One of the core precepts of gardening in an organic way is to watch and learn from nature.
When there is not a spray-able solution to problems you have to make decisions based on experience, an experience that comes from observation.
The plant in the photo is Artemesia "Powis Castle", one of several artemesias I grow for their silvery foliage and woodsy musty smell. It is a plant I use a lot in September and October, lightening arrangements of dark dahlias, seed-heads and berries.
Every year from March, as soon as the weather heats up, it is covered in long ugly strings of black-fly - they cling to the stems and getting them off by hand damages the delicate foliage. If I want to pick artemesia early in the season I have to float the stems in water with a squirt of washing up liquid in it until the aphids float off.
Over the years I have noticed that the black flies do not seem to damage the plants - there is a virus risk, but their sap sucking activities don't seem to effect the vigour of the plants. I have also noticed that the problem seems to clear up towards the end of the year. 
Now I know why - a flotilla of ladybirds have worked their way from one end of the row to the other, they have only 4 plants to go now, and then the black fly will be completely gone.
Complete non-intervention - labour free pest control - suits me.
The bottom photo shows my girls working as labourers, nailing down reclaimed rafters to make the shed floor.
J
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