Fenusa pumila

Adults: Small black sawflies 3.7 mm long. All black except for small amounts of white on each femur, tibia and tarsus. Wings with light brown banding and darker near the body. Antennae shorter than thorax (longer in Fenusa dohrnii).

The larvae mine the leaves of Birch producing a blotch on the upper side of the leaf. It starts near the midrib and extends outwards towards the leaf edge.

The larvae have four black spots underneath - the anterior spots (those near the head on the underside) being large (as shown) which distinguishes it from Profenusa thomsoni.  The feet are relatively long and slender. In the final pre-pupal instar, pigmentation is lost

Several other sawflies mine birch leaves.  

Adult Leafmine (when host plant is identified)

Leaf mine photographs should be backlit so that the frass patterns and larva can be seen.  The host plant must be identified in the comments

Where Birch is present.

The larval mines can be seen when Birch is in leaf.

Status in Britain is difficult to determine due to lack of records.

Probably quite frequent but under recorded in Leicestershire and Rutland.  The Fosse Meadows record of 3rd October 2018 is thought to be the first for VC55.

http://www.leafmines.co.uk/html/Hymenoptera/F.pumila.htm

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/hymenoptera/symphyta/tenthredinoidea/tenthredinidae/heterarthrinae/fenusa/fenusa-pumila/#:~:text=In%20Europe%20F.,1973a%3B%20Smith%2C%201987a).

https://www.sawflies.org.uk/fenusa-pumila/