Nematus septentrionalis (formerly Craesus septentrionalis)
One of four Nematus species in Britain and Ireland characterised by the enlarged basitarsal segment. This species is dulled in the centre of the upper part of the mesopleura by coarse, rugged microsculpture so that the punctures are scarcely discernible. Can be difficult to separate this species from Nematus latipes. In septentrionalis the hind ocelli are closer together than the distance to the hind margin of the head (1:1.2). In latipes the hind ocelli are as far apart as the distance from them to the hind margin of the head (1:1). Note that in Ireland there is a dark race, Nematus septentrionalis var. stephensii, with an entirely dark abdomen and entirely dark hind femora.
Nematus septentrionalis larvae feed gregariously on various trees and shrubs but mainly alders and birches. Eonymphs overwinter in a cocoon in soil, pupating in spring.
Size: 8 to 10mm
Distribution: England, Scotland, Wales, Ireland
Flight period: Bivoltine. May to June and July to September
Plant associations: Acer spp. (maples), Carpinus spp. (hornbeams), Corylus spp. (hazels), Fraxinus spp. (ashes), Populus spp. (poplars), Salix spp. (willows), Sorbus spp. (rowans), Alnus spp. (alders), and Betula spp. (birches)
Benson, R.B., 1952. Handbooks for the Identification of British Insects. Hymenoptera, Symphyta, Vol 6, Section 2(a-c), Royal Entomological Society, London
Liston A, Knight G, Sheppard D, Broad G, Livermore L (2014) Checklist of British and Irish Hymenoptera - Sawflies, ‘Symphyta’. Biodiversity Data Journal 2: e1168. https://doi.org/10.3897/BDJ.2.e1168
Perkins, R. C. L. 1929: Notes on the black Croesus from Ireland. - The Entomologist's Monthly Magazine, Third Series, London 65(11): 18-19