Management of Fall Armyworm
in Pastures and Hayfields

Seasonal Occurrence

Fall armyworms are susceptible to cold, and are unable to survive even the mildest winters in Alabama. Each year, fall armyworm moths, carried by air currents, make their way from Central and South America. The size and timing of the initial moth flights are two factors that influence the outbreak potential of this pest.

Occasionally, severe outbreaks occur as early as mid-April. For example, in 1977 the fall armyworm situation went from bad to worse. Many crops, including pastures and hayfields, were severely damaged before populations slowly diminished from August to late October.

Droughty conditions during the summers of 1999, 2000, and 2006 were also favorable for the fall armyworm. Many bermudagrass and bahiagrass fields were damaged in July, August, and September in those years.

Behavior Patterns

Fall armyworms are typically most active early in the morning, late in the afternoon or in early evening, but on taller, unmowed grass, they can be observed feeding on foliage throughout the day. On closely grazed or recently mowed hayfields, fall armyworm larvae spend the warmer hours of the day deep in the sod.

Fall armyworm damage often seems to appear "overnight." Young armyworms don't eat much. Almost all the damage is caused by the oldest caterpillars which eat more than all the other ages put together (See Figure 1). Therefore, an infestation may have been present but not detected because of the small size of the caterpillars.

Another reason for the sudden appearance of this insect is that the larger fall armyworms will sometimes "march into" (quickly invade) an uninfested area in search of food once an adjacent field has been defoliated. Large armyworms frequently disappear almost as suddenly as they appeared, either burrowing into the ground to pupate or moving on in search of food.

Damage

In hayfields or in pastures where there has been substantial growth accumulation, virtually all tender green material may be removed, leaving only tough stems a few inches long protruding from the soil surface (See Figure 3). Brown patches appear in the field and can rapidly increase in size.

Established, healthy, bermudagrass is rarely killed by fall armyworms, but the complete defoliation caused by a severe infestation weakens plants and deprives livestock of pasture or a hay producer of a hay cutting.

Fall armyworm damage on newly established grasses including winter annuals, tall fescue, or orchardgrass can be an even more serious situation. Seedlings of these fall-seeded plants are small when populations of fall armyworm are at seasonal highs. These crops can be severely stunted or killed if fall armyworms feed too far down on these plants.

Description and Life Cycle

Eggs. The female moths lay eggs at night in masses of up to several hundred on light-colored surfaces, such as fence rails, tree trunks, and the underside of tree limbs. The eggs are light gray and covered with grayish fuzz from the female's body. These masses darken with age, and the eggs hatch within 2 to 4 days. All the eggs within a mass hatch at about the same time.

Larvae (caterpillars). The tiny, light-colored, black-headed larvae spin down to the ground on silken webs and begin to feed. As they grow, their bodies darken and noticeable stripes appear. When fully grown, larvae may be up to 1½ inches long and vary in color from light green to almost black with several stripes along the body (See Figure 4). The "face" is marked with a lightcolored inverted "Y." Just behind the head, on the back of the caterpillar, you will see three thin white stripes running the length of the next segment. Sometimes these lines extend along the length of the caterpillar, as seen in Figure 4. There are usually small dark spots on the top side of each segment of the body. On the next-to-last segment, these spots are arranged like the corner points of a square (See Figure 5).

Pupae. Development from egg to fully grown larva requires about 2 to 3 weeks. At this point, larvae burrow into the soil and form pupae. The moths emerge in about 10 to 14 days.

Management Tips for Perennial Grass Pastures and Hayfields

Damage from armyworms seems to come in "waves," about a month apart. This is because moth activity and egg laying peak periodically even though there is substantial overlap between generations. Fields damaged by fall armyworm should be closely monitored for the rest of the season to determine whether further treatment is required. Two weeks after damage has occurred, start checking for small caterpillars.

Because moths prefer lightcolored surfaces on which to lay eggs, check these first. In pastures and hayfields, fence rails, fence posts, and tree limbs are favored egg-laying sites.

If a hayfield has been heavily damaged, fertilize as recommended to allow for another hay crop. Badly damaged pastures may need to be "rested." If possible, restrict grazing on badly damaged pastures until the grass has regrown.

Scouting. Scouting pastures and hayfields can help detect fall armyworm infestations before they cause economic damage. An easily detectable sign of armyworms is the presence of flocks of birds (especially cattle egrets) feeding in pastures or hay fields. Closely examine areas where birds are congregating.

