This article is part of our Organic Pest Control Series, which includes articles on attracting beneficial insects, controlling specific garden pests, and using organic pesticides.
What Is Insecticidal Soap?
The fatty acids in insecticidal soaps break down the protective cuticles of soft-bodied pests such as aphids, so they quickly become dehydrated and die. Insecticidal soaps provide an effective way to control aphids and other soft-bodied insects if no beneficials are present to do the job.
Which Pests Does Insecticidal Soap Control?
Aphids, mealybugs, spider mites, psyllids, thrips and whiteflies are the most common pests controlled by insecticidal soap. Insecticidal soap may also have some activity against very young earwigs, grasshoppers, harlequin bugs, leafhoppers, sawfly larvae, and squash bugs provided the larvae become well soaked with soap solution.
How to Use Insecticidal Soap
Soap sprays have no residual effect and only kill insects that are sprayed directly. Be sure to thoroughly wet both sides of leaves and all crevices. Repeat applications may be needed every five to seven days as new aphids or whiteflies hatch and form colonies. When using insecticidal soap to control spider mites on cucumber family crops, cover the treated plants with an old sheet for a day after treatment to enhance effectiveness while reducing risk of leaf burn. Always follow label directions for diluting insecticidal soap concentrates. Using too much can cause injury to leaves.
Soap sprays can be made at home by mixing 1 tablespoon of dishwashing liquid per quart of water. Insecticidal soaps are purer, however, and therefore less likely to injure foliage. In the vegetable garden, tomatoes and peas are the most likely plants to be damaged by soap sprays. Be careful when using any soap on leafy greens, which tend to pick up soapy flavors.
If you have hard water, use bottled water when mixing insecticidal soap, or use an already-diluted product. Minerals in hard water can greatly reduce the effectiveness of insecticidal soap.
How to Store Insecticidal Soap
Mix only as much concentrate as you will need. If not used within a few days, dispose of unused solution by diluting it with water and pouring it out in a sunny spot, far from water sources or storm drains. Store insecticidal soap in its original containers on a high shelf, out of the reach of children and pets, in a cool place where temperatures will not exceed 100 degrees. Under good storage conditions, insecticidal soap products may last five years or more.
More information on insecticidal soap is available from Clemson University.