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This year, fall army worms are working their way north across Nebraska at higher than usual levels, and that’s raising questions about how to manage them.
The fall armyworm doesn’t overwinter here — moths migrate up from the south each summer. By late August and September, populations can build quickly. In alfalfa, pasture, and forage systems, the highest risk is in new seedings or tender regrowth, with infestations often starting along field edges or adjacent crops.
Rapid browning in pastures, alfalfa, small grains, or even lawns that seems to appear almost overnight can be a telltale sign. Larvae feed mostly at night or early morning, then hide at the soil surface during the day.
Check the transition zone between green and brown areas, or use a sweep net to confirm presence. When scouting, look for a distinct upsidedown “Y” on the head, stripes running the length of the body, and four black spots in a square pattern on the last segment. Adults are gray-brown moths with a wingspan of about an inch and a half, showing lighter and darker streaks across the wings.
Current weather is playing a role in armyworm presence, too. Warm days and mild nights are keeping larvae active longer and slowing down their migration south. That means scouting now is especially important, since pressure could linger later into the fall than normal.
Thresholds for treatment vary by crop. In new alfalfa seedings, as few as one to two larvae per square foot can cause major loss. In established stands, 10 to 15 larvae per square foot can strip a field quickly. In pastures and hayfields, a working threshold of three or more larvae per square foot is a good rule of thumb. Caterpillars less than threequarters of an inch long are the easiest to control. Larger ones are much harder to kill and may already have done most of the damage, so treatment timing is critical. Control options include several common insecticides, with some products allowing zero-day grazing restrictions. Check the label for your specific circumstance and make sure restrictions match. For organic systems, spinosad or Bt products work best on small larvae. If you’ve already lost a stand, scout for regrowth. Healthy crowns may bounce back if larvae are controlled quickly, but reseeding alfalfa this late isn’t advised. Waiting until spring is usually the better option. For cover crops lost to feeding, small grains like rye, wheat, or triticale are the only real options for reseeding this late in the year.
For those planning fall small grain or cover crop planting, delaying seeding until early October may help avoid pressure, but if you seed earlier, be prepared to scout and protect young seedlings. Lawns can also be hit hard. Products with Bt or Sevin are options, but remember to water them in so granules reach the soil line. Attacked fescue lawns may need reseeding, while mixed or full bluegrass stands usually fill back in.
The takeaway is simple: scouting is critical. Fall armyworm outbreaks can strip a stand within just a few days. Early detection and treatment, timed while larvae are still small, is the best way to protect forage, cover crops, and lawns this fall.
-Ben Beckman is a beef systems Extension Educator serving northeast Nebraska. He is based out of the Cedar County Extension office in Hartington. You can reach him by phone: (402) 254-6821 or email: [email protected]
