Figure 1. Fall armyworm larvae. Image: Blake Layton, MSU-ES.

Figure 1. Fall armyworm larvae. Image: Blake Layton, MSU-ES.

Updated: January 6, 2022
By Amanda Grev, Ph.D.

Managing Fall Armyworm in Pastures and Hayfields

Although fall armyworm (Spodoptera frugiperda) is a native pest to North America and a chronic pest in the southeastern US, reports of fall armyworm activity and outbreaks are unusually high this year. There are numerous reports of heavy fall armyworm activity coming out of Virginia, Kentucky, Indiana, Illinois, Ohio, and other states. In Maryland, there have been cases reported across much of the state so far, including Anne Arundel, Baltimore, Charles, Calvert, Frederick, Howard, Montgomery, Prince George’s, St. Mary’s, and Washington counties. Weather conditions have allowed fall armyworm to flourish this year, so producers are encouraged to be on the lookout for potential problems.

Fall armyworm (Figure 1) is a tropical moth native to warm climate areas of the western hemisphere. They are susceptible to cold and cannot successfully overwinter in more northern areas; however, fall armyworm moths are strong fliers and with the help of air currents they make their way north each year. As a result, populations can show up throughout most of the eastern US in the late summer and fall months. The size and timing of the initial moth flights are two factors that influence the outbreak potential of this pest. The female moths arriving from southern states will seek young, tender foliage in which to lay their eggs. Female fall armyworm moths can lay egg masses of fifty to several hundred eggs, which means large densities of fall armyworms can build up quickly. 

Figure 2. Fall armyworm with  diagnostic “Y” pattern on head. Steve L. Brown, University of Georgia,
Figure 2. Fall armyworm with  diagnostic “Y” pattern on head. Steve L. Brown, University of Georgia,

Fall armyworm larvae may range in color from light green to almost black, with several stripes along the body. The head of the fall armyworm is marked with a light-colored, inverted Y-shape (Figure 2). This “Y” distinguishes the fall armyworm from other armyworm species. Fall armyworm damage is most likely to occur from August through October when populations are at seasonal highs. Droughty conditions are often favorable for the fall armyworm because many of their natural enemies are less active during droughts. Fall armyworms can be found up until the first killing frost although the risk of damage declines as it gets cooler.

Fall armyworms can feed on a number of different host plants, but they typically prefer corn, sorghum, small grains, alfalfa, and forage grasses, including turf, as well as pastures and hayfields. The caterpillars' damage grass by chewing the plant tissue. They are typically most active early in the morning, late afternoon, or early evening. Initially, small larvae will feed on the leaf surface, causing a “windowpane” effect where the green tissue is removed and a transparent membrane remains. Young armyworms don’t eat much, with almost all of the damage being caused by the oldest caterpillars. Under summer conditions, the caterpillars will take about 12 to 16 days to reach full size, with most of the feeding occurring during the last four days when the caterpillars are at their largest size. Eventually, their insatiable appetite can denude alfalfa and other forage crops rapidly before they “march” on to the next field in search of food or burrow into the ground to pupate.

Damage from fall armyworms may vary in appearance and severity. In hayfields or pastures, virtually all tender green material may be removed, leaving only tough stems a few inches long. Brown patches can appear in the field, often resembling drought damage, and the damaged patch may rapidly increase in size as the fall armyworm consumes more foliage. Established, healthy forage stands will likely not be killed by fall armyworms, but the defoliation will weaken the plants and can deprive producers of a grazing or hay cutting. Newly established stands can be severely stunted or killed, as they do not have an established root system and are much more susceptible to fall armyworm damage.

Figure 3. Fall armyworm feeding on grass hay.
Figure 3. Fall armyworm feeding on grass hay.

Producers should monitor their crops that are still green for fall armyworm presence and damage. By the time fall armyworm are larger and on the move, most of the damage will have been done already so it’s important to catch them early on when they are still small. Scouting can be done to help detect infestations. Fall armyworm moths prefer to lay eggs on light-colored surfaces, so checking fence rails, fence posts, and nearby tree limbs can also be useful, and be sure to check areas with dead grass or where birds are congregating. The best way to detect fall armyworms is to use an insect net and sweep the grass, as the sweep net will pick up larvae that may be too small to find otherwise. Sweep the grass in the early morning or late afternoon/evening when they are typically most active. If you find fall armyworms using the sweep net, the next step is to count how many caterpillars you have per square foot. Examine the plant itself as well as any thatch on top of the soil. 

The economic threshold for fall armyworms is typically 2-3 caterpillars per square foot. If you find three or more armyworms per square foot, an insecticide treatment or early harvest may be warranted. There are numerous insecticides that can be used for controlling fall armyworm caterpillars in forages, but rates and restrictions vary by crop so be sure to carefully read pesticide label restrictions by crop and take note of any grazing or harvest restrictions. Some insecticide options may include products containing pyrethroids, chlorantraniliprole, methoxyfenozide, spinosad, or carbaryl. Note that control of larger larvae is less effective with pyrethroids and is sometimes difficult with any insecticide. The label will have a recommended range for application rates; use higher rates when the grass is thick, when fall armyworm populations are high, and when caterpillars are larger. If possible, try to apply insecticides later in the day to coincide with the time when fall armyworms are more active and increase the probability of them encountering a lethal residue.

Harvesting the field for hay is also an option and can be an alternative to insecticides. The harvesting process will kill some caterpillars directly, and others will die from exposure to the high soil surface temperatures after harvest. However, mowing needs to be done as soon as possible and surviving fall armyworms will continue to feed so the faster the hay can be raked and baled the better.

This article appears on September 2021, Volume 12, Issue 6 of the Agronomy news.

Agronomy News, September 2021, Vol. 12, Issue 6

Agronomy News is a statewide newsletter for farmers, consultants, researchers, and educators interested in grain and row crop forage production systems. This newsletter is published once a month during the growing season and will include topics pertinent to agronomic crop production. Subscribers will receive an email with the latest edition.