A small moth with a wingspan of 5.4 to 6.2 mm. Adults difficult to distinguish from other Stigmella species, and more frequently recorded in the larval stage where it creates a leaf mine on the leaves of Aspen (Populus tremula) and occasionally Grey Poplar or White Poplar. The initial corridor is contorted, then widening into a blotch-like corridor; central frass line leaving a clear zone either side. 

Adult Leafmine

Photograph the mine with backlight; note host species in comments

Where the larval foodplants Aspen, Grey Poplar or White Poplar occur.

The larval mine is most cmmonly found in September and October. Adult flght period is June and July.

Larva mines leaves of Aspen, Grey Poplar or White Poplar, over-wintering as a pupa.

Local in wooded areas containing Aspen, Grey Poplar or White Poplar, mainly in south-eastern England and in the Scottish Highlands. In the Butterfly Conservation's Microlepidoptera Report 2011 this species was classified as local.

It appears to be uncommon in Leicestershire and Rutland, where there were only 5 records to autumn 2017. L&R Moth Group status = D (rare or rarely recorded).

http://www.leafmines.co.uk/html/Lepidoptera/S.assimillella.htm

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/lepidoptera/monotrysia/nepticuloidea/nepticulidae/stigmella/stigmella-assimilella/