Adult apterae of Pterocallis maculata are yellowish-green or green, with a pattern of diffuse dark green dorsal cross bands, giving an overall blotchy appearance. The body length of the aptera is 1.4 to 2.1 mm.  The P. maculata alate has a pale green abdomen and dark-tipped siphunculi.

It usually lives in sizeable colonies, often grouped around the mid-rib, and is nearly always attended by ants.

Pterocallis alni is also found on Alder but and they live dispersed on the leaves, not in big colonies like P. maculata. Also P. alni has few or no dark green markings whereas P. maculata has many. P. alni is hardly ever ant attended, whilst P. maculata is nearly always ant attended.

Good close-up photographs required.  Please state the host plant.

Found on Alder.

Summer to Autumn. Apterous males and oviparae occur in September-October.

Ant attended.

Seems to be rather local in Britain and is uncommon in some areas.

Status in Leicestershire and Rutland not known.

https://influentialpoints.com/Gallery/Pterocallis_maculata_green-barred_alder_aphid.htm