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Fatty Acid Soaps/ Insecticidal Soaps
Beyond Pesticides Rating: Least Toxic

Soaps, sodium or potassium hydroxide on a fat combined with vegetable oil, contain fatty acids which act as an insecticide or a herbicide killing pests on contact. These soaps are virtually nontoxic to humans or mammals unless ingested (Olkowski 1991). 

When fatty-acid soap touches the outer body, or cuticle, of an insect or plant tissue it will eventually lead to dehydration and death of the pest. These soaps rapidly biodegrade in soil (Olkowski 1991). 

Commonly used soaps containing potassium and coconut oil are effective in controlling many soft-bodies insects such as aphids, caterpillars, crickets, fleas, flies, and mites. Because fatty-acid soaps can kill a variety of arthropods, including those that are beneficials, outdoor use should be limited to spot treatments. Over use of soaps, like chemical pesticides, can lead to pest resistance (Olkowski 1991). 

Carefully read the label of the fatty-acid soap pesticide product to identify the active ingredient and make sure that they do not also contain toxic pesticides or synergists. 

Reference: 

Olkowski, W. 1991. Common-Sense Pest Control: Least-toxic solutions for your home, garden, pets and community. Taunton Press. Newtown, CT.