According to Prous et al, 2019, there is currently no reliable key or species treatment available for this genus. The description follows Benson.
The head is shiny above without sculpture or hair-warts. A black sawfly with pale tibiae and basal tarsi on all legs. Females of Cladius pectinicornis are difficult to identify. The male antennae have almost equally long projections to the three basal flagellae a shorter projection on the fourth and a minute projection on the fifth. The projection on the first flagella is about as long as the length of the following segment.
Larvae feed gregariously on roses, strawberry and greater burnet.
Size: 5 - 7mm
Distribution: England, Scotland, Ireland
Flight period: Bivoltine, May to September
Plant associations: Rosa spp., Fragaria spp. and Sanguisorba officinalis (roses, strawberries and greater burnet)
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.