Philadelphus aphid attack

Hi,
I have a generally very healthy philadelphus, however a few branches are smothered in black fly... Any idea why it is just a few branches and anything i can do to lessen this?
Many thanks
I have a generally very healthy philadelphus, however a few branches are smothered in black fly... Any idea why it is just a few branches and anything i can do to lessen this?
Many thanks

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Posts
Black and greenfly love the soft emerging growth on plants at this time of year. A good hose usually does the job
Never had them on any philadelphus I've had. I grow the more compact Manteau d'Hermine @B3. Just about to flower. Lovely shrub.
I live in west central Scotland - not where that photo is...