Tropical Sod Webworms on the Move

Rene Schmit  |  8/19/2013 9:13:07 PM

Tropical sod webworms are one of the most destructive pests of St. Augustine grass. The damage looks unusual and shocks the homeowner because it appears to happen overnight. The lawns may have a few chewed up circular areas or browned out areas that exceed a 1,000 square feet. At first glance, you will think disease. However, after close inspection of the turf, you will see that the leaves have heavy chewing damage. Young larvae feed along the mid-rib trough of the leaf. Mid-sized larvae chew notches in the edges of leaves, and older larvae will completely strip off the leaves similar to the damage caused by armyworms. It is the notched edges on grass blades that are the tipoff. Additionally, the worms leave trails of a silky web as they crawl through the grass. You can spot these “webs” in the morning when dew is on the ground. Like armyworms, watch for birds feeding in the area.

Armyworms feed at midday, but sod webworms prefer to feed at night. If you make up a soapy solution in a couple of gallons of water and pour it in a transition area (damaged turf transitioning to good turf), the armyworms present in the area will go crazy. Tropical sod webworms will at least start moving, making them easier to spot. Moths are 0.5 to 0.75 inches long and have snout-like projections on their heads. You see the moths fluttering over the turf and in flowerbed areas nearby. This short-lived moth does not feed on the turf but is responsible for laying the eggs in the grass. The eggs hatch in 7 to 10 days, and the larvae begin feeding on the turf. The larvae (worms) can get up to 1 inch long and are light green to gray-green with dark spots on their body. There is no characteristic inverted Y shape on the head of webworms like you see with armyworms.

The good news is that control is possible, and the same insecticides that kill armyworms work well on tropical sod webworms. Insecticides with the following active ingredients are recommended – bifenthrin, carbaryl, halofenozide, trichlorfon and chlorantraniliprole.

The combination of worm damage, hot temperatures and drought may kill off lawns that were already weakened by disease, low mowing and overall low fertility. However, recovery is likely for lawns that were in good shape prior to the worm damage. Lawns will start to recover once the worms have been killed. The best solution for a late season recovery is to simply raise the mowing height to 3 inches and maintain adequate soil moisture. We have about a month and a half of good growing weather remaining but enough time to provide for at least a partial recovery of lawns that have been heavily damaged by tropical sod webworms.

Rene’ Schmit is the LSU AgCenter county Agent for St. Charles Parish and can be reached at 985-785-4473.
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