According to Prous et al, 2019, there is currently no reliable key or species treatment available for this genus. The description follows Benson.
The body is mainly yellow with the head, upper surface of antennae, middle of the pronotum, the mesonotum and the metanotum black. The wings are yellowish basally. The stigma is brown with darker margins, the costa yellow and the rest of the venation brown to piceous.
Larvae of Cladius grandis feed gregariously on poplars and willows. Eggs are laid into cuts on leaf petioles. Larvae initially feed on the underside of the leaf skeletonizing it before beginning to feed from the edge of the leaf. Cocooning is done in crevices or in the pith of plant stems.
Size: 7 - 9mm
Distribution: England, Scotland, Wales, Ireland
Flight period: Bivoltine, May to September
Plant associations: Populus spp. and Salix spp. (poplars and willows)
Benson, R.B., 1952. Handbooks for the Identification of British Insects. Hymenoptera, Symphyta, Vol 6, Section 2(a-c), Royal Entomological Society, London
Downes, W. 1927: The Poplar Sawfly (Trichiocampus viminalis (Fallén)). - Proceedings of the Entomological Society of British Columbia, N. S., Victoria 22: 26-32
Liston, A. D. 1996: Notes. (Praia taczanowskii, Trichiocampus viminalis, Cephus cinctus). - Sawfly News. - Chalastos, Daibersdorf., Daibersdorf 1(1): 8-9
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.
Sakagami, S. F. & Tanno, K. 1979: Partial Appearance of Nonhibernating Adults and Egg Complements in the Poplar Sawfly Trichiocampus populi (Hymenoptera, Tenthredinidae). - The Institute of Low Temperature Science, Ser. B - Sapporo., Sapporo 37: 73-80
Shimada, K. 1989: Ice-nucleating activity in the alimentary canal of the freezing-tolerant prepupae of Trichiocampus populi (Hymenoptera: Tenthredinidae). - Journal of Insect Physiology, Oxford, London 35(2): 113-120
Tanno, K. 1970: Frost injury and resistance in the poplar sawfly, Trichiocampus populi Okamoto. - Contrib. Inst. low Temp. Sci, Hokkaido Univ. (Biol. ) 16: 1-41.