In established pastures or hayfields, check in and around areas with dead grass or where birds are congregating. If no caterpillars are seen on the grass, look in the thatch at the base of the plants for larvae and green pellets of frass (larval excrement) about the size of bahiagrass seeds. If available, use an insect net to "sweep" the grass in early morning or late afternoon to check for the presence of young fall armyworms. In fields wet with dew, you can find caterpillars stuck on tires of vehicles that have been driven through an infested field.

Soap Flushes. To check areas where fall armyworm populations are suspected, use soap flushes (about 2 tablespoons of lemon-scented dishwashing detergent in 1 gallon of water) to bring larvae to the top of the sod. Since other insects (such as grubs, mole crickets, sod webworms) may be involved, don't give up without checking for these and other soil insects.

Treatment Threshold. The decision to treat for fall armyworms depends on the stage of the armyworms and the intended use of the forage. A population of 3 or more fall armyworms per square foot is a reasonable treatment threshold.

As with other pests, timing is important. If infestations are detected too late, the damage may already have been done.

If necessary, treat with insecticides at the right time. Small fall armyworms are much easier to kill than larger ones. If you check an area properly, you can determine the extent of an infestation, and spot-treat.

Frequently, mowing is the best option for salvaging a hay crop. When this approach is taken it may be possible to avoid using an insecticide.

Insecticidal Control. If insecticidal control is necessary on perennial grass pastures and hayfields, formulations of carbaryl (for example, Sevin XLR), diflubenzuron (Dimilin), methomyl (Lannate), methyl parathion, spinosad (for example, Tracer SC), and zetacypermethrin (Mustang Max) may be effective (See Table 1). Pay close attention to grazing and harvest restrictions. Note that methomyl is registered for use only on bermudagrass.

• Apply insecticides early or late in the day, because fall armyworm larvae are most active at these times.
• Apply sprays by ground in a minimum of 30 to 40 gallons of water per acre, or by air in a minimum of 3 to 5 gallons of water per acre. Control of larvae longer than ¾ inch may be poor. Control of larvae in tall or thick stands of grass may also be poor. If possible graze the affected area before treating.

Table 1. Suggestions for Control of Fall Armyworm in Perennial Grass Pastures and Hayfields.

Insecticide and Formulation	Rate1	Signal Word/ Use Restrictions	Minimum Days from Last Application to Harvest or Grazing	Comments
carbaryl2   Sevin XLR	1.0 to 1.5 lb ai/A 1.0 to 1.5 qt/A	Caution	14 (harvest or grazing)
diflubenzuron2   Dimilin 2L	0.12 lb ai/A 2 fl oz/A	Caution/ Restricted Use	1 (hay), 0 (grazing)	Apply when armyworms are less than ½ inch long because caterpillars can keep eating until their next molt.
methomyl2   Lannate LV 2.4	0.22 to 0.9 lb ai/A 0.75 to 3 pt/A	Danger: Poison/Restricted Use	3 (hay), 7 (grazing)	Use only on bermudagrass. Use higher rate for heavy populations and larger caterpillars.
methyl parathion2   Cheminova methyl 4EC	0.75 lb ai/A 1.5 pt/A	Danger: Poison/Restricted Use	15 (harvest or grazing)	Will not control large caterpillars. No residual activity.
spinosad2   Tracer SC	0.03 to 0.06 lb ai/A 1 to 2 fl oz/A	Caution	3 (hay), 0 (grazing)	Do not allow cattle to graze until the foliage has dried. Use higher rate for heavy populations and larger caterpillars.
zeta-cypermethrin   Mustang Max	0.0175 to 0.025 lb ai/A 2.8 to 4 fl oz/A	Warning/ Restricted Use	0 (harvest or grazing)	Use higher rate for heavy populations and larger caterpillars.
1Amounts listed are for the formulated product, unless otherwise indicated by ai, which is the amount of active ingredient per acre. 2Other products containing this active ingredient may be available.

Additional Information

For the most up-to-date control recommendations on pastures and hayfields, small grains, corn, and other crops, check the annually updated Extension publication ANR -0500-A, Alabama Pest Management Handbook, available from your county Extension office or online at http://www.aces,edu/pubs/docs/A/ANR-0500-A.

Kathy L. Flanders, Extension Entomologist, Associate Professor, Entomology and Plant Pathology; Donald M. Ball, Extension Agronomist, Professor, Agronomy and Soils; and Patricia P. Cobb, Professor Emerita, all with Auburn University