Fall Armyworms on the March Through Piedmont Lawns

— Written By
Lawn damaged by armyworm.

Damage to tall fescue caused by fall armyworm feeding.

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. According to an NC Extension fact sheet, 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.

Experts with N.C. Cooperative Extension report this year is proving to be the worst on record for fall armyworm, which can be a pest of pastures in addition to lawns. In a recent post on the NCSU TurfFiles website, Dr. Rick Bradenburg, entomology professor and Extension specialist, warns this problem may persist well into October.

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. Flocks of birds feeding along the edges of damaged turf areas can be a sign of armyworm infestation, and also a source of natural control for this pest.

caterpillar

Fall armyworm caterpillar. Photograph by John L. Capinera, University of Florida.

If checking for armyworm during midday, you may need to get down on your hands and knees to search the thatch layer (this is the layer of debris that accumulates at soil level) or perform a soapy water flush. To flush out insects from the soil or thatch layer, mix 2 tablespoons of dish detergent in 2 gallon of water then gently pour the water over the area where insect activity is suspected. Watch the area. Within five minutes any armyworms, along with many other insects, in the thatch or upper soil layer should come to the surface. Fall armyworms vary in color from green to brown, to almost black, grow from 1” to 1 ½” long, and have a distinctive inverted “Y” on the head.

Though fall armyworm populations are very high this year, preventive applications of pesticides in established lawns are not warranted or recommended. Even in a record year, only a small percent of lawns will be infested. In most cases, healthy, well-established lawns will be able recover from armyworm feeding without intervention.

But for newly seeded or recently sodded turf an armyworm invasion can be deadly. Armyworm moths find new turf areas particularly attractive for egg laying because these areas are typically well irrigated and fertilized. To prevent loss, scout new turf areas for armyworms on a weekly basis this fall. Treat at the first sign of activity. Insecticides containing organic active ingredients can effectively control fall armyworm if applied when caterpillars are small and recently hatched. For organic control, look for products labeled for application in turf areas that contain spinosad, azadirachtin (a neem oil derivative), or B.t. (note: B.t. is most effective when applied late in the evening) as the active ingredient. Because organic insecticides have very little residual activity, a second application of these products may be necessary.

Many synthetic insecticides marketed for turf pests will control fall armyworm. The use of synthetic products may be necessary to control armyworm infestations that are not caught early, though if caterpillars have already reached 1” to 1 ½” in length, it is too late to treat as these caterpillars will soon stop feeding, enter the soil and pupate. A complete list of control products registered for use on fall armyworm can be found on page 6 of Pest Control for Professional Turf Managers, available online at: http://www.turffiles.ncsu.edu/PDFFiles/004176/AG408PestControl_Professionals.pdf.

For all products, treatment will be most effective if applied when caterpillars are small. For best results, mow the turf before treating. A light irrigation or rainfall prior to treatment can improve effectiveness, as can waiting until late afternoon to apply control products. Chemical control is needed if natural enemies do not keep infestations below the economic threshold of 1 caterpillar per square foot. If possible, do not mow turf or remove clippings for several days after treating. When using any pesticide, always read and follow all label directions. To minimize harm to honeybees, never apply insecticides to open flowers (including weeds) and wait until late evening, after bees have returned to the hive, to spray.

Learn More!

Learn more about fall armyworm from these online Extension resources:

Visit the NC Extension TurfFiles website for more information on turf selection, care, and pest management.

Use Extension Search to find research based information from Cooperative Extension systems across the U.S.

Visit your local Cooperative Extension office to learn more about gardening and landscape care. Go to https://www.ces.ncsu.edu/local-county-center/ to find your county Extension center or post your questions to be answered online via Extension’s ‘Ask an Expert’ widget.

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