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Fall Armyworms: Be Prepared

July 21, 2011
by Rick Brandenburg

Fall Armyworm Moth
Fall Armyworm Moth

Fall Armyworm
Fall Armyworm Larvae

Fall Armyworm Damage 
Turfgrass Damage

by Diane Silcox and Rick Brandenburg

Fall armyworms are sporadic pests in North Carolina turfgrass most years. Dry and hot years often make them a more serious problem as the armyworms will migrate towards irrigated turf since everything else is brown and less desirable. So while the numbers may not be any higher, their concentration around turfgrass can be much higher. The following information provides some insight into their biology and management. We will post alerts if we get any reports of actual damage.

The fall armyworm is a continuous resident of Central America, tropical South America and the West Indies. During mild winters, it may overwinter in coastal areas of southern Florida and Texas. Each spring, the fall armyworm spreads from these areas into the United States east of the Rocky Mountains and westward into southern New Mexico, Arizona and California.

The larvae are green, brown, or almost black. The dark head is marked with a yellow inverted "Y". There is a longitudinal black stripe along each side of the body and a faint narrow middorsal stripe. There are four black dots on the dorsal side of each abdominal segment. Fully grown larvae range between 1.38 and 1.97 in (35 - 50mm) long. The reddish brown - black pupae are about 0.5 in (13mm) long and are found in the soil. Adult moths have front wings that are dark gray mottled with light and dark markings. There is a white blotch near the tip of each front wing. The back wings are white.

The fall armyworm is most commonly associated with damage to bermudagrasses in the southern United States. However, it also feeds on fescue, ryegrasses, bentgrasses, bluegrass, and various small grain and grass crops. Infestations are associated with lush, green, dense grass. Problems most commonly occur in late summer and fall.

Larvae feed on all above-ground plant parts. Younger larvae skeletonize the most tender leaf tissue. Older larvae may consume most or all of the leaf tissue. Larvae move in groups from consumed areas to new areas. After fall armyworm feeding, bermudagrass, with proper management, usually regenerates. Feeding on cool-season grasses such as fescue and bluegrass may result in permanent damage.

Adult moths are most active at night and are attracted to lights. Light-colored objects adjacent to turf, such as flags on golf greens, goal posts, metal gutters and metal fences on athletic fields are favorite sites for egg deposition. Eggs hatch in 2-10 days. After 2-3 weeks, fully grown larvae burrow into the soil to pupate. Moths emerge in 10-14 days.

Flocks of birds feeding consistently in turf areas may indicate fall armyworm presence. If no larvae are seen, examine the turf for green fecal pellets and larvae. Soap flushes can be used to bring larvae to the surface. Except for home lawns and golf greens and tees, infested areas of less than 1,000 ft2 (92.9 m2) are seldom treated.

Control of fall armyworms will be improved if you cut the turf prior to treating. A light irrigation prior to treatment may also help as will treating late in the day. Large fall armyworms are difficult to control. Don't expect 90% control. In warm weather the caterpillar can go from egg to pupa in around 2 weeks. If the worms are very large (inch and a half long) then they will go into the soil very soon to pupate and control efforts may be a waste of time.

The key to finishing out this year with success against fall armyworm is to stay on top of the problem.