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 compressicornis. The description follows Benson.
The head is dulled by fine durface sculpture or hair-warts. The wings are infuscate and the abdomen black usually with a brown spot in the middle of the first tergite. The median facial fovea between the antennae is separated from the frontal basin above by an unbroken ridge.
Larvae of Cladius brullei feed on brambles especially raspberry and also mountain ash. Also recently found on hawthorn. Early instars are pale green with a black head. Prior to moulting, larvae may temporarily cease feeding becoming off-white as the alimentary canal empties. The final instar has a reddish head with a broad dark patch on the vertex and a dark band between the eyes. The torso is pale beneath and dark olive greenish-black above.
Size: 4.5 - 7mm
Distribution: England, Scotland, Wales, Ireland
Flight period: Bivoltine, May to September (multivoltine in hot years)
Plant associations: Rubus spp., esp. Rubus idaeus, Sorbus aucuparia and Crataegus sp. (brambles, esp. raspberry, mountain ash and hawthorn)
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.