Insecticidal soap is an environmentally friendly alternative to common pesticides. It works by penetrating and washing away a soft-bodied insect's protective cuticle. Without this waxy, protective coating, the insect dehydrates and dies. Insecticidal soaps are not harmful to pets or people, but soap is potentially harmful to aquatic life---even if it is free of phosphates---so insecticidal soaps should not be used if there is a risk of the runoff making it into waterways. Insecticidal soap will also kill beneficial insects like ladybirds, which love to eat the aphids you may be trying to kill.
Perform a water test prior to mixing a large amount of the solution. Mix the concentration you intend to use in a jar of water and shake the jar. Wait 15 minutes. If the water stays cloudy, your water is OK to use, but if it makes a scum on the top, minerals present in your water will make the soap less effective. In that case, use a gallon of bottled water.
Drop a bar of unscented, white soap into 2 cups of water and let it sit overnight or until enough soap has dissolved to turn the water white. Remove the bar of soap.
Mix 59.1ml of the soap solution in one gallon of water. The University of Florida advises gardeners that the concentration of the solution is more important than the amount applied. It suggests mixing 5 tablespoons of soap in 1 gallon of water, which makes a solution slightly less than 2 per cent. Mix a weaker concentration for plants that may be sensitive and test it first.
Mix in ¼ cup of light vegetable oil (corn, canola, safflower) and shake well. It will take a bit of shaking to mix in the oil. Oil keeps the insecticidal soap from evaporating too quickly. Light vegetable oil ensures that the leaves aren't smothered.
Fill a spray bottle or a garden sprayer that attaches to a hose with the solution you've made and spray only the affected plants. If possible, spot spray only the insects. It is best to spray plants in the morning so the foliage can dry during the day.