Chapter 12

Insect Pest Management for Organic Crops

Insecticidal Soaps for Controlling Insect Pests

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. This usually means spraying undersides of leaves and other protected sites. Insects that cannot be completely wetted, such as aphids within curled leaves, will not be controlled. Repeat applications may be necessary to achieve control. Once an insecticidal soap spray has dried, there is no residual activity because soaps work only on contact. Results from the application of soap are usually seen in one to three days. Insecticidal soaps are usually diluted with water before applying.

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