Insect I.D.: Sept. 2001 - CROWN & THATCH-INHABITING PESTS: Summing Up Sod Webworms

Species that prefer cool-season grasses are: bluegrass, webworm, larger, western, striped, elegant and vagabond sod webworms, and the cranberry girdler. Some of these may also occur in the warm-season zones, but the imported tropical sod webworm is the principal pest of warm-season grasses.

Sod webworm moths are easy to identify to species by using wing color patterns. Adults of most species rest on grass blades and in shrubs during the day and characteristically roll the forewings tubelike around the body. The head has a snout-like projection extending forward, thus, they are also called snout moths. Tropical sod webworm adults hold their forewings roof-like over the body. Usually two or three sod webworm species cause damage in any given area and species complexes vary across North America.

The larvae may be light tan to light purple and often have a greenish cast due to the chlorophyll contents of their gut. All species have rectangular, shield-like spots in rows down the body. Larvae are difficult to identify to species. Consult an expert if this is needed.

DIAGNOSIS. Generally, cool-season sod webworm larvae construct tunnels in the soil and thatch and line them with silk. At night, the larvae follow the tunnels to the surface and feed on grass blades and stems just above the crown. The severed stems die, leaving dead spots and/or sparse and ragged appearing turf. However, sod webworm larvae rarely kill turf. Brushing or raking away the dead stems exposes the shortened green stems left by the grazing larvae. The tropical sod webworm feeds along the tips and edges of grass blades, similar to armyworms. High populations can literally mow down turf.

An effective method of detecting infestations is to mix two tablespoons of liquid dishwashing detergent in two gallons of water (in our experience, Joy has not caused damage to the turf). Apply the solution uniformly over one square yard of turf using a sprinkling can (= soap flush). The soap solution irritates the larvae, which come to the surface in 10 to 15 minutes. Early morning is the best time to sample because the larvae are close to the surface. Larger larvae may surface first and the smallest last (after 20 minutes). This method is least successful when the thatch and/or upper soil are dry. Preirrigation may help.

LIFE CYCLE & HABITS. In cool-season turf areas, female moths simply drop their eggs as they fly over the turf at dusk and after dark. Eggs hatch in a week to 10 days and about six weeks are required for development from egg to adult.

The most common species on northern turfgrass (bluegrass and larger sod webworms) have two generations each year and overwinter as larvae in silken webs within the thatch or top inch of soil.