This works well, and is actually well known by the old timers. You will need to reapply after a rain or every few days if the plants get morning dew on them.
Taken from Organic Gardening Magazine
1 garlic bulb
1 small onion
1 t. powdered cayenne pepper
1 qt. water
1 TBSP liquid dish soap (I prefer Dawn)
Chop garlic and onion in blender. Add the cayenne pepper and water, Steep one 1 hour.
Strain though cheesecloth
Add liquid dish soap so the spray sticks to plants leaves. Mix well.
Spray both sides of leaves.
Store leftover mixture in the fridge.
CAUTION Certain plants are very sensitive to soaps and can develop leave burn. Always test on a leaf or two the day before spraying the whole plant.