Grass Worms

If you are one of the many that have taken advantage of the frequent rain in order to establish a new lawn, keep an eye open for “grass worms”. Though truly caterpillars, not worms, these destructive, chewing insects can wreak havoc on new sod.

Tropical sod webworm larvae are destructive pests of warm season turfgrasses in the southeastern U.S. especially on newly established sod. Larval feeding damage reduces turfgrass aesthetics, vigor, photosynthesis and density, which is very evident on finer-bladed grasses such as bermudagrass and zoysiagrass.  Adults, a dull brown colored moth about ¾ inch long, rest in sheltered and shrubby areas during the day and are active at dusk.  Females deposit clusters of 10-35 eggs on the upper surface of grass blades.  The eggs hatch in 3-4 days and develop from a 1 mm long caterpillar to one over 11 mm long through six instars within 21 to 47 days, depending on temperature.  Larval feeding occurs at night, leaving the grass looking ragged, shortened and missing.

Control should be against damaging larvae, not the flying moths. However, insecticidal soap applications to moth harboring areas can reduce re-population frequency if such areas are located.  Soil-drenching soap flushes can be used to find the caterpillars, especially in dry and hot grass areas.  Bacterial-based insecticides will control sod webworm caterpillars without impacting beneficial species as long as they are applied with each flush of grass growth.

Excessive fertilizing will lead to caterpillar outbreaks in lawns. Newly installed sod is usually rich in nutrients and rapid growing, which makes it very attractive to sod webworms.  Grass installation over the summer months should be immediately followed by sod webworm treatment.

Fall armyworms are also attracted to newly installed sod. They feed any time of the day or night, but are most active early in the morning or late in the evening.  The 1 ½ inch long gray and white moth lays about 1,000 eggs in multiple masses on any vegetation.  Two to 10 days later, the small caterpillar hatches and begins to grow to nearly 2 inches long over a two week period.  The fall armyworm is easily recognized by its dark head marked with a distinct pale-colored inverted Y and the long black stripe running along each side of its body.  These aggressive feeders “march” rapidly across grassed areas consuming every above-ground plant part.  While bacterial-based insecticides will reduce the numbers, control of armyworms usually requires synthetic insecticides.  Diligent inspection and early pesticide application is critical to establishment of new sod installed during the summer months.