This is an easy and inexpensive aphid killer I found in the book Great Garden Formulas by Benjamin and Martin (editors, 1998). I couldn’t believe that aphids are already swarming my roses. They seem to know it’s the end of May, while weather-wise we are all still back in April. I tried it this week and it appears to be working; time will tell.
Ingredients: 1 pint water, rind from one lemon–grated, cheesecloth, pump spray bottle.
Directions:  Bring the water to boil. Remove from heat and add the lemon rind. Allow the mixture to steep overnight. Strain through cheesecloth and pour into spray bottle. Apply to leaves that have aphids, making sure the liquid comes in contact with the insects.