Sod Webworm Moths Everywhere!

By PR Extension Office

Though the abundance of the moths has tapered in the last week, the sod webworm moths made the simplest jobs difficult, according to MSU Extension Powder River County Agent Mary Rumph.

“Reports of having difficulty seeing and even breathing while on an ATV were common in the last few weeks due to the number of moths flying,” said Rumph. “My horse was even bothered by the pest flying around his face.” Though a common fall pest, the sheer volume of moths was unsettling, she added. The larva feeds on grass and damage was not obvious where the swarms of moths were, often in native range. When large numbers occur on lawn or turf, then management decisions might be warranted.

Profile

Sod webworm moths are one-half to one inch long, depending on the species. Their wings are dull-white or gray with a delicate fringe along the edges of the hindwings. Some species can also have a pattern of dark stripes present on the wings. The moths have long snouts and hold their wings in a tube-like position over their bodies while at rest. Most sod webworm larvae grow three-quarters to one inch long and are brown or gray in color. They have dark spots and coarse hairs on their bodies, and their heads are solid brown.

Overwintering larvae become active in the spring and start feeding on grass blades and stems near the soil surface. Larvae molt several times and then pupate in late spring or early summer. Sod webworm moths emerge several days later to mate and lay eggs in lawns. A second or third generation may occur until fall when colder weather sets in. Cold temperatures cause the larvae to form web-like chambers in the thatch layer of lawns, where they overwinter until the following spring.

Scouting

Sod webworm larvae feeding is first characterized by the observation of small patches in lawns that have a ragged-brown appearance. These areas will often look like they were scalped with minimal amounts of grass present. These areas should be focused on to determine if larvae, silk tubes or webbing is present. Over time, sod webworm damage can result in entire lawns looking thin, drought stressed or having multiple dead patches.

Management

Most lawns are able to tolerate the larval feeding as it occurs during peak growth periods of the grass. However, sod webworm activity during hot weather with drought conditions can lead to large areas of lawns being killed. Consistent watering and fertilizer applications can help lawns recover quickly and outgrow webworm feeding injury.

Birds often play a large role in managing webworm populations, as they will feed on both the developing larvae in the grass as well as the adult moths flying in the air. The use of insecticides for sod webworms should be a last resort and is not necessary unless there are back-to-back years of abundant populations. For healthy lawns, 15 or more sod webworm larvae per square yard is the threshold for insecticidal treatment. Although there isn’t a set threshold for stressed lawns, it is known that much fewer larvae can cause significant damage when other stressors are present (i.e., drought).