On a recent morning pasture walk to collect forage availability data on our tall fescue pastures I stumbled across a fall armyworm. This issue warrants attention as fall armyworm, Spodoptera frugiperda, is a major forage pest in the Southeastern United States. And my timing for a sighting of fall armyworms is about right, as they commonly start to come in during late July or August.
Fall armyworm caterpillars are most numerous in late summer to early fall. They feed on a variety of forage crops including bermudagrass, bahiagrass, pearl millet, sorghum-sudan hybrids, tall fescue, and various winter annuals. Other common crops such as corn, alfalfa, cotton, soybeans, and some vegetable crops have also been reported to be hosts for the pest.
Fall armyworms normally occurs in three or more generations each year. In severe years, outbreaks have been reported as early as April. It is also important to note, with the severe drought in much of the southern U.S. that these conditions create a favorable environment for the fall armyworm. Producers should be forewarned and practice good pest management tactics by scouting pastures early for the pest.
Fall armyworm damage varies in appearance and severity according to forage species. In closely grazed fields, the grass may appear thin and develop brown spots. The spots look burned out due to the result of fall armyworm larvae rapidly dehydrating the plant by chewing off tender shoots. Basically fall armyworm damage resembles drought damage.
In hay and pasture fields with a large amount of forage growth, all green material may be removed with only tough stems protruding from the soil surface. Brown patches will appear in the field and spread rapidly.
Fall armyworms are most active in early morning, late afternoon, or early evening in pastures. During the day they spend most of their time deep in the sod. Most times damage will appear to happen “overnight” and is usually indicative of an infestation that has been ongoing for quite some time as it is the oldest caterpillars that eat more forage than all other ages put together.
Damage usually occurs from August to October when populations are at their largest. During drought, damage can appear even earlier. Damage will appear in “waves,” normally about a month apart. This mainly due to the pest’s life cycle.
To combat the war on fall armyworms field scouting is a important weapon. Scouting may help to detect infestations before they cause economic damage. A common sign of a fall armyworm infestation is the presence of flocks of birds feeding in pastures or hay fields. You might want to check out what all the fuss is about!
In established pastures and hayfields check areas where dead grass is present. If you don’t see any caterpillars on the on the grass take a look at the base of the plant near soil level for signs of larvae or green frass (bug poo). Using a insect net to “sweep” the plants similar to what is done when checking for potato leafhoppers in alfalfa is also a useful too.
The decision to treat pastures is on dependent on life stage of the pest and the intended use of the forage. The most commonly accepted economic threshold for fall armyworms is 3 or more armyworms per square foot. Methods of control will be dependent on the severity of the infestation. Many times mowing is the best option for salvaging a haycrop. However, in severe cases, insecticides are really the only option. Check with your local extension office or agronomic service company for the latest in control methods.