Here's a natural remedy that I've tried and had good luck with.
Crush one bulb of garlic, one small onion and 1 Tbsp of cayenne pepper in a blender and add one quart of warm water.
Blend well and allow it to stand for one hour.
Add one Tbsp of Ivory Liquid soap and mix well before placing the mixture in a spray bottle and lightly misting the leaves.
I hope it works for you, too!