Janus cynosbati - Oak Stem-sawfly

Benson (1951) said this species was found south-east of the Wash/Severn line although there have been several more recent records in northern England, plus single records in Scotland and Wales. The majority of records remain in the south, however, including a fair number in Devon and Cornwall (Musgrove, 2022). One of three species in Britain. The tegulae are yellow and the pronotum is not edged with white behind. In Janus luteipes, the pronotum is edged with white and the tegulae are black or brown.

Janus cynosbati larvae feed inside the twigs of oaks. Females lay a single egg on the end of live green oak twigs and the larvae burrow inside. As the larva grows the stem slightly widens along a length of about 3cm. Any growth after the chamber wilts and dies.

Jump to other Cephidae

Size: 6 - 9mm.

GB IUCN Status: Least Concern
GB Rarity Status: Nationally Scarce

Distribution: England, Scotland, Wales

Flight period: May to June

Plant associations: Quercus spp. (oaks).


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

Musgrove, A.J. 2022. A review of the status of sawflies of Great Britain - Phase 1: families other than Tenthredinidae. Natural England, unpublished