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Insecticidal soap is defined as any of the potassium fatty acid soaps used to control many plant pests. Insecticidal soap is typically sprayed on plants in the same manner as other insecticides. Insecticidal soap works only on direct contact with the pests. The fatty acids disrupt the structure and permeability of the insect cell membranes. The cell contents are able to leak from the damaged cells, and the insect quickly dies.
Insecticidal soap works best on soft-bodied insects such as aphids, mealybugs, spider mites, thrips, and whiteflies. It can also be used for caterpillars and leafhoppers, but these large-bodied insects can be more difficult to control with soaps alone. Many pollinators and predatory insects such as lady beetles, bumblebees, and syrphid flies are relatively unaffected.
Soaps have low mammalian toxicity. However, they can be mildly irritating to the skin or eyes.
Insecticidal soaps may cause phytotoxicity (toxic to the plant) symptoms, such as yellow or brown spotting on the leaves, burned tips or leaf scorch on certain plants. In general, some cole crops and certain ornamentals are sensitive to burn caused by soaps. Multiple applications in a short time interval can aggravate phytotoxicity. In addition, water conditioning agents can increase phytotoxicity. A small spray strip should be applied and observed before a full-scale application is made if there is a question concerning sensitivity. Mountain ash, Japanese maple, jade plant, lantana, gardenia, bleeding heart and crown of thorns are known to be sensitive to soap. Certain cultivars of azalea, poinsettia, begonia, impatiens, ferns, palms and succulents may also be sensitive. When uncertain, spot treat a portion of the plant, and wait at least 24 hours to see if any phytotoxic symptoms develop before treating an entire group of plants. Plants under drought stress, young transplants, unrooted cuttings and plants with soft young growth are more likely to develop phytotoxic symptoms and should not be treated with soap. Soap mixed in hard water may be less effective and more toxic to the treated plants. A precipitate may be formed when the metal ions (e.g., calcium, iron or magnesium) found in hard water bind to the fatty acids in the soap.
The concentration of the spray is more important than the amount of soap applied. Usually insecticidal soaps are used as a 2% solution. If water is increased or decreased, then the amount of soap must be increased or decreased accordingly to avoid phytoxicity.
Insecticidal soaps are used against soft bodied insects and mites such as aphids, thrips, whiteflies, spider mites and immature leafhoppers. Insecticidal soaps have been about 40-50% effective against these pests.
Soap sprays, alone or in combination with horticultural oils or botanical oils, are also valuable in the management of certain plant diseases, most notably powdery mildew. Repeated applications may be necessary to adequately control high populations of pests.
- "Pest Management". National Sustainable Agriculture Information Service. Retrieved 2007-09-07.