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Insecticidal Soap (Potassium Salts)

Potassium salts of fatty acids are produced by adding potassium hydroxide to fatty acids found in animal fats and in plant oils. Fatty acids are extracted from palm, coconut, olive, castor, and cottonseed plants to form this active ingredient

This insecticidal soap is sold commercially for aphid control; these may not always use the word soap, but they will list "potassium salts of fatty acids" or "potassium laurate" as the active ingredient. Certain types of household soaps (not synthetic detergents) are also suitable, but it may be difficult to tell the composition and water content from the label. Potassium-based soaps are typically soft or liquid.

Insecticidal soap is based on potassium fatty acids and is used to control many plant pests. Because insecticidal soap only works on direct contact with the pests, it is sprayed on plants in way such that the entire plant is wetted. Soaps have a low mammalian toxicity and are therefore considered safe to be used around children and pets and may be used in organic farming.

Target Pests:

leafhoppers, mealybugs, scales and whitefly and to suppress aphids and mites

Insect pest management by soap disruption of water balance from emulsification of cuticle wax and cell membranes

Mode of action: Insecticidal soaps work only on direct contact with the target pests. The most common soaps are made of the potassium salts of fatty acids. The fatty acids disrupt the structure and permeability of the insects' cell membranes. The cell contents are able to leak from the damaged cells, and the insect quickly dies. (CT IPM)

Application Tips: Insecticidal soaps should be applied when conditions favor slow drying to maximize effectiveness, e.g., in the early morning hours with dew coverage or in the early evening. Avoid treating with soaps on hot sunny afternoons which promote rapid drying. Thorough coverage is vital for the soap to be effective: Spray thoroughly, but not beyond the point of runoff. Repeat applications may also be needed as determined by follow up scouting. (CT IPM)

Insecticidal soap mixed in hard water with a high mineral content may be less effective and more toxic to the treated plants. A precipitate (soup scum) 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. Because insecticidal soaps are toxic to fish and aquatic organisms, do not use near bodies of water.

Precautions: Insecticidal soaps may cause phytotoxicity (causing plant injury) symptoms, such as yellow or brown spotting on the leaves, burned tips or leaf scorch on certain sensitive plants. Plant sensitivity can be influenced by pest pressure, cultivar, plant vigor, environmental conditions, spray concentration, pH of spray mixture as well as the timing, number and frequency of applications. Plants understress such as hot (greater than 90 °F), humid or drought conditions, young transplants, unrooted cuttings and plants with soft young growth are more likely to develop phytotoxic symptoms and should not be treated with soap.   When uncertain, spot treat a portion of the cultivar, and wait at least 24 hours to see if any phytotoxic (plant damaging) symptoms develop before treating an entire group of plants. Dishwashing soaps and detergents are designed to remove grease from dishes and may cause plant damage by dissolving the waxy cuticle on plant leaf surfaces. There is increased risk of plant injury with the use of dishwashing soaps and detergents (not labeled as a pesticide) when used as a spray. (CT IPM)

    Impact on Beneficial Insects:

    Selected References

    University of Connecticut IPM

    Insecticidal Soaps for Garden Pest Control. Horticulture Extension Agent, Clemson University, 08/15.

    National Pesticide Information Center (NPIC). Potassium Salts of Fatty Acids (General Fact Sheet)

    Technical Evaluation Report. Soap-Based Algicide/Demossers. Compiled by Pesticide Research Institute for the USDA National Organic Program