The larva of the sawfly Heterarthrus ochropoda mine the leaves of Populus species, usually Aspen in Britain, forming a large, deep, upper-surface blotch that starts at the leaf margin, without an accumulation of frass. Around the oviposition site the leaf turns reddish brown. Frass occurs in scattered grains.

The adult sawfly has an all black abdomen.

The larva is white with a brown head, and distinctive markings just beneath the head on the underside. Feet are short and stubby.

Fenusella glaucopsis has dark spots on the thorax underneath and long slender legs; it also mines Aspen leaves, but has not been recorded in VC55.  

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 Aspen is present.

Mines can usually be found in late summer.

Towards the end of the larval period an opening is made in the mine, and both frass and exuvia are ejected. About that time the mined leave is shed, and the larva makes a disk-shaped cocoon within the mine, in which it diapauses until the following spring.

It seems to be uncommon or under recorded in Britain where there are few records.

Rare or under recorded in Leicestershire and Rutland.  The Burbage record of 7th August 2017 is believed to be the first record for this species in VC55.

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

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

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