In late summer, almost every year, caterpillars invade pastures, hay fields, and home lawns throughout the state. Particularly in pastures and hay fields, damage may be severe before the worms are noticed. The grass is usually not killed, but hay yield and forage can be reduced to almost nothing over whole fields in extreme cases. The long-term damage to established lawns, pastures or hayfields is mostly aesthetic, but newly sodded or sprigged areas can be severely damaged or even killed.
Fall armyworms are the larval or caterpillar stage of a small gray moth which overwinters in Florida and the tropics. Each year, storms bring the adult moths north on wind currents. The females lay masses of 50 to 700 eggs on just about everything. The first battalion of females lays eggs in South Georgia. Succeeding generations march northward through the state each summer, traveling on weather fronts and storms. Fall armyworms cannot overwinter in North Georgia. They may survive a mild winter (like this past year) in North Florida and extreme South Georgia.
The hot, dry weather we experienced this summer favored the movement of armyworms into North Georgia in early August. Because of their early arrival this summer, we can expect to see multiple generations of armyworms hatching out this fall every few weeks. The first sign that armyworms are near might be large clusters of birds on your lawn or pasture. Look closer at the grass, and you may see several caterpillars munching on the leaf blades. Although birds eat armyworm caterpillars, they are no match for hundreds or thousands of them.
Young armyworms are one-quarter to three-quarters of an inch long. Mature ones are one and a half inches long. They are greenish when small and become dark brown when fully grown, with several light stripes down the length of the body. The head or “face” has an inverted Y-pattern on it. If you suspect your grass is being infiltrated, but can’t find the caterpillars on the grass, use a soap flush to bring them to the surface. One ounce of dish detergent mixed with one gallon of water can be poured over a one square yard area where an infestation is suspected. The detergent irritates the insects, causing them to come to the surface quickly.
Feeding damage, coupled with damage from the recent dry weather, may justify applying insecticides. In pastures and lawns, finding three to five caterpillars per square foot is a threshold to start treating for fall armyworms. Carbaryl (Sevin), pyrethroids (products that end with “-thrin” in the active ingredient), spinosad and insect growth regulators (Intrepid and Dimilin) are the most common treatment options. Organic products containing B.t. (Bacillus thuringiensis) are effective only on young (a half-inch or smaller) worms. Prevathon is a newer product labeled for pastures that provides longer residual control.
In general, the earlier in the year armyworms appear, the more likely it is that multiple treatments will be necessary. That would make the extra cost of using a longer residual product a good investment if you can treat once instead of incurring the application costs of repeat applications. In years with good growing conditions, many farmers see the last hay cutting as insurance that they will likely not need (and so, not worth the extra cost), while in dry years the late cutting may be the difference between getting through the winter or buying hay later.
If possible, mow and irrigate lawns before treating, to move the caterpillars out of the thatch. Treat in the morning or early evening, when the caterpillars are likely feeding higher on the grass blades. If hay is close to ready for harvest, cut it before treating. Spray coverage is improved with shorter grass than in taller grass. As with all insecticides, be sure to read and follow all labeled application rates and safety precautions. For pastures and hayfields, it’s important to note any grazing restrictions that might affect livestock. Some products have short residual activity and have no restrictions for grazing or cutting hay for forage, while some products such as carbaryl (Sevin) have a grazing restriction of 14 days.
Paul Pugliese is the Extension Coordinator and Agricultural & Natural Resources Agent for Bartow County Cooperative Extension, a partnership of The University of Georgia, The U.S. Department of Agriculture, and Bartow County. For more information and free farm, lawn, or garden publications, call (770) 387-5142 or visit our local website at ugaextension.com/bartow .