Taken from http://www.wikihow.com/Make-Nettle-Spray-for-Aphids
This is an especially good choice when you are removing nettles form your allotment during weeding
1. Cut back some nettles from your allotment. Wear gloves! They sting easily.
2. Place nettles in a bucket and cover with water
3. Cover and leave to break down for 1-2 weeks. The water will turn brown
4. Strain out the plant matter and pour liquid into a spray bottle
5. Spray onto vegetables and fruit trees. The spray will kill aphids and strengthen the plants