Fenusa pusilla agg.

The taxonomy of the sawflies feeding on Ulmus is problematic and unresolved at present. Some authors e.g. Lacourt (2020) consider all the Ulmus feeding leafminers belong to one species - Fenusa pusilla. Fenusa ulmi was recorded in Ireland (2004, Knight) as F. pusilla. In Europe there are at most two species on Elm, F. ulmi and F. altenhoferi and we have decided to treat these as F. pusilla agg. until such time as the taxonomy is resolved. We have provided links to other sites covering this problem.   The species is most likely to be recorded from the leafmine produced by the larva which is a full depth blotch mine on Elm. 

Adult Leafmine (when host plant is identified)

Any records of sawfly mines on Elm should be submitted as Kaliofenusa pusilla agg. But please state whether the mine was found on Wych Elm or one on the small leaved elms such as English Elm.

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 Elm, the larval foodplant is present.

The mines occur from early summer once Elm is in leaf.

Status in Britain is difficult to determine due to the problems of identification.

Status in Leicestershire and Rutland not known.

http://www.leafmines.co.uk/html/Hymenoptera/F.ulmi.htm 

https://www.sawflies.org.uk/kaliofenusa-ulmi/