We received a report that some ryegrass in Blount County (Alabama) was infested with armyworms. From the picture sent to us and the time of year, I suspect these are not fall armyworms. I believe they are a different caterpillar pest, the armyworm, Mythimna unipuncta. This pest is sometimes called true armyworm to distinguish it from other armyworms.
The armyworm occurs in the spring, and is more of a problem during cool, wet weather. It can infest various grasses, including perennial ryegrass, wheat, corn and tall fescue.
The economic threshold for this armyworm in grasses and wheat is around four works with a length of a half-inch to three-quarters of an inch per square foot.
When fully grown, the caterpillar is about 1.5 inches long. By the time caterpillars are 1.25 inches or larger, most of the damage has been done. In forage grasses, scouting and control recommendations are the same as for fall armyworm.