Janus compressus - Constricted Stem-sawfly

Andrew Halstead discovered the first British record in Surrey in 2011 (Halstead 2018), with further Surrey records in 2013 and 2021 by the same recorder. It is uncertain whether or not it is established in Britain. It is a fairly distinctive-looking insect and so it feels unlikely to be widely overlooked. All three records were from parks in close proximity to gardens. The species is known to feed on fruit-trees, suggesting likely importation through the horticultural trade. Its native range includes central and southern Europe (Musgrove, 2022). In both sexes the head is black with some yellow on the mandibles. The thorax is also black marked with yellow on the mesopleuron, mesepimeron and metapleuron. The female abdomen is red to burgundy with a broad, triangular, yellow membranous area in the cente of tergite 1 (red or orange in other members of the genus). The second, ninth and tenth (last) tergites in females are black. The male abdomen is black on the basal tergites and yellow to orange from tergite 4 to the apex. The legs are bright yellow in males, with grey coxae and the hind tarsi are grey to black. In females, the coxae are black and the legs brownish-yellow with the trochanter, the base of the femora and the first third of the tibia being pale or yellow.

Janus compressus larvae feed in the shoots of pears and on occasion apples, medlars, quince and chokeberry. The female makes incisions around the stem shoot causing wilt to occur. Larvae then feed in the wilted shoot from May to September. The shoot becomes blackened. Note that such damage can also be caused by shoot-cutting weevils.

Jump to other Cephidae

Size: Female: 9 - 12mm, male: 6 - 9mm

GB IUCN Status: Not Applicable
GB Rarity Status: Nationally Rare

Distribution: England

Flight period: May to June

Plant associations: Pyrus spp. (pear trees) but also Malus spp. (apple trees), Mespilus spp. (medlar trees), Cydonia spp. (Quinces), Sorbus spp. (Rowans), Aronia spp. (Chokeberries)

The National Biodiversity Network does not currently have a map for this species. (See terms and conditions)

References:

Halstead, A.J., 2018. Two stem-boring sawflies new to Britain, Janus compressus (Hym.: Cephidae) and Cladardis elongatula (Hym.: Tenthredinidae), British Journal of Entomology and Natural History, Vol 31(1), BENHS, Reading.

Lacourt J. Hymenoptera of Europe 2. Sawflies of Europe. NAP Editions; 2020.

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

Validžić, T., Ivezić, M., Raspudić, E., Brmež, M. and Majić, I., 2013: Morfološke osobine kruškine ose srčikarice, Janus compressus Fabricius 1793. (Hymenoptera: Cephidae). Entomologica Croatica 17.