Acronicta rumicis, the knot grass moth, is a species of moth which is part of the genus Acronicta and family Noctuidae. It was first described by Carl Linnaeus in his 1758 10th edition of Systema Naturae. It is found in the Palearctic region. A. rumicis lives and feeds on plants located in wide-open areas. At its larval stage, as a caterpillar, it causes such a large impact as a crop pest that it has received much attention and research. A. rumicis feeds on maize, strawberries and other herbaceous plants.

Acronicta is a genus of noctuid moths containing about 150 species distributed mainly in the temperate Holarctic, with some in adjacent subtropical regions. The genus was erected by Carl Linnaeus in his 1758 10th edition of Systema Naturae. Caterpillars of most Acronicta species are unmistakable, with brightly colored hairy spikes, and often feed quite visibly on common foliate trees. The hairy spikes may contain poison, which cause itchy, painful, swollen rash in humans on contact. The larva of the smeared dagger moth is unusually hairy even for this genus. Acronicta species are generally known as dagger moths, as most have one or more black dagger-shaped markings on their forewing uppersides. But some species have a conspicuous dark ring marking instead.