Fall Armyworms are Making Grass DisappearFall armyworms are caterpillars that appear in large groups and cause damage by feeding on the leaves of green turf (usually bermudagrass).
Hot Springs, Ark. – There are several reports of fall armyworms in central Arkansas on some golf courses, lawns and pastures.
Fall armyworms are caterpillars that appear in large groups and cause damage by feeding on the leaves of green turf (usually bermudagrass). The larvae (worms) can be white, off-white, yellow, or light green in color after they hatch and then darken in color as they develop. Once they mature, the fall armyworms are about 1.5-inches long. They are easy to identify by the presence of an upside-down Y marking on their head with white stripes down their body. The worms can quickly (in a matter of a few days) turn a green turf or a pasture into a brown area by feeding on the leaves across the area. The worms are a problem and a great nuisance until they’re subjected to a hard frost in late October or November.
Fall armyworms do not over winter in Arkansas, instead, the adult moths catch wind currents and gradually move into our state from the south and lay eggs. Fall armyworm damage can appear almost overnight. Infestations can be easily overlooked when the caterpillars are small and eating very little, but once they grow large and eat more, the damage becomes readily apparent.
There are a few tips to remember about fall armyworms. First of all, do not treat your lawn when armyworms are tiny, however, get prepared. Several natural enemies such as parasites, predators and pathogens occur and can possibly eliminate or reduce populations in a very short period of time, thereby saving you the effort. Many of you might remember a few years ago how the parasite, Cotesia marginiventris (a wasp parasitoid), showed up in large numbers and helped control the fall armyworms in many fields. County agents and producers saw the small white cocoon cases (of the pupal stage) that were mistakenly thought by some to be armyworm eggs. We have seen on several occasions that armyworm numbers will often decrease after the population of small larvae of the wasp parasitoid has previously been seen. The fall armyworm has about 6 larval instars (“instar” is the developmental stage of arthropods). The last few, particularly the fifth and sixth instars are when most of the damage to pastures occurs. Of the total foliage consumed, greater than 80–85% will occur at these stages. The best advice is to not get over-anxious and treat before it’s really necessary. But, do not wait until they become too large either. Harvesting an infested hay meadow can often be an option if the hay is mature. Most of the recommended products will work well on medium-sized larvae.
If you decide to treat the armyworms, liquid, not granular, insecticides are usually best. Any sprayable lawn insecticide can be applied in a coarse spray over the grass. Wet the grass thoroughly and do not mow for three days after application.
Several lawn insecticides are effective caterpillar killers, however, products containing B.T. are effective only on small (1/2 inch or less) worms. Irrigate the grass before treating, to move the caterpillars out of the thatch. Treat in late afternoon, as this is when the caterpillars are likely to be feeding. If possible, mow before you treat and then do not mow for 3 days after treatment. Winter cold has a great effect on armyworm populations. If a winter is severe, only a few of them will survive for next year. The good news is that armyworms will not hurt your lawn permanently. Since they feed on the grass blades and not the roots, they very rarely, if ever, kill the grass. But lawns can be weakened by this voracious eater, especially if they have suffered through the harsh growing conditions like we’ve experienced this summer.
For more information on controlling armyworms in pastures, see Extension fact sheet FSA 7083 “Managing Armyworms in Pastures and Hayfields” http://www.uaex.uada.edu/publications/PDF/FSA-7083.pdf. Information about managing fall armyworms can also be found http://www.uaex.uada.edu/counties/prairie/fall%20armyworm%20sheet%202015.pdf.
To contact County Agent Jimmy Driggers, call 623-6841, or email him at firstname.lastname@example.org .
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By Allen Bates
County Extension Agent - Agriculture
The Cooperative Extension Service
U of A System Division of Agriculture
Media Contact: Allen Bates
County Extension Agent - Agriculture
U of A Division of Agriculture
Cooperative Extension Service
236 Woodbine Hot Springs AR 71901
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