From their home in the thatch of your lawn, sod webworms can quickly cause a fair amount of unsightly damage. The brown patches they create are the result of larvae feeding on the lower part of the grass.
While there are several species of insects known as sod webworms or lawn moths, they all respond to the same treatment. In most cases, natural control methods are all you need.
Inspect Your Lawn
Sod webworms leave irregular patches of brown grass cut off close to the ground. The patches start out small and grow to the size of a baseball, but with extensive damage, they can merge together to form large areas of dead grass. You might also find small holes in these patches caused by birds hunting for webworm larvae.
The damage can appear anytime from May through September or even well into autumn because several generations of these worms are born every summer.
The work of sod webworms resembles grub damage, but with one important difference. Grass damaged by grubs can be easily peeled back like a carpet to reveal white grubs, whereas grass damaged by sod webworms can’t be pulled up this way.
Brown patches caused by drought, fertilizer burn, and dog urine are also easily mistaken for sod webworm damage. This is why it’s important to inspect for sod webworm larvae before you treat your lawn for them.
Closely inspect the surface of your turf for silken tunnels and tiny green pellets (worm feces). If you’re still not sure, you can force the worms to the surface using soapy water.
Mix 2 or 3 tablespoons of dishwashing liquid into 2 gallons of water. Sprinkle this over 9 square feet of the affected part of the lawn and wait 10 minutes. The sod webworm larvae will crawl to the surface. These are 3/4-inch brownish worms with brown or green spots.
If your lawn has webworms, you might also see pale tan moths flitting low over the grass on warm summer evenings, especially near lights. These are the adult webworms.
Start with Natural Solutions
Sod webworms live in your lawn’s thatch, the layer of dead grass clippings, leaves, and other organic material that builds up on the soil’s surface. If your thatch is thicker than 1/2 inch, removing it will reduce the webworm population.
You can dethatch your lawn with a dethatcher rake or an electric dethatcher. Doing this every year at the end of the growing season helps discourage sodweb worms.
Next, choose the biological pest control option that’s right for your situation. If you haven’t applied any pesticides to your lawn for the last 60 days, apply Steinernema carpocapsae nematodes. These are small worms that feed on sod webworms, but won’t harm beneficial insects. They’re most effective against newly formed caterpillars, so apply them as soon as you’ve confirmed your lawn has sod webworms.
They’re temperature sensitive and should be applied in the evening when it’s cooler. Don’t water the lawn for two days after you apply the nematodes, but after this period, keep the lawn well watered for the next two weeks. With correct application, the nematodes can form a healthy colony that will defend your lawn for years.
If you’ve used pesticides within the last two months, nematodes are unlikely to survive. In this case, the Bacillus thuringiensis variety kurstaki (Bt-k) is a better choice. This bacteria works best against sodweb worm larvae, so apply it when the webworms in your lawn are in this stage. Even if you apply the bacteria at the optimal time, you might need to reapply it weekly until all the webworms are gone.
While you can apply it any time of day, the evening is usually best unless the directions on the product you choose suggest otherwise. As with nematodes, don’t water the lawn for two days, but water well for the next two weeks after this.
Pesticides are a last resort when biological controls don’t work. Avoid broad-spectrum pesticides, which kill beneficial insects that could help fight the sod webworm infestation. Instead, look for a product designed specifically for controlling sod webworms. A product containing Spinosad or other natural ingredients is usually the safest bet.
Keep Your Lawn Strong
The healthier your lawn is, the better it can tolerate sod webworms and other pests without long-term damage. Sufficient watering is important for deterring sod webworms, but most lawns do fine with 1 inch of water per week. In moderate climates, rainfall is usually all the lawn needs. If you use sprinklers, water only when your lawn begins to wilt slightly. Water during cooler times of the day to minimize stress on the grass.
Apply the right type of fertilizer for your lawn’s age and the level of heat or cold stress it’s under. Dethatch the lawn annually and aerate in the spring, autumn or both.
If you’re planning to re-plant your lawn, consider endophyte-enhanced fescue or ryegrass, which are enhanced with a type of fungi that helps them stand up to a variety of damaging insects. The ‘Windsor’ and ‘Park’ cultivars of Kentucky bluegrass are also somewhat resistant to sod webworms.
Because sod webworms can cause extensive damage to your lawn, they’re not something you can afford to ignore. If natural approaches to getting rid of these worms don’t work or you have webworm problems year after year, talk with a lawn care specialist. A professional can advise you on treating your lawn for sod webworms without harming the grass or the beneficial insects living there.