A cryptic species, not recognised as occurring in Britain until 2016 when a specimen from 1981 was redetermined as this species, newly-separated from Tenthredo colon (Liston 2016). Although only six records are known to date, these are distributed widely from Hampshire to Northumberland, with no obvious habitat features linking the known sites. It feels likely that the species may be widespead at low density but overlooked. More records may come to light if older specimens of T. colon are re-examined.
One of five Tenthredo species with a band of white antennal segments: colon, fagi, ferruginea, livida, semicolon. In colon and semicolon the white is confined to at most segments six to eight. The stigma of the forewing is unicolorous brown and the costa yellowish-brown. The scutellum is black and never white marked. The metapleuron and lateral face of the first tergite are each white marked. The head above is mostly impunctate and shining black.
Tenthredo semicolon can be distinguished from colon by having less sculpture on the mesopleura and mesosternum giving semicolon a more shiny appearance. The hind femur is only black at the extreme apex, whereas in colon it is black for at least the apical third apex. In females, the basal sawteeth are diagnostic being pointed and strongly produced in semicolon and flattened in colon.
Larvae are unknown.
Size: 10 - 12mm
GB IUCN Status: Data Deficient
GB Rarity Status: Unclear
Flight period: May to August
Plant associations: Unknown
Benson, R.B., 1952. Handbooks for the Identification of British Insects. Hymenoptera, Symphyta, Vol 6, Section 2(a-c), Royal Entomological Society, London
Fekete, K. (2018) Beginner’s guide to identifying British Tenthredo, Natural History Museum, London. http://www.nhm.ac.uk/content/dam/nhmwww/take-part/identification-trainers/sawflies-guide-id-trainees.pdf [Accessed 26Apr2019]
Liston, A. D. 2016: Empria camtschatica Forsius and Tenthredo semicolon Mol (Hymenoptera: Tenthredinidae) in the British Isles. - The Entomologist's Monthly Magazine, 152: 131-135.
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
Musgrove, A.J. 2023. A review of the status of sawflies of Great Britain - Phase 2: The Athaliidae and the Tenthredinidae (excluding Nematinae). Natural England, unpublished
Taeger, A., 2013, The type specimens of Tenthredo Linnaeus, 1758 (Hymenoptera: Tenthredinidae) deposited in the Hungarian Natural History Museum, Zootaxa, 3626: 201–244.