Will Armyworms Kill St. Augustinegrass?

When Pests Come Marching In

• Armyworm damage first appears as skeletonized grass blades. As larvae develop, they eat the entire aboveground portion of the grass plant. Turf areas with heavy infestations may develop circular, scalped areas or a line of damaged grass that progresses across the turf area. Armyworms are a problem in late summer or early fall in most areas, but in parts of the southern U.S. they may feed in spring. Armyworms consume only foliage and leave roots intact, allowing healthy, vigorous turf to recover, but after depleting a turf area, these pests may move on to feed on garden crops and other desirable vegetation.

Detecting an Invasion

• Armyworm caterpillars feed at night and extensive damage may seem to appear overnight when a large number of armyworms are present. To determine if or gauge approximately how many armyworms are present, blend about 2 tablespoons of liquid dishwashing detergent -- lemon-scented, if possible -- with 2 gallons of water, and slowly poor the mixture over about a 2-square-foot area where you suspect an infestation. Inspect the surface of the treated section for the pests after five to 10 minutes. Armyworms measure up to about 1 1/2 inches long and are smooth-bodied. Body color varies between species and larval stages, but light or dark stripes are present on the body. The larva of one species, the fall armyworm, has a light-colored "Y" on its head.

Nonchemical Control

• Excellent turf care practices usually allow St. Augustinegrass to withstand and recover from armyworm activity. However, extreme heat or drought stress around the same time feeding damage occurs can kill the grass. Irrigate the grass deeply and infrequently, moistening the top 4 to 6 inches of soil whenever soil 1 to 2 inches below the surface feels dry to the touch. Do not over-fertilize or over-water the grass during the growing season, as overly maintained turf is attractive to the egg-laying female moths.

Biological Battling

• Broad-spectrum, persistent pesticides interfere with naturally occurring armyworm enemies and other beneficial organisms. Biological control including applying entomopathogenic nematodes at a rate of 25 million nematodes per 1,000 square feet of turf, or the bacteria Bacillus thuringiensis applied as granules or a liquid spray at a rate of 1/2 to 4 teaspoons Bt per gallon of water or as directed by the product label, kills armyworms without interfering with beneficial insects. Early detection and prompt treatment is important, as materials such as Bt are most effective against young larvae.