Fenusella nana

The larvae of the sawfly Fenusella nana mine the leaves of various Birch (Betula) species.

The mine starts at the tip of a vein at the leaf edge and a triangle or plug of frass is left at the start of the mine which takes the form of a large pale blotch.

The larva is initially greenish, with dark spots underneath the thorax and a dark patch on the prothorax at the back of the head. It has long slender feet with dark marks at the base. It becomes a darker yellowish colour towards maturity. 

Several other sawflies mine the leaves of birch

Adult Leafmine (when host plant is identified)

Adult: Unless identified by a recognised expert, a photo is required and the specimen should be examined with a microscope. In the comments box, state the key or ID method used and describe the size and identifying characters.  

Leafmine: Unless identified by a recognised expert, a photo is required. If the photo doesn't show the key ID features then in the comments box describe the size and identifying characters you have observed. The larva should be checked and photographed if possible. 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.

Mines may be found when Birch is in leaf.

Pupation occurs outside the mine.

The species seems to be widespread in England and Wales but is not well recorded.

Status in Leicestershire and Rutland not known.

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

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/hymenoptera/symphyta/tenthredinoidea/tenthredinidae/heterarthrinae/fenusella/fenusella-nana/

https://www.sawflies.org.uk/fenusella-nana/