The adult moth is yellowish brown with a chocolate-colored stripe down the length of each forewing. The larvae vary in coloration from greenish to cream-colored, depending on the maturity of the grain they have consumed, but all have longitudinal white and brown lines down each side of the body. Larvae feed on the wheat heads from evening to early morning, typically hanging onto the awns upside down and hollowing out kernels, and resting in the soil at the base of the plant during the day. Damaged kernels appear hollowed out and resemble those damaged by stored grain pests.

The insect passes the winter as a pupa in the soil, emerging as an adult in the spring to lay eggs on a wide variety of grasses, although wheat is highly preferred. Unlike the true armyworm, there is more than one generation per year, but it is the first generation of larvae that feeds on maturing wheat heads and causes direct damage to kernels. Second generation moths emerge over an extended period in summer months and lay eggs on warm season grasses. Fall flights of F. diffusa can be observed well into October and it is possible that some of these represent a third generation, late-developing second generation individuals, or a combination of both. Larvae that complete feeding on maturing warm season grasses in the fall pupate in the ground but remain dormant until the following spring.

There are no established management plans for this pest. Infestations in the field are usually too sporadic and isolated to justify any type of scouting or treatment program.

Infestations are usually concentrated around field margins, so observations along field edges may overestimate population levels. In addition, no economic threshold has been determined and there are no materials specifically labeled for this pest, although materials registered for other armyworms in wheat would likely provide control if applied sufficiently early. However treatment is usually impractical because of the pre-harvest interval requirement of most insecticides, which would result in even greater losses due to delayed harvest. Larvae arriving in storage bins with harvested wheat either die or emerge as moths, but are not a concern in stored grain.