Armyworm Alert: What Ohio Lawn Owners Need to Know

I’ve been pushing mowers across Pickaway, Franklin, and Fairfield counties for more than ten years, and the 2021 fall armyworm outbreak is still the worst single pest event I’ve seen on Central Ohio lawns. By the second week of September that year, lawns I had mowed clean on Friday were brown moonscapes by Tuesday. Some clients lost half their front yard in 72 hours. Phones were ringing nonstop. Nobody had any product on the shelves because the southern states had already cleaned out the supply.

That outbreak taught a lot of us lessons we shouldn’t forget. Fall armyworms are not an Ohio resident pest. They migrate up from the Gulf Coast each summer, and in most years they fizzle out before they cause damage. In bad years, they don’t. Here’s how to spot them early, how OSU Extension recommends scouting and treatment, and when to stop trying to DIY and call a pro.

What did we learn from the 2021 armyworm outbreak?

The 2021 outbreak was a perfect storm of strong southern winds, drought conditions across the corn belt that pushed moths north, and warm late-summer weather that let multiple generations build in Ohio. By August, OSU Extension was issuing alerts. By Labor Day, the damage was widespread across Indiana, Ohio, Pennsylvania, and into Michigan and New York. Some lawns were defoliated in 24 hours.

The biggest lessons:

• Watch the southern reports. Texas and Oklahoma armyworm reports in late July and August predict what hits Ohio in September.
• Armyworms can defoliate a lawn faster than you can buy product. By the time you see damage, you have hours, not days.
• Granular grub products don’t work on armyworms. You need a foliar contact insecticide.
• Birds (especially starlings, robins, and grackles) gathering on a specific section of lawn is often the first visible sign before damage appears.
• The moths fly at night and lay eggs on light-colored surfaces near lawns: fences, sheds, gutters, the underside of porch roofs. A flashlight inspection in late August can give you a heads-up.

On a Columbus property I treated September 8, 2021, the homeowner called Monday morning saying her lawn looked “burned” overnight. Tuesday afternoon I pulled up and the front yard was 60 percent skeletonized. I counted 25+ caterpillars per square foot in one sample. That’s 3 to 4 times the OSU treatment threshold, and that lawn never fully recovered without a complete fall renovation.

The takeaway from 2021 was simple: pay attention to the August reports, and have a plan ready before you see damage.

How do I identify a fall armyworm?

Fall armyworms are 1 to 1.5 inch caterpillars that range from light tan to dark green to nearly black, with a distinctive inverted Y-shape on the head capsule and four small dark dots arranged in a square on the second-to-last body segment. That Y-shape on the head is the field ID.

Other distinguishing features:

• Three light stripes running down the back
• Smooth body, not hairy
• Curls into a C-shape when disturbed (similar to cutworms)
• Feeds in groups, often moving across a lawn in a noticeable “army” front
• Most active feeding in early morning and late afternoon; hides at the soil surface in heat of day

Don’t confuse them with:

• Sod webworm: smaller (1/2 to 3/4 inch), gray-green with dark spots, creates silken tunnels in thatch
• Cutworm: larger and fatter, usually solitary, cuts grass off at the base rather than feeding on blades
• Black cutworm: similar size but solid dark gray-black, feeds at night, makes individual chewed spots rather than expanding fronts
• Japanese beetle grub: C-shaped white larva that lives in the soil and chews roots, not blades

If you find a 1-inch caterpillar above the soil chewing on grass blades during the day, especially in groups, it’s almost certainly fall armyworm. The inverted Y on the head confirms it.

How do I scout for armyworms with a soap flush?

The soap flush is the standard OSU Extension scouting tool for armyworms, cutworms, and sod webworms. It forces hidden caterpillars to the surface where you can count them.

The technique:

• Mix 2 tablespoons of lemon-scented dish soap in 2 gallons of water in a bucket or watering can
• Pour the soap solution evenly over a 1 foot by 1 foot section of lawn at the edge of suspected damage
• Wait 5 to 10 minutes
• Count caterpillars that come to the surface
• Repeat in 3 to 5 areas across the lawn, especially at the edge of damaged sections and in adjacent healthy turf

The lemon scent isn’t necessary, but it cuts the residue and seems to work as well or better than fragrance-free soap. Don’t use a degreaser or laundry detergent. Dawn or any standard dish soap is what you want.

On a Lancaster property in August 2024, a client called concerned about a small brown spot near her air conditioner. I did a soap flush at the edge of the spot and pulled 4 fall armyworms out of one square foot. Not yet over threshold, but close. We scheduled a follow-up scout in 5 days and were ready to spray. Counts dropped naturally as cooler nights moved in, and we never had to treat. That’s why scouting matters: you spray when the count justifies it, not on speculation.

What is the OSU treatment threshold for armyworms?

