I get these on my small willow, though I haven't particularly noticed them yet this year (haven't looked either). Last year there were a lot of ladybirds and larva on the willow, and the wasps were very interested towards the end of the season too, but I'm not sure if they were collecting aphids or just honeydew. I cut my willow every year for basketry, and I haven't noticed any real detrimental effect. Not even cosmetic damage.
