That is almost always an indicator of sod webworms. Even if you’re not seeing moths, you really should examine sickly looking yards for tiny worms or caterpillars.
You could also be experiencing cutworms, especially if you’re seeing moths in the evening hours. Sod webworm moths are usually seen only in the early morning hours.
These critters are night-feeding caterpillars, even though the moths are seen at different times. They feed around a small burrow or tunnel in the grass, and they carry leaf blades down into it. Silken threads can be seen in the early morning covering the tunnel of a sod webworm. The cutworm is often found on golf greens after the grass is aerated. The aeration holes provide an ideal habitat for the larvae during the day, and they feed around the hole at night.
Adult sod webworms are small, white-to-gray moths with a snout-like projection on the front of their heads. While resting, the wings of the moth are closely folded about the body. The females scatter eggs at random early in the morning as they fly over the grass. Apparently, the moths are attracted to dark-green, healthy turf. The eggs hatch in 7-10 days, and the larvae begin feeding on grass leaves. As they mature, the larvae build silk-lined tunnels through the thatch layer and into the soil. The slender larvae reach ¾ inch in length and are light brown with several rows of dark spots along the entire length of the body.
The first signs of sod webworm damage are areas of unevenly clipped grass and patches of brown or closely clipped grass. The larvae remain active for several weeks, and then pupate. Adults appear about a week later. Their life cycle is completed in 5-6 weeks with several generations per year.
Cutworm moths, on the other hand, are grayish-brown to black and have a wingspread of 1-2 inches. They’re usually only active at night. Females lay their eggs singly or in groups of 2 or 3 on the leaves and stems of grass. Their eggs also hatch in 7-10 days and the larvae begin feeding. Cutworm larvae are grayish-black, smooth “worms” that curl into a ball or “C” shape when disturbed. The cutworms grow to a length of 1-2 inches, are usually plump (in contrast to slender sod webworms) and may be spotted or striped. Several generations of cutworms are produced annually. And while they most often appear in early spring, if the weather is just right they will appear in the fall.
Another problem, armyworms, are appropriately named because they can be seen moving across turf in large numbers. In contrast to sod webworms and cutworms, armyworms feed day and night and leave the grass with a white skeletonized appearance.
Sod webworms and cutworms are both readily controlled by most liquid insecticides approved for turfgrass — bifenthrin, malathion or any of the synthetic pyrethroids or carbamates out there. However, these are short-residual materials, and repeat applications are required to control the next generation. Just as in controlling chinch bugs, three applications of liquid insecticide spread over two weeks usually does a great job of breaking the egg cycle.
If you must stay organic and can confirm the issue is cutworms or sod webworms, spray liquid BT (Bacillus thuringiensis) products. These organic worm and caterpillar controls are not an altogether new type of treatment, and you will find BT in ready-to-spray bottles that just hook on the end of a hose. Years ago you would probably only find BT liquids in concentrates and trigger-spray bottles.
I should caution you that brown patch can also rear its ugly head thanks to August rains. Many people seem to be somewhat successfully treating the worm problem, but still see growing patches of unhealthy grass. Could it be brown patch? If you think that’s possible, check the brown patch tip sheet which covers both sod webworms and fungal diseases.