The body length of Cavariella theobaldi apterae is 1.8 to 2.8 mm. For further details please follow the ‘Further info:’ link below

Unless identified by a recognised expert, a photo is required. If the photo doesn't show the key ID features, then in the comments box describe the size and identifying characters you have observed. Please provide a note of the food plant. 

Wherever the host species* occur (*see under ‘Life History:’ below)

Aphids may be found whenever the host species are in leaf.

Cavariella theobaldi host alternates between Willows (Salix species) and some umbellifers mainly Wild Parsnip (Pastinaca) and Hogweed (Heracleum).

The species appears to be widespread in Britain, but true status is difficult to determine due to lack of records.

Status in Leicestershire and Rutland not known.

https://influentialpoints.com/Gallery/Cavariella_theobaldi_Willow-parsnip_aphid.htm