The adult female is about 7 mm (0.3 in) long and mainly black, with the exception of the first two segments of the thorax, which are red, and the underside of the tips of the antennae, the inner side of the fore-tibia and the basal part of the hind femur, which are whitish.

The head has numerous large puncture marks while the thorax has scattered smaller ones; the abdomen has faint transverse sculpturings.

Adult males are unknown in North America, and rare in Europe, the females breeding by parthenogenesis.

[3][4] The alder sawfly is native to Europe, but has been accidentally introduced into North America, both the Atlantic and Pacific coasts.

[3] The adult female inserts her ovipositor into the upper surface of a leaf near the midrib to deposit her eggs.