"Soaps" and Detergents: Should They Be Used On Roses?
Raymond A. Cloyd, Assistant Professor,
Extension Specialist in Ornamental Entomology/Integrated Pest Management
University of Illinois
Department of Natural Resources and Environmental Sciences
384 National Soybean Research Laboratory
1101 West Peabody Drive
Urbana, IL 61801
Phone: (217) 244-7218, Fax: (217) 333-4777
Pest control materials including insecticides, miticides, and fungicides are routinely used to manage pests on roses. Insecticides and miticides classified as biorational or "reduced risk," including insect growth regulators, horticultural oils, and insecticidal soaps may be used in certain situations because they generally leave minimal residues, are less toxic to humans and natural enemies, and are short-lived in the environment because they degrade rapidly. In addition, soaps are generally non-toxic to the user unless ingested at high doses.
Insecticidal soaps are used to control a variety of insect and mite pests including aphids, scales, mealybugs, and twospotted spider mite. A soap is a substance derived from the activity 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.
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 now available as a hand soap or shampoo, and has been shown to be effective, as an unlabeled insecticide, in controlling soft-bodied insects including aphids.
Soft-bodied pests such as aphids, scale and mealybug crawlers, thrips, whiteflies, and mites including twospotted spider mite are most susceptible to soap applications. Soaps generally have minimal activity on beetles and other hard-bodied insects although this is not always true, as soaps have been shown to kill hard-bodied insects such as cockroaches. Soaps are effective only when insects or mites come into direct contact with the spray. Dried residues on plant surfaces have minimal insect or mite activity as soap residues degrade rapidly.
The mode of action of soaps is still not certain, as there may be three ways that soaps kill insect and mite pests. First, soaps may work by penetration of the fatty acids through the insect's outer covering (cuticle) and dissolving or disrupting cell membranes. This disrupts cell integrity causing cells to leak and collapse, destroying respiratory functions, and resulting in dehydration and death of the insect. Second, soaps may act as insect growth regulators interfering with cellular metabolism and the production of growth hormones during metamorphosis. Third, soaps may block the spiracles (breathing pores), interfering with respiration.
There are a variety of fatty acids; however, only certain fatty acids have insecticidal properties. This is based on the length of the carbon-based fatty acid chains. Most soaps with insect and mite 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 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.
There is a misconception that any soap or detergent can be used as an insecticide. Although, as already discussed, only a few select soaps have insecticidal properties, many common household soaps and detergents including PalmoliveÒ , DawnÒ , IvoryÒ , JoyÒ , TideÒ , and DoveÒ , which are unlabeled insecticides, have some activity on many soft-bodied insects when applied to plants as a 1% or 2% aqueous solution. However, reliability is less predictable than soaps formulated as insecticides.
Examples of research demonstrating the effectiveness of various dishwashing liquids and detergents on insect and mite pests are provided below:
1) PalmoliveÒ , DawnÒ , JoyÒ , IvoryÒ , and DoveÒ effectively reduced the numbers of sweet potato whitefly, green peach aphid, cabbage aphid, and twospotted spider mite on a variety of vegetable crops.
2) Dawn UltraÒ dishwashing liquid was found to be effective on German cockroach causing 100% mortality.
3) IvoryÒ liquid dishwashing soap tested at 0.4 to 3.0% concentrations was effective in controlling spider mites, aphids, and psyllids.
4) IvoryÒ liquid dishwashing soap was effective against aphids, spider mites, psyllids, and thrips at 1 and 2% concentrations.
5) New DayÒ dishwashing detergent when used at 2.0 ml/L was highly active on whiteflies providing 95% mortality of silverleaf whitefly nymphs. New Day contains cocamide DEA and dodecylbenzene sulphonic acid as the active ingredient.
6) Aqueous solutions (0.1 to 2.0% concentrations) of two "soft" soaps caused nearly 100% mortality on two household insect pests: cricket and cockroach.
7) IvoryÒ liquid dishwashing soap and TideÒ detergent were effective in reducing populations of aphids, citrus red mite, psyllids, and greenhouse thrips on landscape plants.
Despite these examples, dishwashing liquids and laundry detergents are primarily designed to dissolve grease from dishes and clean clothes; not to kill insects. These materials may cause plant injury by dissolving the waxy cuticle on the leaf surfaces. Registered, commercially available insecticidal soaps are less likely to dissolve plant waxes than household cleaning products. Additionally, plants with pubescent (hairy) leaves may be more susceptible to phytotoxicity from dishwashing liquids and detergents. Dishwashing liquids and laundry detergents, like insecticidal soaps, lack any residual activity and thus more frequent applications are needed. However, too many applications will harm certain plant types. In addition, detergents are chemically different from soaps and may cause phytotoxicity. In fact, many hand soaps are not necessarily pure fatty acids. Most importantly, these solutions are not registered insecticides. Soap companies don't intend for their products to be used as insecticides as 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 insecticides. Even more important is that a pest control company will generally stand behind a product when there is a problem. If a dish or laundry soap is used and plants are injured--there is no recourse.
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