Hayden Schug, Agriculture Agent | hschug@umd.edu
University of Maryland Extension, Charles County
Corn earworms, also known as podworms, headworms, bollworms, and several other names, have increased in Maryland following flights up from the South and higher local emergence. Growers should remain alert for damage in double-cropped soybeans and sorghum. While this pest is a familiar concern in sweet corn, it can also cause serious yield losses in other summer crops when populations are high.
In soybeans, larvae feed directly on pods and developing seeds, with the greatest risk occurring between the R1 and R5 stages. Fields that are flowering or setting pods are especially attractive to egg-laying moths. Female moths typically deposit eggs in clusters near the upper canopy. Larvae may be green, yellow, or brown, with faint stripes along the body and small spines on the skin.
When scouting, ensure that earworms are identified correctly, as they have a different economic threshold than other caterpillars commonly found in fields, such as soybean loopers and cloverworms (Figure 1). Damage can be detected by looking for chewed pods or flowers. However, the absence of visible chewing damage does not mean the pest is absent. Soybeans often abort young pods or flowers that have been damaged, redirecting energy toward intact pods and flowers. Earworms are voracious eaters, it has been shown that one large earworm per row foot in wide-row soybeans can reduce yields by 1.93 bushels per acre.
In sorghum, earworms target the developing grain heads. Damage resembles that caused by sorghum webworm but tends to be more extensive, with larvae consuming the soft kernels directly. Yield loss is tied to the number of larvae per head and the crop’s stage at infestation. Sorghum is vulnerable from bloom to milk stages, with one corn earworm per head causing 5–10% yield loss.
Scouting should be conducted carefully and often. In soybeans, use sweep nets to sample across the field, not just along field edges, since moths may concentrate in certain spots. A common threshold is an average of 2–3 medium to large larvae per 15 sweeps, though the NCSU online calculator is recommended for adjusting thresholds based on row spacing, control costs, and crop value, https://go.umd.edu/soybeanthreshold.
In sorghum, examine at least 10 heads in each of several locations. Treatment is typically justified when an average of two or more large larvae are found per head during flowering and early seed development. Texas also provides a dynamic threshold calculator that accounts for field conditions and larval size, https://extensionentomology.tamu.edu/sorghum-headworm-calculator/.
Because some corn earworm populations have developed resistance to pyrethroid insecticides in parts of the Mid-Atlantic, growers should consider products with alternative modes of action if treatment is warranted or rotate among different modes of action. Group 28 insecticides and indoxacarb products have shown good efficacy and can be used in rotation with pyrethroids if needed.
Monitoring flights, identifying larvae correctly, and scouting fields at vulnerable stages are key to avoiding late-season losses. With higher-than-normal moth pressure this year, fields should be checked regularly through September or until crops are no longer susceptible to damage.
References available upon request.
