The adult is 2.7 to 3.6 mm. The larva of Orchestes testaceus mines the leaves of Alder species. Initially a gallery is formed which runs towards the leaf edge from a vein or midrib. The gallery widens quickly and then becomes a blotch - a spherical cocoon develops in this. The mine is brown.

Adult Leafmine

Adult: Unless identified by a recognised expert, photographic evidence is required and the specimen should be examined with a microscope. In the comments box, state the key or ID method used. Note the beetle's size and describe the identifying characters. It is advisable to retain the specimen in case further checks are needed.

Leafmine: Unless identified by a recognised expert, good photos are required that show the key ID features.

On Alder.

Early summer.

Uncommon in Britain and a UKBAP species.

Rare in Leicestershire and Rutland.

http://www.leafmines.co.uk/html/Coleoptera/o.testaceus.htm

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/coleoptera/polyphaga/cucujiformia/curculionoidea/curculionidae/curculioninae/rhamphinini/orchestes/orchestes-testaceus/