondary pests such as sap beetles or ear rots may take advantage of feeding injury. While treatment is no longer an option at this stage, monitoring ear quality can help with harvest decisions and grain management.
As you finish scouting for the season, note fields that consistently experience late-season pest pressure. These areas may warrant extra attention in 2026, as overwintering populations can build from this fall. Maintaining scouting records is an important part of a long-term IPM strategy.
This article appears in September 2025, Volume 16, Issue 6 of the Agronomy News.