I have a young Hazel tree, about four years old, and all the new leaves have been infested with black aphids this summer (never happened before) - is there any way to get rid of them organically or should I resort to chemicals?
Do you have a hose? You could try just putting it on the jet setting and blasting them off. Failing that, you can try a soap and water mix (Ecover washing up liquid and water) or neem oil - though you have to be careful to target only the aphids with them. Both of those, and the ladybird suggestion above, will be much better for your garden than using insecticide, which may decimate really beneficial insects too.