It is time for hay and livestock producers to consider prevention and protection for hayfields and pastures from fall armyworms. The fall armyworm, Spodoptera frugiperda, is known as a chronic pest in the Southeast region of the US. Climates in South Carolina favor the development of fall armyworms, with hot, dry summers being typical. As the name implies, fall armyworms are most numerous in late summer and early fall. The caterpillars feed on a variety of forage crops, such as bahiagrass, pearl millet, and sorghum sudan hybrids, but the most damage is usually seen in lush, green, well-fertilized bermudagrass.
Fall armyworms are susceptible to cold, making late summer/early fall prime time for the development of fall armyworms, which means this is also the prime time to prepare for prevention/treatment of an invasion. Often the pest is present but goes undetected because of the initial small size of the eggs/larva. Mature caterpillars often cause the most damage because of the vast amount of foliage they consume, and damage often appears overnight. Damage may vary in appearance and severity according to the type of grass and management practices. In a pasture situation where the grass has been grazed closely, the grass may seem to thin out and develop brown spots. All tender green material may be removed in hayfields, leaving only tough stems a few inches long. Established, healthy bermudagrass is rarely killed by armyworms. Still, the complete defoliation caused by severe infestation weakens plants and deprives livestock of pasture or a hay producer of a hay cutting. Fall armyworm damage on newly established grasses can be a more severe situation. The crops can be severely stunted or killed if fall armyworms feed too far down on these plants. Most commonly fall armyworms forage during early morning and late afternoon, making this the recommended time to scout the fields for the pest or signs of damage.
Scouting hayfields and pastures should begin in July. While scouting, the following descriptions are stages of the fall armyworm that you may notice. Fall armyworm eggs are creamy-white and dome-shaped with a flat base. Eggs are laid in clusters of 25 to 100, with a single adult female moth laying as many as 2,000 eggs on lower leaf blades. The egg clusters will have a hairy/fuzzy appearance. The newly hatched larva is light green to cream-colored with a dark head capsule. The newly hatched larva is tiny and often goes unnoticed. As the larva develops, it becomes darker with light-colored lines down the side of the body. The head capsule will be dark with a light-colored inverted Y marking on the front. Another identifying mark will be the four dots on the next to last abdominal segment. Once fully grown, the larva measures about 1 ½ inch long. Large armyworms frequently disappear almost as suddenly as they appeared, either burrowing into the ground to pupate or migrating in search of food. The adult fall armyworm is an ash-gray moth with a 1 ½ inch wingspan. It will have whitish spots near the tip of the front wings, and the hind wings are whitish with narrow brownish wing edges. With the rapid rate of development of the fall armyworms during late summer/early fall, it is not uncommon to get 5 or more generations produced each year. Although generations can overlap, most of the fall armyworms in one area will be similar in life stage development, making damage seem to come in waves that follow the cyclic development of the fall armyworms. Some signs that indicate fall armyworms are cattle egrets and/or geese feeding in concentrated areas, foliage that has been chewed on the underside only, and forage that appears to be ‘frosted.’ Once damage becomes more severe; the field will have dead spots appear. It also pays to notice the condition of your neighboring fields in that this pest will sometimes invade an area in search of food once an adjacent field has been defoliated.
Scouting pastures and hayfields can help detect fall armyworm infestation before they cause economic damage. If fields are already damaged from fall armyworms, these fields should be closely monitored for the rest of the season to determine whether further treatment is required. If a hayfield has been heavily damaged, fertilize as recommended to allow for another hay crop, whereas severely damaged pastures may need to be rested.
If you detect fall armyworms, there are certain insecticides that are labeled for pastures and hayfields that can be used for treatment. The decision to treat and the amount of insecticide to apply for fall armyworms depends on the stage of the fall armyworms and the intended use of the forage. A population of 2-3 or more fall armyworms per square foot is a reasonable treatment threshold. As with other pests, timing is essential. Apply insecticides early or late in the day because fall armyworm larvae are most active at these times. Below is a link with insecticides labeled for fall armyworms. As always, read the label before use to ensure proper precautions are taken, such as any grazing or haying restrictions, and to ensure the product will treat the intended target.
For more information on pasture and hay insect management, visit: https://www.clemson.edu/extension/agronomy/pestmanagement21/2021pmhpastureinsectcontrol.pdf
Contact your local Livestock and Forages agent for further assistance.
Sources:
Loftin, Kelly, Gus Lorenz, Ricky Corder. Managing Armyworms in Pastures and Hayfields (publication number FSA7083). University of Arkansas Cooperative Extension. Accessed July 15, 2015, from https://www.uaex.edu/publications/PDF/FSA-7083.pdf
Marshall, Mike, et al. 2021 South Carolina Pest Management Handbook. Clemson University Cooperative Extension.
Submitted by: Amber Starnes, CUCES Area Livestock and Forages Agent
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