Sod Webworms

Order: Lepidoptera

Family: Cerambidae

Description

The small moths are white and tan and their wings tend to curl downward. They can be observed flying around lawns during late spring through early summer. The moths fly at dusk and lay eggs individually on turfgrass while in flight. After hatching, young caterpillars are brown or gray and 6.5–25.5 mm long.

Quick Facts

  • In Colorado, two species of sod webworm are considered pests of turfgrass. They are the cranberry girdler (Chrysoteuchia topiaria) and vagabond crambus (Agriphila vulgivagella).
  • Sod webworm moths commonly lay eggs in turf. After hatching the caterpillars spin silk webs for protection and feed on leaves of grass.
  • Healthy lawns receiving adequate fertilizer and irrigation are generally able to withstand sod webworm attack.
cranberry girdler larva
Cranberry girdler in Kentucky bluegrass (Poa pratensis L.)
David Cappaert, Bugwood.org

Sod webworms are a complex of commonly occurring insect pests of lawns in the Rocky Mountain region. There are two species of sod webworm of significance in Colorado, the cranberry girdler and vagabond crambus. Healthy lawns can usually withstand small to moderate infestations, and there are several management options for heavily infested areas.

Vagabond crambus moth
Vagabond crambus moth
Whitney Cranshaw, Colorado State University, Bugwood.org
damage to turf
Damage to turf caused by sod webworm.
Photo by Michigan State University

Life History

Young webworms live in silk tunnels during the day and emerge at night to feed on leaves of grass. Most species of sod webworm overwinter as fully grown larvae within their silk tunnels and have two to three generations each year. The greatest damage to lawns is usually done by overwintered larvae.

After emerging in spring, the caterpillars feed for a brief period, pupate, and then emerge as adults. The second-generation larvae tend to feed in July and August, and by late summer there can be overlapping generations with all life stages present in turf.

Injury

Caterpillars damage turf by feeding on leaves of grass which causes patches of yellow or brown discoloration on the grass blades. When inspected more closely, these areas will appear scalped or grazed. Early signs of damage include small, brown patches of closely clipped grass. The patches will gradually increase in size as the caterpillars continue to feed and grow.

The presence of sod webworm caterpillars in turfgrass will also attract large numbers of blackbirds to the area. Birds probe the grass and soil for the caterpillars, forming unsightly holes in turf.

Monitoring

Look for large numbers of birds feeding on lawns. The presence of green fecal pellets is another indicator of sod webworm, as well as the presence of feeding injury on grass blades. In addition, the presence of silken tubes and webbing is a good indicator that sod webworm is present.

Brown patches in lawns can be caused by several lawn diseases. Therefore, when infestations are suspected, it is important to confirm the presence of sod webworm before treating. To do this, one to two fluid ounces of liquid dishwashing soap should be mixed with one gallon of water and poured evenly over one square yard of turf. Webworms will rise to the surface within 10 minutes. When 15 or more webworm larvae are present per square yard in healthy turf, or when 5 webworm larvae are present on stressed turf, then insecticidal applications should be considered.

Cultural and Biological Management

Proper irrigation is usually sufficient in allowing turf to outgrow moderate webworm feeding injury.

Sod webworms are also susceptible to the parasitic nematode, Stenernema carpocapsae, and toxins of the bacterium Bacillus thuringiensis. However, products containing these natural enemies are only effective if applied against smaller, young webworms.

Other natural enemies include tachinid flies, braconid wasps, earwig, rove beetles, paper wasps, ants, robber flies and vertebrate predators.

Insecticidal Control

Lawns can be treated for sod webworm with insecticides labeled for “lawn insect control”. It is important to mow lawns before applying insecticides to remove flowering weeds and help protect foraging bees. Applying insecticides in the late afternoon or early evening when sod webworm larvae tend to be more active is also recommended. Before treating, irrigate the lawn with ½ to ¾ inches of water to cause webworms to move closer to the surface. Since young larvae are most susceptible, applying insecticides two to three weeks after peak moth flights is recommended so that eggs are allowed to hatch.

References

Colorado State University (n.d.). Plant Talk 1514 – Insects. Colorado State University – Extension. Available https://planttalk.colostate.edu/topics/lawns/1514-insects/#:~:text=The%20sod%20webworm%20is%20one,late%20spring%20and%20into%20summer.

University of California. 2009. Sod Webworms. University of California – IPM. Available https://www2.ipm.ucanr.edu/agriculture/turfgrass/sod-webworms/

University of Nebraska-Lincoln (n.d.). Turfgrass – Sod Webworms. University of Nebraska-Lincoln: Department of Entomology. Available https://entomology.unl.edu/turfent/documnts/swebwrms.shtml#:~:text=Sod%20webworm%20moths%20do%20not,layer%20and%20into%20the%20soil.

Vitum, P. 2011. Sod Webworms. University of Massachusetts Amhurst – Center for Agriculture, Food, and the Environment. Available https://ag.umass.edu/turf/fact-sheets/sod-webworms