Insecticidal soap

Insecticidal soap is based on potassium fatty acids and is used to control many plant pests. Because insecticidal soap only works on direct contact with the pests, it is sprayed on plants in way such that the entire plant is wetted. Soaps have a low mammalian toxicity and are therefore considered safe to be used around children and pets and may be used in organic farming.

Insecticidal soap should be based on long-chain fatty acids (10–18 carbon atoms), because shorter-chain fatty acids tend to be damaging for the plant (phytotoxicity). Short (8-carbon) fatty-acid chains occur for example in coconut oil and palm oil and soaps based on those oils. Recommended concentrations are typically in the range 1–2 percent. One manufacturer recommends a concentration of 0.06% to 0.25% (pure soap equivalent) for most agricultural applications.; another one recommends concentrations of 0.5 to 1% pure soap equivalent. In the European Union, fatty acid potassium salts are registered and allowed as insecticide at a 2% concentration.

Insectidal soap is most effective if it is dissolved in soft water, since the fatty acids in soap tend to precipitate in hard water, thereby reducing the effectivity.

Insecticidal soap is sold commercially for aphid control; these may not always use the word soap, but they will list "potassium salts of fatty acids" or "potassium laurate" as the active ingredient. Certain types of household soaps (not synthetic detergents) are also suitable, but it may be difficult to tell the composition and water content from the label. Potassium-based soaps are typically soft or liquid.

The mechanism of action is not exactly understood. Possible mechanisms are:

Insecticidal soap works best on soft-bodied insects and arthropods such asaphids, adelgids, mealybugs, spider mites, thrips, jumping plant lice, scale insects, whiteflies, and sawfly larvae. It can also be used for caterpillars and leafhoppers, but these large-bodied insects can be more difficult to control with soaps alone. Many pollinators and predatory insects such as lady beetles, bumblebees, and hoverflies are relatively unaffected. However, soap will kill predatory mites that may help control spider mites. Also, the soft-bodied aphid-eating larvae of lady beetles, lacewing, and hoverflies may be affected negatively. According to one study a single soap application killed about 15% of lacewing and lady-beetle larvae, and about 65% of predatory mites (Amblyseius andersoni).

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Wikipedia