If large areas of your lawn have suddenly disappeared or turned brown, fall armyworm is likely the cause. Established lawns often recover from this injury with little lasting damage, but recently seeded or sodded areas can be more seriously harmed by armyworm feeding. When deciding whether or not to treat, consider the age and health of your lawn and check to make sure fall armyworm are actively feeding before taking action.
Where do armyworms come from?
Fall armyworm is a type of caterpillar that matures into a small moth with gray and brown wings. Fall armyworms do not overwinter in North Carolina. Instead, egg laying moths migrate northward from Florida and the Gulf Coast each spring and summer, typically arriving in NC by June. Once they arrive, each female moth lays around 1,000 eggs in masses of 50 or more. The eggs are typically laid on houses, shrubs, trees, fences, mailboxes, or similar structures. Two to 10 days later the caterpillars emerge and strike out in search of food.
At first, feeding by small armyworms is not very noticeable or damaging. After a couple of weeks of growth, the larger caterpillars feed voraciously as they march across turf areas, eating all above ground green leaf tissue and leaving behind large areas of thatch and brown turf. After feeding for 2 to 3 weeks, the caterpillars dig into the top inch of the soil to pupate. Within 2 weeks, a new population of moths emerges and usually flies several miles before laying eggs to start the cycle all over again. It is possible in some years to have as many as 4 generations of fall armyworm in North Carolina.
To treat or not to treat
Often, by the time fall armyworm damage is noticed the caterpillars have finished feeding, moved into the soil and pupated (formed a cocoon). At this point the damage is done. Pesticides will have no effect on the pupae, which do not feed on grass leaves or roots. Therefore, the first step in determining whether or not to treat is to determine if fall armyworms are still actively feeding. If they are still feeding, armyworms can be found at the edges of the damaged area, feeding on green leaf tissue. Armyworms feed most actively in the early morning and late afternoon.