Recent outbreaks of fall armyworms have been observed in pastures, lawns and on golf courses.
Armyworm infestations are favored by cooler temperatures and wet or humid conditions, particularly where lush tender growth of grass plants is readily available.
Area forage producers and ranchers are encouraged to inspect pastures and hay meadows closely for armyworms. Homeowners should also be alert to the potential for armyworms to cause extensive injury to lawns.
Armyworms have four life stages: egg, larva, pupa and adult. Eggs are very small, white, laid in clusters of 50 or more and are covered with grayish, fuzzy scales from the body of the female moth. The eggs are seldom seen in grasses and are usually laid at the base of host plants. Lush plant growth is preferred by the adults for egg laying and larvae (caterpillars) are very small when they emerge from the egg. Larvae will feed for two to three weeks and can be 26-37 mm (1-1.5 inches) long with various color patterns depending on the species. The larvae have five instars (stages when molting occurs) and sometimes hide in debris on the soil surface in the middle of the day. When full grown, larvae enter the soil and form the pupal stage. Adult moths emerge from pupae. Moths mate and lay eggs, thus starting the life cycle over again.
Several generations (a generation is the development from egg to adult stage) occur each year and typically take about 28 days to complete. Generation time can be extended if cooler temperatures occur and can last up to several months. Fall populations of larvae often blend together several generations and may appear to be continually occurring.
The fall armyworm overwinters in the pupal stage in the southern regions of Texas. The adult is a moth that migrates northward as temperatures increase in the spring. The adult moth has a wingspan of 32-40 mm (about 1.5 inches). The hind wings are silver-white; the front wings are dark gray, mottled with lighter and darker splotches. Each front wing has a noticeable whitish spot near the extreme tip on the males. Larval color can vary from light tan to shades of green. The head is brown or black with a prominent white line between the eyes that forms an inverted “Y.” The fall armyworm has four large spots on the upper surface of the last segment of its body. Along the middle of the larva’s back is a wide, yellowish-gray band with a dark, black stripe just below the yellowish-gray band.
Armyworms attack many different kinds of plants and, when food is scarce, they will move to plants that are not normally attacked. Thus, armyworms can be found on nearly any plant as they migrate in search of edible foliage. Plants attacked by armyworms include Bermuda grass, fescue, grain and forage sorghum, corn, small grains, sweet potato, beans, turnip, clover, spinach, cucumber, potatoes, tomatoes, cotton and cabbage.
The small larvae will chew the green layer from the leaves, creating a “window pane” effect. The first three instars cause very little feeding damage while the last two instars consume 85 percent of the total foliage consumed.
Although armyworm outbreaks are memorable when they occur, in reality, the outbreaks are usually small in scope. Weather and natural enemies usually act together to keep populations under control.
Armyworms should be controlled when they occur in large numbers or plant damage is becoming excessive. Fall armyworm outbreaks usually occur late in the summer and early fall.
Treatment thresholds in improved pastures and lawns vary with conditions but treatment should be considered when counts average three or more worms per square foot.
Insecticide choices vary with the crop but the following (with product names and grazing restrictions in parentheses) are labeled for use in pastures: carbaryl (Sevin) (14 days), malathion (zero days), methomyl (Lannate) (seven days), Mustang Max (zero days), Tracer (let spray dry) and various biologicals such as Dipel (zero days).
Insecticide choices labeled for use in lawns and turf include halofenozide (Mach 2), bifenthrin (Talstar), cyfluthrin (Tempo, Bayer Advanced), carbaryl (Sevin) permethrin (multiple brands) and spinosad (Conserve and others).
Glen Moore is the county extension agent for intergrated pest management.