Resistant Varieties of Grass
There are a number of steps you can take to reduce the chances of a recurrence of a armyworm problem next year.
Make your lawn armyworm resistant by overseeding with one or more of the new grass varieties that contain "endophytic" fungi that discourage armyworms and some other insects from feeding on the grass. A number of perennial ryegrasses, such as `Citation II', `Commander', `Repell', and `Sunrise', and a few tall fescues with this fungi are already available. More new varieties of grasses containing these endophytes are due, so keep an eye out for your favorite type of grass and use the brand that contains this built-in protection. Lawns can be overseeded almost any time, but the fall is the best time.
Build Healthy Turf
A healthy lawn can withstand minor infestations of armyworms. To keep it healthy aerate it at least once, preferably twice, a year. Add about a 1/4 inch of organic material such as peat moss, composted municipal sludge, or sifted compost each year to virtually eliminate the buildup of thatch that serves as home for armyworms. If thatch does accumulate, remove it when it exceeds 1/4 inch thick.
Because armyworms prefer turf that is dry and warm, do not allow your lawn to get too dry in the hot summer months. Water it well when rainfall is sparse. Watering in the late morning provides moisture to the grass and also keeps the soil surface cooler, making the area less attractive to armyworms. Cut the grass no shorter than 2 inches. Longer grass blades shade the soil and keep it cool.
Encourage Insect Predators
Maintaining a healthy lawn and avoiding the use of powerful insecticides encourage the buildup of a strong population of predator and parasitic insects that attack and control armyworms, especially at the egg stage. Braconid wasps, vespid wasps, earwigs, carabid and rove beetles, ants, and mites all attack armyworms. Of course, use of a general purpose, broad spectrum insecticide on the lawn will destroy this natural line of defense, leaving the lawn open to attack by other pests.
Above the soil another natural predator of armyworms are birds. Robins and Starlings will eat a lot of armyworms if they are in your lawn. Many seed eating birds prey on caterpillars while raising their young. Encourage birds to set up housekeeping in your yard by providing them shelter and water.