The larvae mine the leaves of various species of Alder creating an upper surface gallery, narrow at first but widening considerably towards the end.  Frass is in two rows.

See the National Agromyzidae Recording Scheme website for further details of the mine, larva and adult insect.

ref.: Warrington, B.P. (2023).  Agromyza alnivora. Accessed at https://agromyzidae.co.uk/species/agromyzidae/agromyzinae/agromyza/agromyza-alnivora-spencer-1969/ on 13/12/2023)

Stigmella species of micro-moth make similar gallery mines, but with a single row of frass, or dispersed frass.

Leaf-mine Adult  

Provide backlit photographs of the whole leaf-mine in the leaf, and the larva, and note the host plant. 

Where Alder, the larval foodplant is present.

Click on the 'further details' link in the 'Description' section above.  Then click on the Phenology tab which will provide phenology details for both adult and larval stages.

Common

Warrington, B. P. 2022. A provisional review of the status of Agromyzidae in Great Britain & Northern Ireland

Fairly frequent in Leicestershire and Rutland.

http://www.leafmines.co.uk/html/Diptera/A.alnivora.htm

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/diptera/brachycera/agromyzidae/agromyzinae/agromyza/agromyza-alnivora/