Eriosoma lanuginosum is an aphid which causes the formation of a leaf gall on Elm which takes the form of a bladder looking like a pale green egg or pear which may be 6 or even 8 cm long and nearly as wide. The gall is more often seen than the adult aphid.

Gall  Adult

state the host plant, and provide a photo

On elms, usually small leaved elms

Look for galls in June and July although they may persist over winter in a hardened form.

Aphids feed inside the gall protected from predators.

Uncommon in Britain.

Uncommon in Leicestershire and Rutland.

https://influentialpoints.com/Gallery/Eriosoma_lanuginosum_elm_balloon-gall_aphid.htm