Early August of 2024 was a tough time for turf and pasture managers. An invasion of fall armyworm, Spodoptera frugiperda, caterpillars (larvae) marched across lawns and pastures throughout the region. We got inundated with calls from frantic folks wondering what they could do for these worms. Unfortunately, by the time they are big enough to notice it may be too late to treat. Let’s go over some marching orders to help make sure you’ll be prepared if we’re invaded by fall armyworms!

What are fall armyworms?

Fall armyworms are actually a type of DBM, or darn brown moth. It is hard to tell the fall armyworm moth from other DBMs because their size and markings are similar to other species. According to AggieTurf (Texas A&M), Fall armyworm caterpillars range in color from shades of brown to gray, green or yellow-green. Their most distinguishing characteristic is a whitish inverted Y between the eyes, and three whitish stripes on the pronotal shield behind the head.

They have four lifestages: egg, larva (or caterpillar), pupa, and adult (or moth). The adult moth lays her eggs on structures and tall grass near turf, pasture or agriculture fields, and the tiny caterpillars hatch and begin to feed on members of the grass family (Poaceae), like turfgrass, pasture grass, corn and wheat. The caterpillars molt about five times over the course of 14-21 days, but it is the final, largest size that they can eat more than 90 percent of the grass they need to go on to become a pupa and eventually a moth. This is also when they are most noticeable and most destructive.

Once the caterpillars have had their fill they burrow into the ground and pupate for 10-14 days and then emerge as a moth to fly and mate, and start the cycle back over again!

When Should You Worry About Fall Armyworms?

Fall armyworms are a native insect, but are a known pest because they negatively affect our crops and turf areas. They can be devastating to a corn crop or golf course, if you are not scouting for them. You should be worried about fall armyworms in July, August and September in North Carolina if you are raising corn; have unhealthy pasture or turfgrass; you have recently seeded a turf area and the grass is young; or you have recently installed sod.

They do not overwinter here in North Carolina because our typically cold winters kill any caterpillars or pupa in the ground. Instead, the populations in southern Florida and Texas are who eventually can make their way north as the weather warms across the Southeast. This is why we do not usually see fall army worms until mid-July (at the earliest) to early August. If we have strong storms that come from the south (like Hurricane Helene of 2024), then you can almost guarantee that some of those fall armyworm moths got blown up as well, and they will be looking for a place to lay their eggs!

What to Do About Fall Armyworms

Fall armyworm populations ebb and flow, and some years can be “bad” with heavy infestations and other years there are only a few populations that bubble up. The key with all integrated Pest and Pollinator Management Strategies is scouting! For Fall Armyworms, you need to be paying attention to the weather starting in mid-July for storms and winds coming from the south and east. That is when the moths can be blown up from the south and start laying eggs.

It is important to properly identify a caterpillar, or larvae, before a pesticide application. According to AggieTurf (Texas A&M), Fall armyworm caterpillars range in color from shades of brown to gray, green or yellow-green. Their most distinguishing characteristic is a whitish inverted Y between the eyes, and three whitish stripes on the pronotal shield behind the head. Photo by Ivan Cruz, CC BY-NC-SA 2.0

The time to pull out the chemical pesticides is when you know you have a situation where the plants cannot successfully overcome being munched on: a corn crop about to be harvested; turfgrass that has been stressed out; recently installed sod; recently seeded turf areas; or small pastures where ranchers rely on that pasture to feed their animals and cannot wait three or more weeks for forage to recover. Apply a pre-emptive chemical application on sensitive areas once a storm has blown over, making sure to read the label to understand when a chemical should be applied for maximum effectiveness. The small caterpillars will be clustered together, feeding as a unit. Most of the time it is too late for a spray application to be effective once folks start noticing the damage from fall armyworms because the caterpillars are too large for the chemical to work on them.

Healthy turf, especially rhizomatous (or running) turfgrass species, and pasture can recover from a fall armyworm invasion if it is healthy and well-managed. If you can tolerate the damage, just leave the caterpillars alone. Plenty of wildlife eat the eggs and caterpillars and they will be gone in about three to four weeks. If you are worried about reinfestation, then you can apply a pre-emptive chemical application after you stop seeing the large caterpillars.

You should be working to make sure that the turf or crops you have are healthy and well-maintained, and make sure to do what you can to reduce stress from heat- and weather-related events. This means give your plants a little more water if it is hot and dry in the mornings, or a little more fertilizer (but not too much!) if they have been damaged from other events. Too much fertilizer can result in too much growth, which is just a ‘free buffet’ sign to insects, but not enough fertilizer can stress plants out!