The fall armyworm is a native species that can’t tolerate cold very well. They only overwinter in South Florida and the southernmost regions of Texas. Since they can not tolerate cold, we typically have about four to five generations per year. They begin to show up in late May to early June, but the worst damage typically occurs in August. They migrate north yearly, but their populations are lower for some years, and their damage is minimal. Damage seems worse following droughts.
Lawn is infested with fall armyworms (Spodotera frugiperda).
Justin Ballew, ©2024, Clemson Extension
The adults do not feed and only live for two weeks, so the key to controlling fall armyworms in your landscape is to target the caterpillars. Do this by scouting with a soap flush. Use two tablespoons of lemon or orange-scented dish soap to one gallon of water, and pour in an area that looks drought-stressed. If the caterpillars are present, they will surface within five minutes. Look for the distinctive Y-shaped mark on the tops of their heads. Apply recommended insecticides when no irrigation or rain is expected for 24 hours. Recommendations can be found in HGIC 2159, Armyworms. Scout every 7 to 10 days because generations can overlap, and fall armyworms can be present in the landscape throughout November if temperatures stay mild.
Armyworms (Spodoptera spp.) can be easily identified in larval stages by the inverted “Y” shape on their heads.
Adam Gore ©2021, Clemson Extension
Preventative spraying is not recommended for homeowners because most insecticides do not provide long-term control. During field research, some commonly available insecticides only lasted for one week. Populations of fall armyworms will all vary from year to year, depending on weather events in South Texas and South Florida.
The best defense against fall armyworms is to reduce the attractiveness of your lawn. Use a slow-release form of nitrogen fertilizer instead of water-soluble nitrogen fertilizer on your lawn. The rapid, lush growth associated with quick-release fertilizers is highly attractive to female moths looking to lay eggs. Irrigate deep and infrequently and control thatch. Information on calibrating your sprinkler system and effectively watering your lawn can be found in HGIC 1207, Watering Lawns.
Fall armyworms have several natural enemies, so reduce unnecessary insecticide applications. Have insects correctly identified and use pesticides that have minimal impact on nontarget species.
If fall armyworms have damaged your lawn, how should you proceed?
Fall armyworm infestations do not kill established warm-season grasses. Caterpillars only eat the blades of grass and do not destroy the stolons and rhizomes. Patience is key for warm-season grasses. Bermuda and other warm-season grasses are preparing for fall dormancy, and mid-September is too late for a fertilizer application, so wait until the grass has fully greened up next spring to fertilize the lawn. Mowing at the lower end of the recommended height range will encourage your grass to send out more runners and thicken up. The lower mowing height can be used through May to mid-June. Once excessive heat is forecasted, raise the mowing height to encourage drought tolerance in the lawn. Mowing height recommendations for various types of grass can be found in HGIC 1205, Mowing Lawns. I also do not recommend overseeding warm-season lawns with winter rye because winter rye is also highly attractive to fall armyworms, and the rye will compete with your established warm-season grass for water, nutrients, and light later in the spring.
Tall fescue lawns may need to be reseeded since they are a tillering grass, and fall armyworm feeding can damage growing tips. Seed in mid-September at a rate of 5 to 6 pounds of seed per 1,000 square feet. Use a slow-release nitrogen fertilizer once the grass is about two inches tall. For more information, see HGIC 1201, Fertilizing Lawns and HGIC 1219, Tall Fescue Maintenance Calendar.
