Fall armyworm marching from US South to Midwest, damaging pastures, soybeans, lawns

The fall armyworm outbreak of 2021 is going to be one to remember. It's not just hitting the south this year, but fall armyworms are marching north into the Midwestern states like Illinois, Indiana, Ohio and Iowa.
calendar icon 6 September 2021
clock icon 3 minute read

My colleague, Dr. Gus Lorenz in Arkansas called it “epic”, writes Scott Stewart, IPM Extension Specialist at the University of Tennessee. We knew something was different this year when another colleague in Texas, Dr. David Kerns, gave us a warning over a month ago.

So what’s different?

  1. Fall armyworm flight is several weeks early
  2. Larval infestations have been unusually high
  3. For whatever reason, control with pyrethroid insecticides has been unusually poor (ranging from sometimes acceptable to a complete failure)

It almost goes without saying that bermudagrass pastures are at high risk. It’s not every field, but some double-cropped soybean fields are heavily infested, and it’s even worse where volunteer wheat or other grasses were present that attracted the moths.

Even bermudagrass lawns are being infested, and homeowners and landscapers need to be especially alert on newly sodded or seeded lawns (fall armyworms really love the tender new growth).

What little sorghum that is out there, including sorghum Sudangrass, is also at risk.

Scout in morning or evening

According to Department of Entomology team at Iowa State University, scouting for fall armyworm should occur during the early morning or late in the day, since this is when larvae are most active. Start in the most valuable crops first. The best way to confirm the presence of fall armyworm in pastures or hayfields is to use a sweep net. The net will pick up larvae that are too small to find other ways.

If you find fall armyworm larvae in the sweep net, determine how many caterpillars are present per square foot by looking on plants and in the litter on the soil. If you don’t have a sweep net, check areas that look dead, wilting or drought-stressed, or where birds are congregating. Sometimes, you can find the frass (insect excrement), which are green pellets. The generic threshold is 2 or 3 caterpillars per square foot.

So what can you do to stop fall armyworm?

Pastures - Refer to the 2021 Insect Control Recommendations at https://extension.tennessee.edu/publications/Documents/PB1768.pdf (pages 61-62) for insecticide recommendations, but avoid using the pyrethroid insecticide like Baythroid XL, Mustang Max, and Warrior. The alternatives will be more expensive but you may make up the differences in making fewer applications in the long run.

Soybeans - Avoid using pyrethroid insecticides, and if you do use them, tank mix with a minimum of 0.5 lb of Acephate and check back within 2-3 days to make sure it worked. A more trustworthy suggestion is to use products containing chlorantraniliprole (e.g., Besiege, Prevathon, Vantacor, Elevest), or Intrepid, Intrepid Edge, or Blackhawk. For the most part, you should be able to use the lower end of the recommended label rates. The defoliation threshold for soybean prior to bloom is 30-35%, but don’t wait on this to happen if large numbers of armyworms are present. Catching 10-15 per 25 sweeps on small soybeans should catch your attention, especially if you already are finding 15-20% defoliation. If you are seeing a broad mix of small and larger larvae, that’s a clue that pressure is continuing. The point is, it will require a little judgement to make treatment decisions.

Sorghum - Follow the recommendation at https://guide.utcrops.com/sorghum/, but again, avoid products containing only a pyrethroid insecticide.

Lawns - Options are more limited for homeowners trying to control armyworms in their lawns, refer to the following article from Mississippi State University, http://extension.msstate.edu/publications/control-insect-pests-and-around-the-home-lawn.


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