Insecticidal soaps contain the active ingredient potassium salts of fatty acids, which penetrate and disrupt the cuticle that holds moisture inside insects' bodies. When sprayed with soap, many soft-bodied insect pests, such as aphids, dry out and die. Some pests, especially beetles with hard bodies, remain unaffected, however. To make soaps more effective, some products combine soap with pyrethrins, a botanical insecticide that's described later in this chapter.
Insecticidal soap is nontoxic to humans and other animals, and breaks down quickly in the environment. If you use a concentrated product, dilute it with soft water before use for the best effect. Hard or mineral-rich water decreases its effectiveness.
The downside to insecticidal soap is that it also disrupts the waxy cuticle on some plants, making it toxic to young and thin-leafed plants, especially tomatoes. If you aren't sure of the plant's sensitivity to the product, always test it on a leaf or two, and allow a couple of days to pass before spraying a whole plant. Follow the label directions carefully.
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