All soaps are long chain fatty acids, but not all soaps have insecticidal properties. Insecticidal soaps are specifically formulated to have high insect-killing properties, while being safe for most plant species.
Insecticidal Soap is a Contact Material
Insecticidal soaps kill susceptible insects by washing away the protective coating on the surface of the insect and by disrupting normal membrane functions inside the insect. The insects must come into direct contact with the spray droplets for the material to be effective. Good coverage is essential. The soaps have no residual activity toward insects, but repeated applications may have damaging effects on some types of plants.
Water Quality and Insecticidal Soap Effectiveness
Water hardness reduces the effectiveness of insecticidal soaps. Calcium, magnesium and iron precipitate the fatty acids and render them useless against the insects. It is important to use the purest water possible. Conduct a "jar test" to determine if your water is compatible with the soap. Mix the concentration of soap that you intend to use with water in a glass jar. Mix and allow to stand 15 minutes. If the mix remains uniform and milky, then your water quality is adequate. If a scum develops on the surface of the water, then conditioning of the water will be necessary. The water can be conditioned using a commercially available anionic buffering and conditioning agent. Some readily available products such as "Calgon" may be used. Insecticidal soaps may foam; if your sprayer has an agitator, a defoaming agent may also be added.
Good spray coverage is essential for adequate results. Spray equipment must be clean and operating at peak efficiency. The proper configuration of nozzles for good coverage must by utilized in order to wet both sides of the leaves and growing points of the plants. Some of the new spray technologies that create a "fog-like" spray may also improve coverage. Spraying in the evening or early morning hours so that the spray droplets do not dry out quickly may also improve the effectiveness application.
Insecticidal soaps may cause a burn on the foliage of sensitive 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.
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.
Insecticidal soaps are used against soft bodied insects and mites such as aphids, thrips, white flies, spider mites and immature leafhoppers. Insecticidal soaps have been about 40-50% effective against these pests. Repeated applications may be necessary to adequately control high populations of pests, and close attention should be paid to all details outlined above to achieve maximum control.
Some entomologists have concerns about impact of soaps on soft-bodied immature predators, such as lady beetle and lacewing larvae. No real data has turned up that substantiates these fears. Until such data surfaces, perhaps refraining from using soaps where there are numbers of these beneficial larvae present would be advisable.
Even though soaps have low toxicity to humans, they should always be used with caution. Read and follow all label directions.
Trade names are not intended as an endorsement.
For further information, contact your local WSU Extension Office.