If you are not into pestisides you can try either washing them off with a jet of water from the hosepipe or you could try washing up liquid mixed with water and spray them with that, I have found that it works.

I agree with John, I use a diluted washing up liquid in a spray bottle on my Dahlias and it really works!

If you add a drop of vinegar to the soapy liquid that really sees the little blighters on their way - and it doesn't harm the plant.