I used to spray blackfly with a systemic insecticide which worked effectively unlike the plot holders around me who trusted in washing up liquid which seemed to have little effect. The effect of using washingup liquid seemed purely psychological, giving them great piece of mind that they had 'solved' the problem with a targeted approach. Someone then told me that he sprayed them off with a hose. I was initially sceptical but tried it and found it actually worked without the cost and pollution of a systemic insecticide. If there are still blackfly the next day, repeat. It is an obvious solution, costs nothing and makes sense. If there aren't any blackfly on the plant, there is nothing to suck the plants. Ladybirds never overtake the spread of the blackfly and ants merely milk the blackfly.