What Is Insecticidal Soap?

Insecticidal soaps are ideal for use on soft-bodied insects like aphids.

Colorado State University Extension classifies insecticidal soap as a "selective insecticide," which means that is effective at killing harmful garden pests while having minimal adverse effects on other garden organisms. Insecticidal soaps contain potassium salt of fatty acids, much like liquid hand soaps. It is applied to food and non-food plants and is safe to use even on the day of harvest. The soap rinses easily from plants, fruits and vegetables.

How it Works

Insecticidal soaps wash away the protective coating (cuticle) on an insect's body. Without this protection, the insect quickly dehydrates and dies. The insect must come in contact with the soap and the best way to achieve this is to cover the tops and bottoms of the leaves on an infested plant, sometimes with multiple applications.

Insects

Insecticidal soaps should be used to target soft-bodied insects like aphids, spider mites, thrips, whiteflies and mealybugs. The soaps are somewhat effective at controlling caterpillars, leaf hoppers, boxelder bugs and Japanese beetles, but can also harm beneficial insects like lady bugs, lacewing larvae and parasitic mites. Beneficial insects eat the soft-bodied insects you are targeting with the soap, so avoid using the soap when those insects are present.

Usage

Dilute the soap to a 2 or 3 percent solution: mix 2 ounces in 1 gallon of softened or distilled water. The concentration is more important than the amount used. Spray the soap on the tops and bottoms of leaves. Soaps can be used on outdoor and indoor plants. Apply the soap in the morning or late in the day to avoid overly rapid drying. Introduce beneficial insects (such as ladybugs) once the plant foliage has dried. Do not use insecticidal soaps near a water source or on a day with high humidity.

Phytotoxicity

Insecticidal soap may cause a burn on the foliage of sensitive plants. This is called phytotoxicity. Some geraniums, impatiens and poinsettias have shown sensitivity to the soaps, as well as the young growth of shrubs and evergreens. See the product's label for a detailed list of sensitive plants. Avoid treating plants in direct sunlight, which can also increase the risk of phytotoxicity.

Household Soaps and Detergents

Some household soaps and detergents are also efficient insecticides, but use of household products brings a greater risk of harming the plant. Dish soap and laundry soap are both too harsh for use on plants, but if you choose to use a hand soap, dilute it more than 2 or 3 percent (which is recommended for insecticidal soaps) and wash the plants with clean water within a few hours after application.

Old Soap Uses

Before using soap leftover from last season, mix a small amount in a jar with water. The mixture should look slightly cloudy, but you should still be able to see through it. If the mixture results in a completely white solution, it has oxidized. Old, oxidized soap will burn plants (see Reference 4). If you have leftover soap at the end of the season, use it to clean the inside of a greenhouse or pesticide sprayers.

References

• Colorado State University Extension: Insect Control: Soaps and Detergents
• Penn State: Insecticidal Soap
• Washington State University Extension: Guidelines for Insecticidal Soap
• Living With Bugs: Insecticidal Soap