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Don’t Get “Washed Up” Using Soaps :: Raymond A. Cloyd
Why detergents and dishwashing liquids don’t equal an insecticidal soap
Most growers would love to fix their pest problems by lathering up with soaps. However, managing pest problems isn’t that easy. Insecticidal soaps, which are classified as biorational or “reduced risk” insecticides, may be used in certain situations, as 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 nontoxic to the user unless ingested at high doses (unrelated to your mom’s threats to “wash your mouth out with soap”).
You can use insecticidal soaps to control a variety of insect and mite pests, including aphids, scale, mealybugs and two-spotted spider mite.
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’s now available as a hand soap or shampoo, and it has been shown to be effective as an unlabeled insecticide in controlling soft-bodied insects.
Soft-bodied pests, such as aphids, the crawler stage of scale and mealybugs, thrips, whiteflies and mites are most susceptible to soap applications. Soaps generally have minimal effect on beetles and other hard-bodied insects; although, this isn’t 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 unknown; however, soaps may kill insect and mite pests in one of three ways: First, they may work by penetrating the fatty acids through the insect’s outer covering (cuticle) and dissolving or disrupting cell membranes; second, soaps may act as insect growth regulators, interfering with cellular metabolism and the production of growth hormones during metamorphosis; and third, they may block the spiracles (breathing pores), interfering with respiration.
Soap is a general term for the salts of fatty acids. There are a variety of fatty acids; however, only certain fatty acids have insecticidal properties. This is related to 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 fewer) have herbicidal properties that can kill plants. There’s a general misconception that any soap or detergent can be used as an insecticide. This is not true. Although, as already discussed, only a few select soaps have insecticidal properties, many household soaps and detergents have some activity on many soft-bodied insects when applied to plants as a 1% or 2% aqueous solution. Examples demonstrating the effectiveness of various dishwashing liquids and detergents on insect and mite pests are provided in Culture Notes, page XX.
Despite these examples, dishwashing liquids and laundry detergents are designed primarily 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. Additionally, plants with pubescent (hairy) leaves may be more susceptible to injury from dishwashing liquids and detergents. Dishwashing liquids and laundry detergents, like insecticidal soaps, lack any residual activity, and thus you need more frequent applications. 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 aren’t necessarily pure fatty acids. Most importantly, these solutions aren’t registered insecticides. Soap companies don’t intend for their products to be used as insecticides, as they have not gone through EPA registration processes.
Growers using biological control agents or natural enemies, including parasitoids and/or predators inadvertently may disrupt their biological control programs using soaps, even insecticidal soaps. Soaps may be directly and indirectly harmful to natural enemies. For example, one study showed insecticidal soap to be harmful to the predatory mite, Phytoseiulus persimilis. In another study, a 4% application rate of insecticidal soap was moderately harmful (80% to 99% mortality) to the western flower thrips predator, Neoseiulus (=Amblyseius) cucumeris.
The type of fatty acid, length of the carbon-based fatty acid chains and concentrations in many laundry and dish soaps aren’t known. In addition, coloring agents or perfumes in these products may compromise the insecticidal effectiveness. This often 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, you should avoid their use in greenhouses primarily because they aren’t registered insecticides. Even more important is that a pest control company generally will stand behind a product when there’s a problem. If you use a dish or laundry soap and plants are injured, there’s no recourse. As such, you may get washed up using soaps indiscriminately.
—Raymond A. Cloyd is assistant professor, extension specialist in ornamental entomology/IPM, University of Illinois, Urbana; rcloyd@uiuc.edu.