Tea tree oil shampoo is recommended to prevent headlice - I use it any time I think any of my students have nits and I've never (crosses fingers and touches wood) caught them. I suppose the active ingredient might be just as unpleasant for aphids.

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You are a child of the universe,
no less than the trees and the stars;
you have a right to be here.
Max Ehrmann, Desiderata

blog: http://allyheebiejeebie.blogspot.com/ and my (basic!) page: http://www.allythegardener.co.uk/