LSU AgCenter Horticulturist
(09/18/20) 2020 has definitely been one for the history books. COVID-19, murder hornets, Hurricane Laura and now Hurricane Sally hitting our Gulf neighbors to the east. This week is no exception for our losses. LSU AgCenter offices across the state are getting call after call as lawn after lawn is experiencing devastating loss of turfgrass.
Sod webworms are the main culprit this year, and fittingly, our weed and turfgrass extension specialist Ron Strahan said, “The numbers are biblical. We have observed nearly every house on a single street with damage in the lawn.” Cue the locusts.
You might first take notice of the small moths that are light brown to dark brown with striping on the wings. You will see them flying around as you walk through your lawn or around outdoor lights at night. This is the adult of the sod webworm. The moths lay eggs on grass blades.
Larvae hatch a week or so later, maturing into adult moths in three to five weeks. There can be two or more generations each year. Larvae are amber in color but become greener as they feed on the blades of grass at nighttime. They are the cause of the damage we see in the lawn.
If you missed the moths but are seeing yellowing and browning patches of dead lawn, you need to inspect further to determine if it is sod webworms. One way to tell is to investigate individual grass blades. They will have a chewed appearance, with pieces of blades missing or chunks bitten out. The caterpillars are making a feast of your lawn.
Another clue to look for while you are down there at ground level is the worm castings. That’s a fancy term for caterpillar poop. The castings are digested grass, and they appear as light beige pellets at the base of the plants just above the soil level.
Yet another sign are the webs that are visible in the early morning when the dew is still on the ground. The water droplets from the dew will be trapped in the webbing, and this is where sod webworms get their name.
If you dig thoroughly enough at the soil-grass interface, you can usually find a tiny caterpillar about 1/2 to 1 inch long and the diameter of Q-tip and the castings of the insect. Sod webworms seem to especially love St. Augustine grass.
Heavy infestations can cause a feeding frenzy of birds to the lawn. If you’ve noticed more birds pecking around in your turfgrass, it’s usually an indicator that some good food — nutritious sod webworm caterpillars — is out there.
Heavy infestations can lead to stress, causing your lawn to be more susceptible to fungal diseases such as gray leaf spot and large patch in addition to other insects such as chinch bugs and armyworms. A combination of these problems can lead to the death of turfgrass.
To help control sod webworms, use an insecticide with the active ingredient bifenthrin. AgCenter entomologist Sebe Brown recommends liquid rather than granular applications for better control. You will need to retreat the lawn again in seven days to kill any newly hatched eggs. Spray will not control the moths. It is most effective on the main culprit doing the damage — the caterpillar.
Treat the infested areas and extend 3 to 4 feet past where you see browning. Moths will continue to lay eggs, so continue to monitor the lawn. Eggs hatch every seven days. The hopefully cooler weather of fall will slow down the generation interval but not kill the worms already in the lawn. Last year’s mild and short winter is likely the cause of the large populations we’ve seen this summer.
If you are like me, you don’t like to use chemicals because insecticides don’t discriminate for the most part and will kill other beneficial bugs. You can let Mother Nature help you out with the birds who feast on the caterpillars instead.
Additionally, consider transitioning large areas of lawn to garden beds that you fill with native plants or other ornamental perennials, shrubs and trees. Lawns, mainly turfgrass, account for 30% of water use in a typical U.S. home, so go easy on the turf in home landscapes. With water scarcity on the horizon, by reducing the size of your lawn you can reduce water consumption, water bills and the amount of time you spending mowing your grass; save fuel; lessen your carbon footprint; and expand natural ecosystems by adding diversity to your landscape.
The good news, if there is any, is that the grass will recover from live stolons and roots. Water your lawn during extended periods of drought that are especially common in October here in Louisiana to help the grass recover before going into winter dormancy. It’s also time to stop fertilizing the lawn and wait until spring to apply any new applications.
Sod webworms are tiny, amber green caterpillars. Photo by Heather Kirk-Ballard/LSU AgCenter
Sod webworm damage appears as yellowing to brown patches in the lawn. Photo by Heather Kirk-Ballard/LSU AgCenter
Upon closer inspection of grass blades, you will notice the chew marks as evidence of sod webworms. Photo by Heather Kirk-Ballard/LSU AgCenter
Sod webworm castings are tiny, tan and sand-like. Photo by Heather Kirk-Ballard/LSU AgCenter
LSU AgCenter turf specialist Ron Strahan and research associate David Sexton scout for sod webworms at a home visit in Baton Rouge. Photo by Heather Kirk-Ballard/LSU AgCenter