Tropical Sod Webworm

Description

The tropical sod webworm, Herpetogramma phaeopteralis Guenee (Lepidoptera: Crambidae), is a common insect pest that affects turfgrass in Louisiana. Larvae injure turfgrass by feeding on leaf tissue and stripping foliage. Injury caused by tropical sod webworm usually appears as yellow or brown turfgrass that may completely die if already weak or stressed. Injury is observed in mid-summer to early fall. Though most turfgrass species are susceptible, tropical sod webworms commonly attack St. Augustine grass, bermudagrass and centipedegrass.

Identification

In Louisiana, the tropical sod webworm can complete two or more generations in a year. Females lay eggs on leaves, and larvae emerge after about one week, depending on temperature. Larvae can mature into adult moths after three to five weeks but may take longer if temperatures are cooler. Tropical sod webworm larvae are translucent amber in color but begin to appear green as they feed. Mature larvae are about three-quarters of an inch in length. Adult moths are light to dark brown with wavy lines across their wings.

Indicators of insect presence

The webworm’s larval stage determines the type of injury to turfgrass:

• Young larvae chew leaf blade edges, leaving an uneven appearance.
• Older larvae chew entire leaves.
• Larvae cut grass blades at night, pull them underground and feed on them during the day.

Injured areas appear scalped and form yellow or brown patches.

• Leaf height is noticeably shorter in areas where larvae are feeding.

Injury typically observed in mid-summer to early fall.

Flush test

Use the flush test to determine whether certain insects are present in the lawn. Mix 1 tablespoon of lemon-scented soap per 1 gallon of water. Slowly pour the soapy water onto healthy grass surrounding the injured areas. In wet conditions, drench a 1-square-foot area with soapy water. In dry conditions, drench a 4-square-foot area. Then, for five to 10 minutes, closely watch the area to see if insects come to the surface. Repeat as desired in other areas to better determine insect presence.

Cultural control practices

One way to reduce insect injury and accelerate turfgrass recovery is to maintain a healthy lawn through proper fertilization and irrigation and regular mowing. Never apply more than 1 pound of nitrogen per 1,000 square feet per application, and always follow soil test recommendations for proper fertility. Irrigate as needed while taking rainfall into account. Mow regularly, but never remove more than one-third of the leaf blade height at one mowing. Thatch can develop over time and may need to be reduced through vertical mowing. Compaction can form more quickly on finer texture soils and in areas where there is high traffic. Dethatching or aeration need to be performed in late spring to summer when the turfgrass is actively growing. Properly maintaining a lawn through these cultural practices promotes dense and vigorous turfgrass and can increase tolerance to insect injury.

Chemical control practices

In addition to cultural practices, insecticide applications may be required to achieve effective insect control. Treat with insecticides when tropical sod webworm injury is excessive or large numbers of larvae are found during the flush test. When using any insecticide, you must follow the manufacturer’s labeled concerning all application parameters.

For more information regarding insecticides for turfgrass insect pests, please reference the Louisiana Insect Pest Management Guide.

Download here: Louisiana Home Lawn Series: Tropical Sod Webworm 3624-T