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Armyworms can strike in most regions of the United States

Armyworm illustration


(crown & thatch inhabitant)

The fall armyworm (Spodoptera frugipeda) is a good example of a pest that can sneak up on you. When it does, the results can be disastrous. Fall armyworms can strike in most regions of the United States and seem to have been relatively serious during the past few years. No one can accurately predict what this pest will do in coming years. However, if it does strike, you must learn two things. First, you should understand the pest's biology. Second, you need to know how to most effectively control them.

These caterpillars feed on a variety of grasses including agricultural grass crops such as small grains and corn. Turf grasses are not commonly infested. Mature larvae reach 1-1/2" — 2" in length. Larvae are a dull yellow to gray with stripes running lengthwise along the body

Armyworm MothThe larvae feed at night on grass blades. The caterpillars feed on a variety of plants. Notably, warm-season grasses such as bermudagrass, St. Augustinegrass and some others are commonly attacked. Among the cool-season grasses, bluegrass, ryegrass, fine fescue and bentgrass are preferred cool-season turfgrasses.During the day the larvae hide in silk-lined tunnels or burrows at or slightly into the soil surface. Some species damage plant crowns or roots as well as blades. Heavy infestations may seriously damage large areas of turf. Look for dew sparkling on the webs in the early morning or at dusk. Use the flotation method to force the caterpillars to the surface, where they can be counted.

In the flotation method, a soapy solution is poured inside a topless and bottomless can. The soapy solution is made by adding one ounce of mild dishwashing detergent to one gallon of water. It is best to scout for sod webworms in June and again in early August, since sod webworms have two generations per year. Tolerance is around 12 larvae/ft2. Water the lawn thoroughly a day or so before applying an insecticide. Then delay further water for at least three days after treatment.

Populations arrive as annual flights from extreme southern populations. They are similar in size to armyworms. Populations of fall armyworms are typically kept in check by natural means, though population booms can occur, generally after a drought. Thresholds are not well developed.

Armyworms and loopers may be present during the spring, summer or fall. Armyworms and loopers differ from sod webworms in size and feeding habits. Armyworms, cutworms and loopers grow to about -1/2" in length, or about twice the length of the full grown sod webworm.

Armyworm Damage

Armyworm injury is similar to that of webworms; however, the damage is usually more scattered and not confined to patches as with sod webworm infestations. It is not unusual to have populations of armyworms, webworms, and other lawn caterpillars all feeding at the same time in the same location. Armyworms and loopers feed during the day and do not rest in a curled position while cutworms feed during the night and remain concealed during the day like webworms.


Fall armyworms are a little more difficult to kill than cutworms and as the worms become larger, the challenge greater. Many turf managers rely on pyrethroids for control of armyworms, but other chemicals are effective as well.

The damage creates a frosted appearance or a brown area (look for this symptom late in the summer or early fall) or in the case of severe infestation, the turf may be completely denuded. Fall armyworms rarely kill the turf, even if left untreated, but they can make it look very unsightly. Damage usually moves in from the edges and often proceeds in a relatively straight line as the armyworms march across the lawn. This line may progress at 1' — 3' per night.