Tropical Sod Webworms are Wreaking Havoc on Louisiana Lawns

Ronald Strahan  |  9/23/2010 9:02:36 PM

Tropical sod webworm (http://turfgrass.com/pics/pics39.html)

Tropical sod webworms completely devastated a St. Augustinegrass lawn in Vermillion Parish.

St. Augustinegrass lawn devastation in Baton Rouge.

Soapy water helps in diagnosis of chinch bugs and worms in lawns

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Tropical sod webworms are creating havoc on home lawns, especially St. Augustine grass. We are getting calls from all over the state, but particularly in the Baton Rouge area. We are seeing sod webworms and to a lesser extent armyworms in the same yards. At least every other St. Augustine grass yard in Baton Rouge has some degree of damage. These two moth larvae together have caused more damage than I have ever seen in home lawns.

Tropical sod webworms are one of the most destructive pests of St. Augustine grass and carpet grass. Just drive around and look at all the damage. So far centipede grass and Bermuda grass and zoysia have been less affected. In fact, I have seen St. Augustine grass completely devastated growing next to zoysia that was totally unaffected. I have had no calls concerning sod webworm damage in Bermuda grass sports fields or golf courses.

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.

How do you know if it is tropical sod webworm 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. I have found that just looking carefully through the grass during midday may not turn up any webworms. However, 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.

Life Cycle

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 tropical sod webworm completes its development from egg to moth emergence in 5 to 6 weeks. The species overwinters as larvae in the soil. We could get 3 to 4 generations in a season.

Control Options

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.

Lawn Recovery

The combination of worm damage, hot temperatures and drought may kill off lawns that were already weakened by cold damage, 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.

Fertilizing with nitrogen-containing fertilizer will help lawns recover quickly, but it is risky for another reason this late in the summer. Normally, it is not recommended that you fertilize lawns in Louisiana with nitrogen-containing fertilizer past August 15. Applying nitrogen fertilizer this late may lead to increased incidences of another serious problem – brown patch disease. If you decide to fertilize the lawn, be willing to go back with fungicides for brown patch disease as part of the overall treatment. Otherwise, raise the mowing height to 3 inches and maintain adequate soil moisture. We are still in Louisiana with Popsicle-melting heat well into November in most years. You have a month and a half of good growing weather that will give you some time for at least a partial recovery.

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