Soap has been used as an insecticide for many years by home gardeners. In the past, the origin of these soaps varied considerably, and in some cases, resulted in plant injury, says David Robson, University of Illinois Extension horticulture educator, Springfield Center.

In the 1970's a major breakthrough occurred when scientists were able to unravel the insecticidal properties of certain naturally occurring plant oils and animal fats. These natural compounds are known as "fatty acids."

Fatty acids play important roles as basic energy sources for living organisms and are major components of cell membranes. Before utilizing the benefits of these fatty acids from the hundreds found in nature, researchers had to find those that would not harm the plant. In the mid 1980's this task was completed, and there are now formulations available for pest control. Common insecticidal soaps now found on the market are normally prepared from natural human food items, such as milk and peanut oil.

The soap itself actually consists of approximately 50 percent potassium (K) salts of fatty acids and is formulated by treating selected acids with a potassium base. The inert ingredients are a combination of water and alcohol, which aids in the mixing of the soap concentrate. Most insecticidal soaps do not contain any organic solvents or dangerous inerts.

Insecticidal soaps are considered biodegradable and are exempt from residue tolerance by the EPA.

Because soaps are formulated from specific fatty acids, they are relatively selective against pest insects and usually are not lethal to beneficial insects. Their special formulations reduce their phyto-toxicity, or ability to damage plants. Homemade soap sprays may damage or kill plants at the same time they reduce the insect pests.

Soaps are considered a contact insecticide, and the insect or mite pest must receive a direct application of the spray in order for it to be effective. Soaps are only effective in the liquid state, and there should be no insecticidal activity present once the material has dried on the application surface.

Insecticidal soaps differ from conventional insecticides in that the latter specify the amount of active ingredients per acre or per 100 gallons of water. With insecticidal soaps, the relative concentration of the soap in the spray solution is important.

Follow label directions for the desired concentration necessary to control the pest and to cover the foliage thoroughly.

Source: David Robson, University of Illinois Extension horticulture educator, Springfield Center, (217)782-6515