Eudonia pachyerga is a moth in the Crambidae family. It was described by Meyrick in 1927. It is found in New Zealand. The wingspan is about 24 mm. The forewings are brown, mixed with blackish-fuscous. The first and second lines are whitish, edged with a blackish shade posteriorly. The hindwings are pale brassy-greyish. Adults have been recorded on wing in January.