Hayden Schug, Agriculture Agent | hschug@umd.edu
University of Maryland Extension, Charles County
As we move into July and August, insect pressure tends to ramp up across Maryland row crops, particularly in soybeans and corn. These months are critical for monitoring pest activity and making timely, threshold-based decisions. Several pests are commonly active during this period, including corn earworm, stink bugs, Japanese beetles, other defoliators, and spider mites.
Corn earworm moths typically migrate into Maryland in larger numbers in mid to late July, although some can overwinter here, and early flights have already been reported moving up the coast. In soybeans, larvae can cause significant injury during the R1 to R5 growth stages by feeding on pods and developing seeds. When I ran the numbers, the threshold was approximately 2.5 larvae per 15 sweeps, but thresholds can vary depending on market value and treatment cost. You can use the NCSU dynamic threshold calculator to run your own field-specific estimates: https://go.umd.edu/CEW. It’s important to sample evenly across the field, as female moths may lay eggs in clusters, leading to localized infestations. Many corn earworm populations show resistance to pyrethroids, so growers should consider using alternative insecticide chemistries when treatment is needed.
Defoliating insects, such as Japanese beetles, bean leaf beetles, grasshoppers, green cloverworms, and soybean loopers, can all be active during July and August. Last year in Southern Maryland, there were soybean fields that reached defoliation thresholds from grasshopper feeding alone. In many cases, damage was more severe in the center of the field, so it’s important to walk the entire field and not rely solely on edge scouting. Defoliation thresholds are based on cumulative injury from all feeding insects. Economic thresholds are 30 percent defoliation before bloom (V1–V6), 15 percent from bloom through pod fill (R1–R6), and 25 percent from full seed to maturity (R7–R8). A helpful visual guide and estimator tool is available through the University of Nebraska (Figure 2).
Stink bugs, including brown and brown marmorated species, are another priority pest in soybeans and corn during mid-summer. They damage pods by piercing and feeding on developing seeds, often resulting in aborted or shriveled beans. Scouting is most important from R3 to R5. The general treatment threshold is five stink bugs per 15 sweeps, but this drops to three per 15 sweeps in fields grown for seed or planted early.
Finally, spider mites may become a problem during periods of hot, dry weather, especially along field edges. These pests cause stippling and bronzing of leaves and can spread quickly under drought stress. Treatment should be considered when active mites are present on the undersides of leaves and bronzing or stippling begins to move into the upper canopy. Threshold estimates follow the 15 percent defoliation threshold, and plants should be treated when that level of injury is reached.
In corn, Japanese beetles can clip silks and interfere with pollination. The treatment threshold is 3 or more beetles per ear, and pollination is still ongoing. Almost all pollination occurs within the first 4-5 days of silking, so any damage done by silk clipping after that period should not affect yield. It is important to look at ears throughout the field as Japanese beetles tend to congregate on field edges.
Regular scouting during the next several weeks will be essential to managing pest pressure effectively. Making decisions based on economic thresholds helps protect yield while avoiding unnecessary insecticide applications. If you would like assistance with scouting, pest identification, or choosing control options, contact your local Extension office.
