Insecticidal soaps are made from the salts of fatty acids.  Fatty acids are components of the fats and oils found in plants and animals.  These soaps should not be confused with our ordinary cleaning soaps which do not contain the fatty acids.  Insecticidal soaps kill only what they come in contact with and are effective against soft-bodied pests such as aphids, thrips, crawler stage scales, mites, whiteflies and leafhoppers.  Always spray the under sides of leaves where many of these pest hide.  When using insecticidal soaps you should test on a single leaf to make sure it will not burn the plants, as some plants are more susceptible than others, particularly those with hairy leaves.