If you want to be organic then either a milk or bicarb. Spray it with 10 parts water to one of milk, or for bicarb 2 tsp/litre with a few drops of washing up liquid. I've tried both and they work milk is probably slightly kinder to plants but you need to use it up -smells vile and clogs up the sprayer otherwise.
I was reading elsewhere that some people use a 100:1 solution as a preventative spray -might give it a try.
Jeremy
