According to Prous et al, 2019, there is currently no reliable key or species treatment available for this genus and this species is inseparable from Cladius brullei. The description follows Benson.
The head is dulled by fine durface sculpture or hair-warts. The wings are subhyaline and the abdomen black without a brown spot in the middle of the first tergite. The ridge that separates the median facial fovea between the antennae from the frontal basin above is more or less broken through.
The male antennae lacks any protrusions and antennal segment 3 is straight. The penis valves are asymmetrical.
Cladius compressicornis larvae feed on a wide range of shrubs and herbs of the rose family.
Size: 5 - 8mm
Distribution: England, Scotland, Wales, Ireland
Flight period: Bivoltine, May to September
Plant associations: Rosaceae including Craetagus spp., Fragaria spp., Laurus spp., Prunus spp., Pyrus spp. snd sorbus spp. (the rose family including thorns, strawberry, laurels, pear and rowans)
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
Prous, M., Liston, A., Kramp, K., Savina, H., Vårdal, H. and Taeger, A., 2019. The West Palaearctic genera of Nematinae (Hymenoptera, Tenthredinidae). ZooKeys, 875, p.63.