On Viburnum dentatum. Aphis, 5/13/12 - Aphis viburniphila
Willow Grove, Montgomery County, Pennsylvania, USA May 13, 2012
Close up of adult and nymph.
I hope that we can find out whether it is Ceruraphis viburnicola mentioned by EoL.
Another possibility is Aphis viburniphila.
Update: iNaturalist ID it as Aphis viburniphila Higher resolution image