· by Michael Blair, Nashville's Lawn Care
Fall armyworms are the caterpillar of a moth that blows north from the Gulf states every summer. In Middle Tennessee they usually show up from August into early October, and in a bad year they arrive in waves. They eat grass blades down to the crown, they do it fast, and they move across a lawn in a visible front, which is where the name comes from. The damage looks like drought at first glance, and treating it as drought is how a lawn gets lost.
Telling armyworms from drought
Drought browns a lawn evenly and slowly, worst on the high, sunny spots. Armyworm damage has an edge: a ragged line between chewed and untouched turf that moves day by day. Blades on the damaged side are eaten down or notched rather than dry and curled, and the crown is still green.
Birds are the other tell. A flock of starlings, robins or grackles working a lawn intently in late summer is feeding on something, and armyworms are the usual answer. Wasps hunting low over the grass mean the same thing.
Finding them before the damage shows
Mix a little dish soap into a bucket of water and pour it over a square yard of lawn at the edge of the suspect area. Caterpillars in the thatch come to the surface within a few minutes. They are striped, up to an inch and a half long, with an inverted Y on the head. A handful per square yard is enough to justify treating.
They feed mostly in the cool of early morning and evening, and hide in the thatch through the heat of the day, which is why a midday walk across the lawn can miss a heavy population. Check at dusk.
What works, and how quickly it has to happen
Small caterpillars are easy to kill with a labeled lawn insecticide; large ones are harder and have already done most of the eating. The useful window is a few days from first sighting. Biological products based on Bt or spinosad work on small larvae and are gentler on everything else in the lawn; synthetic pyrethroids work faster on larger ones. Follow the label and treat the whole lawn, not just the brown part, because the front is ahead of the damage.
Mow before treating if the grass is tall, so the product reaches the thatch, and hold off watering for a day afterward unless the label says otherwise. Anyone treating your lawn commercially needs the Tennessee pesticide applicator license for it.
- Soap flush at the edge of the damage to confirm and count
- Treat the entire lawn the same day if the count is high
- Check again a week later; a second wave is common
- Keep the lawn watered afterward so the crowns can regrow
Whether the lawn comes back
Bermuda and zoysia almost always recover, because armyworms eat the blades and leave the crowns and stolons. Water and a light feeding bring them back within a few weeks while the weather is still warm. Fescue is the concern. A fescue lawn eaten to the crown in September, on top of the summer it has just been through, may not have the reserves to regrow, and a lawn that was thin already can be lost.
The consolation is timing. September and October are when fescue is overseeded here anyway, so an armyworm loss on a fescue lawn is repaired with the same seeding you might have planned, done a little heavier. Treat first, seed after the caterpillars are gone, and keep the seedbed moist.