OSU Extension’s treatment threshold for armyworms on cool-season lawns is 5 or more caterpillars per square foot in a soap flush, on an actively growing lawn. Below that, the lawn can usually outgrow the feeding. Above it, you’ll see visible damage within days and treatment is justified.

Decision guide:

• 0 to 2 per sq ft: no treatment, continue monitoring weekly
• 3 to 4 per sq ft: marginal, scout again in 5 days
• 5+ per sq ft: treat
• 10+ per sq ft: emergency treatment, expect existing damage to expand fast

The threshold drops for newly established lawns. If you overseeded 4 weeks ago and the seedlings are still tender, 3 caterpillars per square foot is enough to treat. Mature, vigorous lawns tolerate more.

What products work on fall armyworms?

Three active ingredients work fast on armyworms when applied as a foliar spray: bifenthrin, lambda-cyhalothrin, and spinosad.

Bifenthrin (Talstar, Ortho Bug B Gon Insect Killer for Lawns, several others) is the most common pro and consumer option. It’s a synthetic pyrethroid with fast contact knockdown. Apply at label rate, water in lightly (a tenth of an inch, not enough to wash it off the blades), and expect kill within 24 hours. Residual lasts 7 to 14 days.

Lambda-cyhalothrin (Demand CS, Scimitar) is similar to bifenthrin but with slightly faster knockdown. Pro use only in most cases.

Spinosad (Conserve SC, Captain Jack’s Deadbug Brew) is a biological-derived insecticide that’s much safer for pollinators and beneficials. It works slower than the pyrethroids (full kill in 2 to 4 days) but is the right choice on properties with bee hives, vegetable gardens, or chemical-sensitive families.

Application rules:

• Spray late afternoon or early evening when caterpillars are active and bees are not
• Don’t mow for 24 hours before or after
• Don’t apply under drought stress
• Water in lightly (an eighth-inch) to move product into the canopy where caterpillars feed
• One application is usually enough; rescout 5 days later

Granular grub products with imidacloprid or chlorantraniliprole DO NOT control armyworms effectively. Don’t waste money applying grub products thinking they’ll cover an armyworm outbreak.

On a Washington Court House lawn September 12, 2021, we sprayed bifenthrin at the labeled rate on a 12,000 square foot front lawn at 6 p.m. By 8 a.m. the next morning, the dead caterpillars were visible on the surface. That property still had significant defoliation damage but the active feeding stopped immediately and the lawn recovered with a fall overseed.

When should I call a pro instead of DIY?

DIY armyworm control is feasible for most homeowners on a residential lot if you can:

• Confirm identification with a soap flush
• Get product within 24 hours (problem during heavy outbreaks)
• Apply with a calibrated sprayer at the correct rate
• Treat the entire lawn, not just the visible damage

Call a pro when:

• The outbreak is severe (10+ per square foot) and damage is spreading hourly
• You don’t have a sprayer or experience applying product evenly
• You have a large property (over 15,000 square feet)
• You also have pollinator hives, livestock, or sensitive ornamentals to work around
• The lawn is newly seeded and vulnerable to over-application damage

During an active outbreak year, my own approach on client lawns is to scout weekly starting mid-August, spot-treat at threshold with bifenthrin or spinosad, and plan fall overseed for any damaged areas in September.

If you’re seeing damage right now and don’t know where to start, call before the damage doubles. Armyworm outbreaks don’t slow down on their own.

Quick armyworm checklist for Central Ohio

• Subscribe to OSU Extension’s BYGL (Buckeye Yard and Garden Line) email for in-season alerts
• Watch southern state armyworm reports in late July and August
• Run a soap flush scout at the first sign of unusual damage, bird gatherings, or “lawn looks burned” symptoms
• Treat at OSU threshold of 5+ caterpillars per square foot
• Use bifenthrin, lambda-cyhalothrin, or spinosad foliar spray, not granular grub products
• Apply late afternoon, water in lightly, rescout in 5 days
• Plan fall overseed for damaged areas in September
• Keep a pro on speed dial during outbreak years

Want a written quote?

If sorting through pest emergencies isn’t how you want to spend a September weekend, Lawn Harmony Landscaping handles lawn care across Pickaway, Franklin, Fairfield, Ross, and Fayette counties. We’re locally owned and operated, licensed and insured, with a 5.0-star Google rating.

Call (614) 425-9789 or email LawnHarmonyOhio@gmail.com for a free quote. Commercial properties can request a walkthrough at /commercial.

Service area: Circleville, Columbus, Grove City, Bexley, Upper Arlington, Pickerington, Canal Winchester, Groveport, Lancaster, Baltimore, Chillicothe, Washington Court House, and Jeffersonville.

Related reading: lawn mowing service, aeration and overseeding, commercial grounds maintenance.