Chapter 11

Insect Pest Management for Organic Crops

Insecticidal Soaps

Insecticidal soaps are sprayable liquid formulations of potassium salts of fatty acids that are specifically intended for insect control. Insecticidal soap products work by disrupting the cuticle (skin) layer and suffocating soft-bodied insects. To be effective, the spray solution must contact and thoroughly cover the targeted pest. Soaps work best against soft-bodied pests such as aphids, scales, whitefly, mealybugs, thrips, spider mites, and the immature stages of other pests; predatory mites and the soft-bodied larval stages of beneficial insects such as ladybird beetles (“ladybugs”) and hoverflies are also likely to be affected. Soaps are ineffective against insect eggs and harder-bodied adult insects, such as beetles. Some soap-based products are formulated with neem oil, and these tend to have a broader range of action (see previous section on neem).

Application

Target pests must be directly contacted by the spray; once the residue dries on the plant, it is no longer effective. For this reason, it is more effective to spray in the early morning when the insects are becoming active but temperatures are still relatively cool so that drying is less rapid. Application must be thorough and completely wet the pest.

Water Quality

Environmental factors also can affect use of soaps. In particular, soaps are affected by the presence of minerals found in hard water, which results in chemical changes producing insoluble soaps (soap scum). Control decreases if hard-water sources are used.

Phytotoxicity

One of the most serious potential drawbacks to the use of insecticidal soaps is their potential to cause plant injury—their phytotoxicity. Certain plants are sensitive to these sprays and may be seriously injured. For example, most commercial insecticidal soaps list plants such as hawthorn, sweet pea, cherries, and plum as being sensitive to soaps. Portulaca and certain tomato varieties also are sometimes damaged by insecticidal soaps.

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