Maybe check your dark leafed Sambucas.

I don't normally have a problem with black fly. Apart from an Euonymous Alatus Red Cascade which can get a small amount that's about it in my garden, or it was! I have just been round the garden seeing how things are coping with the torrential rain we've been having and noticed my 3 Sambucas Black Lace are heaving with black fly. I hadn't noticed before because with the stems of the shrubs being so dark the flies weren't easy to spot. I then checked my golden leafed Sambucas and they are fine so it looks like it's only the dark leafed ones which are affected. Maybe it's only mine that have fallen prey but perhaps others would be wise to check theirs. I have squashed what I can (yuk) but will have to wait until it stops raining to spray them.