While coppicing willows at the Wildlife Trust’s Pavenham Osier Beds reserve, SP990552, on Tuesday (19/11/13) we encountered large quantities of large aphids on the stems.
I’ve just keyed these out as Tuberolachnus salignus.
There is a nice write-up about this species at http://www.nhm.ac.uk/nature-online/life/insects-spiders/common-bugs/aphid-watch/index.html which ends with “This large aphid is very distinctive and there is nothing else like it, so if it doesn't have a large thorn-like process on its back then it is not T. salignus”. These do have the large spinal tubercle, so the ID seems OK?
If you read that article and also this one http://simonleather.wordpress.com/tag/tuberolachnus-salignus there seems to be a mystery as to where these aphids go for a third of the year. And, no-one has ever found a male of the species apparently, or found out what the thorn is for...
It established itself at Rothamsted about 10 years ago according to this article: http://www.rothamsted.ac.uk/news/worlds-largest-aphid-species-found-rothamsted-research