Fall armyworm caterpillars damage grasses by chewing plant tissue. The caterpillars prefer to feed on plants in the grass family, including turf, corn, sorghum, and grass pastures. When hungry, they will also attack most field crops. Interestingly, there are two strains of fall armyworm, one which prefers to feed on rice and pasture grasses, the other which prefers corn and sorghum. When fall armyworms eat all the available food in a field, they will migrate in large numbers to other fields. At this stage, much of the damage is already done, and the caterpillars are large and are harder to kill.
     Reports of damaging populations usually come in late July and early August, when later generations of the pest are present. However, this year, there are reports of damaging populations of fall armyworms now. The fall armyworm moths migrate north from Central and South America each year. By early summer, they are laying eggs in Alabama pastures. This insect has several generations per year. There can be considerable overlap between generations later in the season.
     Fall armyworms can be found feeding on the foliage at any time of day. When fully grown, they can be 1½ inches long. Fall armyworms are always striped, but are variously colored. The background color ranges from light green to almost black. Fall armyworm caterpillars can be identified by the four black dots on the back of the tip of the abdomen. Larger caterpillars typically have an upside-down "Y" shape on the head.
     More information on biology and habits of fall armyworms can be found in Circular ANR-172, Controlling Fall Armyworms on Lawn and Turf, by Patricia P. Cobb.

Control considerations

     Fall armyworms need to be treated when they are small (about ½ inch in length). Detecting infestations when the caterpillars are small gives more time for control measures to be implemented. When fall armyworms are fully grown, they are less susceptible to insecticides and therefore harder to kill. In addition, if most of the caterpillars are nearly grown, there will be little benefit of control, as most of the damage will already have been done.

Scouting for Fall Armyworm in Pastures

     Because of the need to detect infestations when the caterpillars are small, pastures should be checked weekly, usually from late July through October, to see if damaging numbers of fall armyworms or other pasture pests are present. This year, they should be checked earlier, as reports of high fall armyworm populations have already occurred.
     Walk into the pasture from all four sides, or walk in an "X" across the field to make sure you check a large enough area.
     Stop at about ten places in the field and look closely for small caterpillars feeding on the grass. If you find them, estimate the number per square foot. An easy way to do this is to make a sampling hoop out of a wire coat hanger. Pull the hanger into a circle, and it will have an area of about 2/3 of a square foot.

Decision Making

     Fall armyworm control is probably justified when the population exceeds three ½ inch caterpillars per square foot (an average of 2 caterpillars per sampling hoop).
     If fields are almost ready to mow for hay, consider doing so a bit earlier if damaging levels of fall armyworm are found.
     Suggestions for chemical control of fall armyworm in grass pastures are given below. Pay careful attention to the grazing interval, the time required from application until livestock can be put back in a pasture.
     Some populations of fall armyworm have been reported to be resistant to Sevin. However, Dr. Patricia Cobb has found that increasing the gallonage of water in the spray can improve control of fall armyworm with Sevin in turf. When treating pastures, try to use 25-30 gallons of water per acre for ground treatments, to improve coverage of the foliage. When treating by air, 3-5 gallons of water per acre is recommended when grass is tall and the fall armyworm population is high.
     If the efficacy of a chemical is in doubt, treat a small test area one day, and come back and check for percent control the next day.

Suggestions for control of fall armyworm in perennial grass pastures