Insecticidal soap has been used by home gardeners for generations as an alternative to chemical pesticides. It is 40 per cent to 50 per cent effective in killing soft-bodied insects such as aphids, thrips, whiteflies, spider mites and immature leaf hoppers. Hard-bodied insects, including most beneficial insects, are not harmed by insecticidal soap. It has no residual insecticidal value and must come into contact with an insect to kill it. Homemade insecticidal soap is safe for use on most plants in the vegetable garden, though it may cause harm to some varieties of tomatoes and other hairy leaf plants.
Purchase a dish-washing liquid that contains no detergent or degreaser. Do not use washing powder or automatic dishwasher soap.
Mix 1 tsp dish-washing liquid with one pint of water in a jar and shake or stir to mix. Let it stand for 15 minutes. If the solution remains milky the water is pure enough to make the soap. If scum forms on the top, the mineral content is too high and will reduce the effectiveness of the insecticidal soap. Add a water softener to water with a high mineral content before mixing the insecticide or use purified water.
Combine 5 tbsp dish-washing liquid with one gallon of pure water, or water that has been treated with a softener. Stir to combine.
Pour the insecticidal soap into a labelled gallon jug and seal. Use this to fill spray bottles as needed for application in the garden. Spray bottles should also be labelled for safety purposes.
Insecticidal soap should be tested on a small part of a plant to determine if the solution will cause damage. Insecticidal soap should not be applied to the entire garden, but used as a spot treatment for insect control. This insecticidal soap is slightly less than a 2 per cent solution, which is recommended by the University of Florida, IFAS Extension for home gardens. A 1 per cent solution (2 1/2 tbsp dish-washing liquid per gallon of water) should be used on tomatoes, hairy leaf plants and other plants that are fragile. Repeated applications may be necessary to control large infestations. Insecticidal soap should not be used in full sun, and only when the outside temperature is lower than 32.2 degrees Celsius.
Insecticidal soap may leave spots on hairy leaf plants which will make them more susceptible to burning in the sun. Insecticidal soaps should not be used on any plant that is stressed. Insecticidal soap should be used only on plants that have hardened off. It may damage young foliage. Gardenia, crown of thorns, maidenhair fern, nasturtiums, lantana, and Easter lilies are sensitive to soap sprays.