Soap causes a disruption of the cell membranes of the insect. It may also cause excess water loss by removing the protective waxes that cover the insect.
Several insecticidal soaps are commercially available for control of insects and mites. The active ingredient of each of them is potassium salt of fatty acids. While chemically similar to hand soaps, insecticidal soaps have several key differences from dishwashing liquids or soaps that are sometimes substituted. Commercially distributed insecticidal soaps are selected to:
Some household hand soaps and liquid dishwashing detergents also make effective insecticides. They can be substantially less expensive, however, there is increased risk of plant injury with these products. They are not designed for use on plants. Do not use dry dish soaps or clothes-washing detergents as these are too harsh to be used on plants. Do not use soaps with a citrus base as these will also damage plants. Many soaps and detergents are poor insecticides. Regardless of what product is used, soap-detergent sprays are always applied diluted with water, typically at a concentration of around 2 to 3 percent.
Insecticidal soaps are considered selective insecticides because of their minimal adverse effects on other organisms. Lady beetles, green lacewings, pollinating bees and most other beneficial insects are not very susceptible to soap sprays. However, predatory mites, important in control of spider mites, are an exception. They are a beneficial organism easily killed by soaps.
In order to thoroughly wet the pest it is usually necessary to spray the undersides of leaves and other protected sites. Insects that cannot be completely wetted, such as aphids within curled leaves, will not be controlled. Because of the short residual action, repeat applications may be needed at relatively short intervals (four to seven days) to control certain pests, such as spider mites and scale crawlers.
One of the most serious drawbacks to the use of soap-detergent sprays is their potential to cause plant injury. Always test soap-detergent sprays on a small area of leaf tissue a day or two before an extensive area is treated.
Plant injury can be reduced by:
Environmental factors also can reduce the effectiveness of soaps and detergents:
Soaps and detergents can offer a relatively safe and easy means to control many insect pests. As with all pesticides, however, there are limitations and hazards associated with their use. Carefully follow all label instructions.
|Table 1: Approximate mix to produce various dilute soap sprays.|
|Percent dilution desired||Approximate amount of soap to add to water to produce:|
|1||2 1/2 Tbsp (-)||2 tsp (+)||1 tsp (+)|
|2||5 Tbsp (-)||4 tsp (+)||2 tsp (+)|
|3||8 Tbsp (+)||2 Tbsp (+)||1 Tbsp (+)|
|4||10 Tbsp (-)||2 1/2 Tbsp (+)||4 tsp (+)|
|(+) Will produce a solution of slightly higher concentration than indicated.
(-) Will produce a solution of slightly lower concentration than indicated.
Based on Publication by W. S. Cranshaw, Colorado State University Cooperative Extension entomologist and professor, bioagricultural sciences and pest management.
Use of Insecticidal Soaps in the Low Desert
Last Updated October 12, 2005
© 1998 The University of Arizona, College of Agriculture and Life Sciences, Cooperative Extension, in Maricopa County. Comments to Maricopa-Hort@cals.arizona.edu 4341 E. Broadway Road, Phoenix, AZ 85040, (602) 470-8086