One of two 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.
Size: 6 - 9mm.
Flight period: May to June
Plant associations: Quercus spp. (oaks) (1)
(1.) Benson, R.B., 1952. Handbooks for the Identification of British Insects. Hymenoptera, Symphyta, Vol 6, Section 2(a-c), Royal Entomological Society, London
(2.) 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