Adults: Underwing moths possess characteristic wing patterning that makes them easy to identify (Catocala). The forewings are a mottled grey, white and brown, resembling many other species. However, when the forewings become extended they reveal the distinctive black hindwings with vivid blue/ lilac stripes. Females: Mature female moths are always bigger than males. A Blue Underwing female can have a wingspan as large as 9.5cm. Females can sometimes be seen resting on walls, fences and tree trunks. The caterpillars can vary but generally resemble the patterning seen on the forewings of the adults. These are strongly associated with species within the Populus genus. They can vary anywhere between 4-9cm. Larvae: The caterpillars are greyish-blue and large in size (length: 7- 8cm) with 3 pairs of legs near the front of the body; 4 pairs of prolegs (unsegmented legs) towards the rear. Eggs: Description currently unavailable.

Caterpillars may be seen on the leaves of plants within the Populus genus. Won't cause severe damage to plants.

Crepuscular