The fall armyworm (Spodoptera frugiperda) annually migrates northward, invading much of the continental United States and Canada. Many lawns in the month of August of 2021 are being eaten to death in the greater Dayton, Ohio area. The larval (caterpillar) stage of the fall armyworm moth can cause severe damage to lawns. In summer, the moths lay clusters of eggs in cotton-like, greenish-white masses on structures (houses, mailboxes, fences, signs, etc.), shrubs and trees, and the newly hatched larvae drop down to the ground and “march” like an army to feed on the closest grass.

While the larvae are still small, damage may not be readily apparent. But when they reach their full size (about 1-1/2” long), they can eat the grass down to the soil. Significant infestations can defoliate a lawn entirely.

Lush green grass is the fall armyworm’s preferred food, so irrigated lawns of any type (warm- or cool-season) are at most risk. So are newly laid sod and newly seeded or overseeded lawns since these must be kept moist/irrigated until the new grass establishes roots.

Controlling fall armyworms is difficult since several generations can occur in one summer-through-fall season. Products that contain carbaryl (such as Sevin) or pyrethroids will be most effective while the larvae are still small. Mowing the turf and light irrigation before treatment can increase control. Repeat treatments may be necessary.

Turfgrass will recover with a little help from properly timed fertilizer applications if the insecticide applications were made quickly enough to protect a substantial percentage of the turfgrass canopy. However, if the canopy has been completely removed, the crowns need to be protected from dehydration through irrigation.

Resources:

https://turf.cals.cornell.edu/pests-and-weeds/insect-management/#caterpillars

Potter, D.A. and S.K. Braman. 1991. Ecology and management of turfgrass insects. Annu. Rev. Entomol. 36:383–406.