Fall Armyworms Infesting Some Oklahoma Lawns

By Donald Stotts, OK State University

First posted on 09-05-2014

Oklahoma homeowners should be on the lookout for the presence of fall armyworms defoliating their lawns, an unwanted pest problem that is being reported across parts of Oklahoma.

Fall armyworms are surface-dwelling, “climbing cutworm” caterpillars that prefer to feed on grasses. Their body color can range from green to brown or black. They have a distinct stripe along each side of their body, and their head capsule features a prominent, inverted white “Y” at the front.

Tall fescue is typically their favorite meal, but they also feed on bermudagrass and other turfgrass species.

“Small larvae don’t consume the entire leaf; instead, they scrape off all green tissue, leaving a clear membrane that gives the blade a ‘window pane’ appearance,” said Eric Rebek, Oklahoma State University Cooperative Extension entomologist. “Larger larvae can chew through the entire leaf.”

One generation of fall armyworms can develop in about 18 to 28 days, depending on temperature, and infestations may occur until the first killing frost of the year. In Oklahoma, there are typically two to three generations present from late July through late October.

“The feeding activity by flocks of birds can serve as a sign that armyworms are present,” Rebek said. “Fall armyworms can be detected through close examination of the turf, or by using a soapy water flush.”

A soapy water flush involves mixing one tablespoon of lemon-scented dish soap per gallon of water and pouring the solution over several small areas of damaged turf. If present, larvae should be visible within 60 seconds as they become irritated by the flush and leave their hiding places in the thatch.

If three to four larvae per square foot are discovered, treatment may be warranted in commercial turf or golf courses. However, the Oklahoma Cooperative Extension Service recommends homeowners carefully consider the need to control fall armyworms.

“Some cool-season turfgrass could recover from a fall armyworm infestation late in the year without treatment, and bermudagrass and zoysiagrass lawns may only be slightly damaged and not warrant treatment,” Rebek said.

Unfortunately, early indications this year suggest chemical treatment will be needed in many areas, especially for protecting cool-season turfgrass. Insecticides registered for control of fall armyworm generally provide excellent results.

Rebek said products containing microbial-active ingredients such as Bt (Bacillus thuringiensis subsp. kurstaki) or spinosad should be applied when caterpillars are small to promote maximum effectiveness.

“If choosing between granular and liquid applications, keep in mind that granular products are a bit slower acting and require watering for activation,” he said.

A listing of recommended insecticides is available online by accessing the Aug. 20 OSU Cooperative Extension Pest E-Alerts, vol. 13, no. 27 at http://entoplp.okstate.edu/Pddl/ via the Internet.

As always, read the selected insecticide’s label for important information regarding safe and effective use of the product.