Sod Webworms Create Large Brown Spots In Lawn

Patricia M. Arledge, Sharpe, Kenneth W.  |  9/14/2016 3:12:30 PM

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News Article for September 9, 2016:

My most frequent call this week by far relates to large brown spots in the lawn with very little grass left.

I have talked to lots of people with this problem, seen lots of the damage and I have the problem myself.

Sod webworms are the culprit. They are caterpillars that like to feed on grass blades. One of the unique features about them that make it hard for people to identify the cause is that sod webworms feed at night. When you see the brown spots in the daylight hours you do not see any worms feeding.

Some people are reluctant to believe that this is caterpillar damage because they are not seeing the worms. Look for chewed foliage and if you have missing foliage or chewed leaf blades, do not overlook the obvious.

I have seen some of the worms down in the thatch layer of the grass during the day. They bury themselves under the thatch to avoid the sunlight and heat. The mature worms that I have seen are approximately 1 inch long and emerald green. They are little smaller than the diameter of a pencil.

While down on their knees looking for caterpillars a number of callers report that the soil looks like earthworms have been working. What they are seeing is the casting of the caterpillars, the digested grass blades that have passed through the sod webworms.

In the early morning with the dew on the grass, you will see the webbings that are left by sod webworms. They will spin a web trail as they feed and you can see the webs when they have water droplets attached.

Moths are another way to identify that you have sod webworm potential. I turned the outside porch light on just after dark and went back in about 30 minutes to find hundreds of moths swarming around the night lights. Sod webworm moths are light brown to gray colored, they are ½ to ¾ inches long, have a wing span of about 1½ inches and have a snout like projection on their heads.

A heavy infestation can kill grass. We had a huge infestation in 2010 and many people who did not treat were left without lawn grasses.

I would spray with an insecticide such as Sevin, Ortho Bug B Gon, Talstar, Spinosad or a pyrethrin to control sod webworms. I would spray the brown spots plus a buffer of 3-4 feet past the brown area. Continue to watch for more outbreaks as the moths continue to lay eggs, as many as 60 per day, and those eggs will hatch and begin feeding in about 7 days in hot weather. Moths do not feed on grass and will not be controlled by the spray.

My observation is that sod webworms have a preference for carpet grass and then St. Augustine. They will eat other grasses such centipede and Bermuda also.

Most of those grasses will have live stolons and roots and if we continue to get rain the grasses will recover. If we do not continue to get rain you might want to water those damaged areas but do not apply nitrogen fertilizers until next spring.

For more information on these or related topics contact Kenny at 225-686-3020 or visit our website at www.lsuagcenter.com/livingston.
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