Fall armyworm, Spodoptera frugiperda, is an occasional field crop pest in the Midwest. It feeds on a wide range of plants, including corn, sorghum, small grains, pasture grasses, and forage crops. It is a distinct species from other armyworms encountered in Ohio, such as true armyworm, beet armyworm, and yellowstriped armyworm. Fall armyworm is a tropical species, and in the United States it can overwinter in only the warmest places—southern Texas and South Florida. In early spring, populations begin to build on weeds, crops, and forages in the Southern states, and adults migrate farther north, often assisted by strong winds.
The wrong combination of events—unusually high early-season populations in the South, strong sustained wind patterns that bring adults to our region, and hot local conditions (a hot summer and a long, warm fall)—contribute to outbreaks in Northern states. Such an outbreak was seen in Ohio in 2021 (Figure 1), resulting in significant fall armyworm damage to alfalfa, clover, and turfgrass, and lesser damage to fall cover crops and small grains (Figure 2). Though rare, such outbreaks are likely to become more common as a warming climate contributes to a greater frequency of these weather-related events and the winter survival of fall armyworms across a wider range of the Southern United States.
Life Cycle and Biology
The adult fall armyworm is a moth that does not feed. Moths lay eggs that hatch caterpillars—the crop-damaging stage of the fall armyworm life cycle. When the caterpillars complete their development, they pupate (the transformation stage preceding moth emergence) in the soil, later emerging as adult moths. The duration of their life cycle varies depending on temperature but takes approximately 30 days in hot weather—meaning that more than one generation can propagate in Ohio during an extended warm fall before freezing temperatures kill them. Research is underway to better understand the life cycle development times of fall armyworms in Ohio’s climate.
Fall armyworm adults are gray mottled moths with a wingspan of about 1 ½ inches (Figure 3). They resemble other armyworm moth species and can be difficult to distinguish without study. Adults will not typically appear in Ohio until July or August, after migrating from further south. These adults lay egg masses on various surfaces, such as plant leaves, fence posts, and telephone poles. A female moth can lay several egg masses in her lifetime.
Fall armyworm eggs are individually very small (less than 1/32 inch wide) but are laid in clusters ranging from a few to hundreds. These clusters usually have a fluffy-looking cover, making individual eggs harder to see (Figure 4). Egg color begins as pale yellow, darkening as eggs mature. Eggs will appear almost black just before they hatch. The development time to hatching varies with temperature but is about four days.
The caterpillars (larvae) pass through six or seven growth stages (instars) before pupation. Initially quite small (about ¹⁄₁₆ inch), the caterpillars may be as long as 1 ½ inches in the final instar. Caterpillar color varies from light green to brown or almost black, often with white lines along the body. The best way to identify fall armyworm caterpillars is by an inverted “Y” marking on the head capsule, and four black dots in a square pattern near the end of the body (Figure 5). The caterpillar stage is the feeding stage, thus the crop-damaging stage.
In Southern states, fall armyworm may spend about three weeks as caterpillars during the summer months. In Ohio, it is likely a bit longer; research is underway to understand this better. After the final instar, caterpillars fall to the ground, burrow down a few inches, and pupate. Fall armyworm caterpillars are voracious feeders, and the final instar is the most damaging of all—eating as much plant material as all the earlier instars combined. When farmers observe fields being eaten “almost overnight,” the caterpillars have actually been feeding quietly for a few weeks, with defoliation snowballing rapidly as they approach maturity. Thus, management relies heavily on scouting to detect the caterpillars while they are still relatively small.
Early-instar caterpillars are easy to miss because they are small and eat relatively little. As the caterpillars age, they grow quickly and eat progressively more. By the time they reach the final instar, they are up to 1 ½ inches long, eat a huge amount, and damage crops very quickly. Compounding the problem is the fact that larger caterpillars are not very susceptible to insecticide and are thus difficult to kill. Chemical control should take place when caterpillars are ¾ inch long or smaller (the first three instars). Freezing temperatures will quickly kill fall armyworms. If frost is expected soon, control may not be needed.
Scouting and Threshold
The Ohio State University conducts a sampling network in several counties, using pheromone traps to monitor for common corn pests, including western bean cutworm, and less common pests including fall armyworm. In the summer, results are reported weekly in the Ohio State agronomic crops newsletter. This monitoring is used to provide a general warning if or when a pest is found in Ohio—it is not a substitute for scouting. You can subscribe to the Ohio State agronomic crops newsletter at lists.osu.edu/mailman/listinfo/corn-out.
Thresholds are not yet researched in Ohio, but a rule of thumb based on guidelines in Southern states is to take management action when three or more caterpillars are present per square foot of crop.
In lower vegetation (e.g., forage, soybean, and cover crops), you can sample with a sweep net or beat sheet, both available from suppliers such as Great Lakes IPM in Vestaburg, Michigan. To use a sweep net, swing the net in a 180-degree arc with the rim of the net in a close to vertical position to maximize the vegetation it comes into contact with. Sweep from left to right, step forward, and then reverse the sweeping direction. After a 10-sweep set, hold the net out with its opening facing the sky, give it a few good shakes to knock the contents to the bottom, and then empty the contents into a sealable bag or onto a tray for examination. Perform 10-sweep sets in several parts of the field. A beat sheet (also called a drop cloth) is a piece of sturdy fabric mounted on a dowel frame, usually about 3 feet long. Place the cloth under the plants, parallel to a row of vegetation. Use your extended arm to shake the vegetation vigorously over the cloth, and then quickly examine the fallout.
In field corn, choose five locations from areas throughout the field. In each location, examine 20 plants in a row. Fall armyworms may feed on leaves and tassels or ears, with a raggedy feeding pattern. Many pests feed on corn foliage, so inspect the plant carefully to find and identify the caterpillars. Insecticide application in field corn is likely to be economical only under heavy infestation, with 75% of plants infested and caterpillars less than an inch long.
If caterpillars are longer than ¾ inch, chemical control is likely too late to be effective. In forage fields, if hay can be cut, early harvest is recommended. If caterpillars aren’t longer than ¾ inch, Southern states recommend mixing chemicals with two modes of action:
- a pyrethroid for knockdown
- a product with a longer residual such as chlorantraniliprole (which has good rainfastness) or diflubenzuron or methoxyfenozide (insect growth regulators that work well on small caterpillars, but which won’t provide good control after rain)
For more information on field crop insecticides, visit the Michigan/Ohio Field Crops Insect Pest Management Guide at aginsects.osu.edu/extension-publications/msuosu-ipm-guide.
Some beneficial organisms attack fall armyworms, including parasitoids in their year-round locations, ground beetles, soldier bugs, birds, rodents, and fungal and viral diseases. While such natural enemies reduce background numbers of armyworms, they are often not sufficient to prevent outbreaks during times of high adult colonization.
For More Information
For a good overall review of fall armyworm biology, damage, and management, read the following article:
Hardke, J. T., Lorenz III, G. M., & Leonard, B. R. (2015). Fall armyworm (Lepidoptera: Noctuidae) ecology in southeastern cotton. Journal of Integrated Pest Management, 6(1): 10.
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