Extension News

Control Houseplant Insect Pests Safely With Insecticidal Soap

Note to media editors: This is the ISU Extension Garden Column for the week of Nov. 23, 2007.

11/19/2007

By Linda Naeve
Program Specialist
iowa State University Extension

Homemade remedies have been around almost as long as the common cold. Native Americans and early settlers used plant parts and extracts of native plants to treat many human health problems and to control pests on their crops. Although the ingredients and uses have changed over the years, some homemade remedies are still being used in homes and gardens.

About 15 years ago, a celebrity gardener promoted the use of all types of household products to green up a lawn, control insect pests and many other garden problems. I never heard from anyone who had success with those recipes, but I do know local grocery stores couldn’t keep enough ammonia, dish detergent and cheap beer on the shelves. Eventually, this fad passed and people went back to more conventional, effective fertilization and pest control methods.

One homemade remedy from a common household product – insecticidal soap – stood the test of time with its convenience and effective control of specific insect pests on plants. When diluted and sprayed on plants, household soaps or detergents are an effective insecticide to control spider mites and soft-bodied insects on plants, such as aphids, young scales, whiteflies and mealybugs. While they aren’t effective on larger insects such as caterpillars and beetle larvae, soaps and detergents will control boxelder bugs when they are still in the small nymph stages.

Soaps and detergents work on contact which means they are only effective when they cover or coat the insect. They have no residual effect and must be applied several times at weekly intervals for best control.  It is believed that soap kills insects by disrupting their cell membranes. It also removes the protective waxes that cover the insect, causing it to die by excess water loss. Although it controls many insect species, insecticidal soap is considered a selective insecticide because of its minimal adverse effect on beneficial organisms, such as lacewings, lady beetles and bees.

For years, extension specialists, including myself, routinely gave out the recipe for an effective, homemade insecticidal soap which consisted of a specific amount of liquid dishwashing detergent, diluted in water to create about a 2 percent solution and sprayed on the surface and undersides of leaves. We even used it on our own plants.

Like other effective natural remedies, commercial companies have developed their own version. Several commercial companies, such as Safers®, have developed and marketed specific formulations of insecticidal soap that contained potassium salt of a fatty acid as its active ingredient. You may ask “Are these ready-to-use products worth the extra cost and are they more effective or safer to use?” Today, the answer is “yes”, they are probably the recommended choice because the formulations are consistent and there is a minimal potential for plant injury (phytotoxicity).

The problems with homemade insecticidal soaps made with current dishwashing detergents have to do with the concentration, formulation and other ingredients found in these “new and improved” liquid detergents.  They aren’t simple detergents anymore to cut grease and grime, they now contain air fresheners, “power scrubbers,” anti-bacterial agents, fragrances, lotions and other ingredients. Most detergents also contain surfactants to reduce the surface tension of water and make it “wetter.” These added products may be phytotoxic to sensitive plants and cause their leaves to be deformed or discolored, or even die back. Also, many liquid dishwashing detergents are now sold as concentrates in smaller containers. The variable concentrated formulations make it hard to recommend a general dilution ratio that is effective against the pests yet will not injure the plant.

Environmental factors can affect the usefulness of homemade insecticidal soaps. For example, soaps are affected by minerals found in hard water, which can result in chemical changes producing insoluble soaps, commonly known as “soap scum.” The effectiveness of the soap as an insecticide decreases if hard-water is used. 

We all like to save money when possible using homemade products, however, it is probably easier and less risky to purchase the convenient, effective and dependable commercial insecticidal soaps available at most garden centers.

 

--30--

Contacts :

Linda Naeve , Entomology, (515) 294-8946, lnaeve@iastate.edu

Jean McGuire , Extension Communications and Marketing, (515) 294-7033, jmcguire@iastate.edu

There are no photos for this week's column.