Cutworms are one of the main problematic white grubs for lawns in North America, especially in the Pacific Northwest. They infest the thatch layer directly on top of the soil and consume any grass they can get to from there. A cutworm problem can grow in severity until the grass begins to die, from the top of the blade down to the bottom.
Cutworms or white grub larvae are moth larvae, and they are only about one or two inches long. Cutworms are typically a shade of brown or tan and may have some striping along their middles. The larvae spend the summer feasting on the grass. In the fall, they become dark-colored moths, as opposed to white grubs that are actually beetle larvae. Cutworms are almost completely nocturnal and will eat the grass during the night hours rather than in the daytime. This, along with their small size and dirt-like color, can make them hard to spot in the lawn.
While cutworms can do a significant amount of damage to a lawn, they are not particularly hardy or hard to kill. Almost any contact herbicide will work to get rid of them. Just spray it directly onto the grass blades. Then, when the cutworms come out to eat the grass, they will ingest the pesticide too, which will kill them. For the most effective pest treatment, apply the herbicide just before the sun completely sets. The cutworms will be out soon after to start eating.
It may take more than one application to rid the lawn of cutworms. Once the cutworms are gone, the grass usually will go back to being green and healthy on its own. But, if there was greater damage, then reseeding will probably be needed. Keeping the lawn in good condition and in good health will decrease the likelihood of a repeat cutworm infestation.