By Raymond A. Cloyd
Insecticidal soaps may provide control of a variety of insect and mite pests of roses including aphids, thrips, scales and the twospotted spider mite (Tetranychus urticae). A soap is a substance derived from the synthesis of an alkali such as sodium (hard soap) or potassium (soft soap) hydroxide on a fat. Fats are generally a blend of particular fatty acid chain lengths. Soap is a general term for the salts of fatty acids. Fatty acids are the primary components of the fats and oils present in plants and animals.
Soaps may be combined with fish, whale, vegetable, coconut, corn, linseed or soybean oil. For example, “Green Soap” is a potassium/coconut oil soap that was used widely as a liquid hand soap in public restrooms. It is currently available as a hand soap, shampoo and/or treatment for skin disorders. However, it has also been shown to be effective, as an unlabeled insecticide, in controlling soft-bodied insects including aphids.
Soft-bodied pests such as aphids, scale crawlers, thrips, whiteflies and mites including the twospotted spider mite are, in general, susceptible to soap applications. Soaps typically have minimal activity on beetles and other hard-bodied insects due to the insect’s thickened cuticle, which is more resistant to penetration. However, this is not always the case since soaps have been shown to kill hard-bodied insects such as cockroaches. Soaps are effective only when insect or mite pests come into direct contact with the wet spray. Dried residues on plant surfaces have minimal activity on insect or mite pests because soap residues degrade rapidly — especially under sunlight. Insecticidal soaps may also be harmful to natural enemies including parasitoids and predators. For example, ladybird beetles and green lacewing larvae, are killed by wet sprays when present on treated plants.
The mode of action of soaps is still not well-understood although there are four ways by which soaps may kill insect and mite pests. First, soaps may penetrate through the fatty acids present in the insect’s outer covering (cuticle) thus dissolving or disrupting cell membranes. This impairs cell integrity causing cells to leak and collapse, destroying respiratory functions, and resulting in dehydration and death of the insect or mite pests. Second, soaps may act as insect growth regulators interfering with cellular metabolism and the production of growth hormones during metamorphosis (equals change in form). Third, soaps may block the spiracles (breathing pores), interfering with respiration. Fourth, soaps may uncouple oxidative phosphorylation or inhibit the production of adenosine tri-phosphate (ATP), which reduces energy production.
There are a variety of fatty acids; however, only certain fatty acids have insecticidal properties. This is solely based on the length of the carbon-based fatty acid chains. Most soaps with insect and mite pest activity are composed of long chain fatty acids (10 or 18-carbon chains) whereas shorter chain fatty acids (9-carbon chains or less) have herbicidal properties, so using materials that have short chain fatty acids can kill rose plants. For example, oleic acid, an 18-chain carbon fatty acid, which is present in olive oil and other vegetable oils, is very effective as an insecticidal soap. In fact, most commercially available insecticidal soaps contain potassium oleate (potassium salt of oleic acid).
There is a misconception that any soap or detergent can be used as a pesticide (insecticide or miticide). Although, as already discussed, only a few select soaps have insecticidal or miticidal properties; many common household soaps and detergents including Palmolive, Dawn, Ivory, Joy, Tide and Dove, which are unlabeled pesticides, may have activity on some soft-bodied insect and mite pests when applied to plants as a one percent or two percent aqueous solution including the sweet potato whitefly (Bemisia tabaci), green peach aphid (Myzus persicae), cabbage aphid (Brevicoryne brassicae), mites, psyllids and thrips. However, reliability is less predictable than soaps formulated as pesticides.
Despite this, dishwashing liquids and laundry detergents are primarily designed to dissolve grease from dishes and clean clothes; not kill insect and mite pests. These materials may cause plant injury (phytotoxicity) by dissolving the waxy cuticle on leaf surfaces. Although the leaves of roses tend to have a thickened cuticle and the flowers are waxy there is still a risk of phytotoxicity. Registered, commercially available insecticidal soaps are less likely to dissolve plant waxes compared to household cleaning products. Dishwashing liquids and laundry detergents, like insecticidal soaps, lack any residual activity and thus more frequent applications are needed. However, too many applications may damage the leaves or flowers of roses. In addition, detergents are chemically different from soaps. In fact, many hand soaps are not necessarily pure fatty acids. Most importantly, these solutions are not registered insecticides or miticides. Soap companies don’t intend for their products to be used against insect or mite pests because they have not gone through the Environmental Protection Agency (EPA) registration process.
The type of fatty acid, length of the carbon-based fatty acid chain, and concentration in many laundry and dish soaps is not known. In addition, the insecticidal effectiveness of these products may be compromised by the presence of coloring agents or perfumes. This often times leads to inconsistent results. Certain laundry and dish soaps will precipitate in hard water thus reducing their effectiveness.
Despite the activity of some dishwashing liquids and laundry soaps on insect and mite pests, their use should be avoided on roses primarily because they are not registered pesticides; they don’t have an EPA Registration Number. Even more important is that a pesticide company will generally stand behind a product when there is a problem. If a dish or laundry soap is used and roses are injured — there is no recourse.
If anyone has questions or comments regarding this article they may contact the author via email (rcloyd[at]ksu[dot]edu) or phone (785-532-4750).
Raymond A. Cloyd
Professor and Extension Specialist in Horticultural Entomology/Integrated Pest
Department of Entomology
Kansas State University
123 Waters Hall
Manhattan, KS 66506-4004