Insecticidal soap vs. dishwashing liquid

On the radio last week a caller read the label of an insecticidal soap over the phone, and wondered about the ingredient: “potassium salts of fatty acids?” The caller also wanted to know if dishwashing liquid could be used as a substitute for insecticidal soap.

I explained to the caller that these potassium salts of fatty acids are created by adding  potassium hydroxide to fatty acids obtained either from  animal fat or in plant oils. The resulting “soap salts” are most effective in controlling  soft-bodied pests such as aphids, scale and mealybug crawlers, thrips, whiteflies, and spider mites. Generally, they have little effect on beetles and other hardbodied insects (an exception being cockroaches). The soaps must come into direct contact with the pest to be effective.  The soap penetrates the outer cuticle of the insect’s body and dissolves or disrupts the cellular membranes causing dehydration and death.  Soaps can also block the spiracles or breathing pores in the insect’s body which interferes with respiration. In some cases soaps may also act as an insect growth regulator, affecting the metabolism of cells and metamorphosis.

Certain common dishwashing liquids and laundry detergents when mixed with water have also shown insecticidal and miticidal properties. When applied to an assortment of vegetable crops, Palmolive®, Dawn®, Joy®, Ivory®, and Dove®, for example, have effectively reduced populations of whitefly , aphids, and spider mites. However, dishwashing and laundry detergents are not labeled as insecticides. Although they may be insecticidal, they are chemically different from  the registered insecticidal soaps.  Furthermore they may prove phytotoxic, causing injury by dissolving the waxy cuticle of the plant’s leaf surface. I told that caller that it’s better to save these soaps or the washing of clothes and cleaning of dishes for which they were designed.

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