Adults: Small black sawflies. Females only 4.5 mm long. Black thorax with yellow pronotum and tegula, white legs but with coxa and basal half of femur black. Forewing with slight brown band.

Leaf-mine:  The older part of the mine is a reddish colour and most of the frass is ejected through a slit. The larva pupates in the mine in a disc-shaped cocoon.

Larva:  Brown or blackish marks on the prothorax, and some brown spots on the ventral side of the thorax, and short stubby legs. As with other sawfly miners, the pre-pupal stage has no pigmentation.

Several other species of sawfly mine birch leaves

Adult records should be accompanied by a good photo.

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

Around Birch and other deciduous trees.

Larvae: Creamy white with distinct brown head capsule about 8 mm long at the last feeding stage. Food plants Birch, but also Sycamore and Field Maple.

Uncommon

Uncommon in Leicestershire and Rutland.

http://www.leafmines.co.uk/html/Hymenoptera/H.nemoratus.htm

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

https://www.sawflies.org.uk/heterarthrus-nemoratus/