Heterarthrus vagans

The larvae of the sawfly Heterarthrus vagans mine the leaves of Alder species.  The white larva has ringed feet and a distinctive black mark near the head followed by 3 spots (but see 'Life History' below).

It creates a large blotch that normally starts at, or near, the leaf margin, where some or much frass is ejected. The mine is brownish and the full grown larva makes a disc-shaped cocoon in which it pupates.

The larva has a dark patch on the back of the head, and dark markings under the head.  There are black dots on the thorax underneath, and the feet are short and stubby.   The pigmentation is lost in the pre-pupal stage in the cocoon

The mines can be very similar to Fenusa dohrnii on Alder, so the larval characteristics should be checked; these can usually be seen in the mine if it is backlit.  Fenusa dohrnii does not pupate inside a discoid cocoon in the leaf. 

Adult Leafmine+larva  

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

Where Alder is present.

Larval mines may be seen whilst Alder is in leaf especially towards autumn.

The species is univoltine with larva being present towards autumn. Late in the year a circular disc is created in the mine and this is where pupation takes place. The  prepupal stage larva looses its darker markings and is a more uniform pale greeny-white colour.

Widespread, but less common than Fenusa dohrnii which also mines Alder species.

Status in Leicestershire and Rutland not known.

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

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

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