Insecticidal soap is a popular organic means of controlling harmful garden pests. It is highly regarded because it kills harmful insects while not affecting beneficial ones. Furthermore, it is generally safe to use around humans, pets, other animals and plants. Still, some care must be taken to ensure that it is being used effectively. It may also be harmful to some sensitive plants, for which additional precautions are necessary.
Insecticidal soap kills harmful insects such as aphids, thrips and spider mites by coating their cellular membranes with soapy oil. It washes their natural protective coatings off, making it difficult or impossible for the insects to retain their natural hydration. Harmful insects die as a result of dehydration.
Insecticidal soap is best applied early in the morning, well before the sun is at its full power. Many insects are active at this time and will be available on the plants for gardeners to kill directly by use of the soap. The soap spray will remain moist (and therefore effective) for a longer period of time, as the sun will not dry it out so quickly.
Insecticidal soap does not harm or kill all insects. It only works on small, soft-bodied insects—which are, luckily, mostly pests that are harmful to garden plants. Caterpillars and other larger insects are not harmed and beneficial insects such as lacewings and lady beetles are usually not harmed either.
Although they are benign to most plant life, insecticidal soaps can be harmful to some sensitive plants. Gardeners who are uncertain whether or not insecticidal soap is the right choice for them should test the soap on a small part of a plant first. They should observe it for a couple of days, and perhaps even do multiple applications before moving on to spraying a whole crop.
One exception to the rule about insecticidal soap generally not harming beneficial insects is predatory mites. Predatory mites are helpful to gardeners with spider mite infestations. Unfortunately, they may be killed by insecticidal soaps as well. Gardeners with serious spider mite infestations may want to consider other methods of pest control.
Insecticidal soaps have little to no effect on the insects they are meant to kill when they have dried. To be their most effective, insecticidal soaps must be sprayed while insects are active on affected plants and directly on those insects. Furthermore, multiple applications in a short time period (over a course of four days or so) may be necessary to fully combat serious infestation problems.
Recipes for insecticidal soaps can be found in organic gardening books, magazines and websites. However, retired Oregon State University entomologist Dr. Jack De Angelis warns that mixing insecticidal soaps at home may not have the desired effects. Because soap formulations vary so widely, many soaps commonly used in the home are not suitable for this purpose, and may harm plants and beneficial insects irreparably.