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Understanding Insecticidal Soaps and Detergents

Don’t Get “Lathered:” Understanding Insecticidal Soaps and Detergents

There continues to be a growing interest in the use of pesticides (in this case, insecticides and miticides) that are both effective and compatible with biological control agents or natural enemies such as parasitoids and predators. These may be referred to as “alternative” pesticides and include insect growth regulators, insecticidal soaps, horticultural oils, feeding inhibitors, beneficial fungi and bacteria, and other micro-organisms. Many “alternative” pesticides are used in greenhouses and conservatories, and generally leave minimal residues, are less toxic to humans, and are short-lived in the environment. In addition, “alternative” pesticides are non-toxic to the user unless ingested at high doses. Commonly recommended and used pesticides, in both greenhouses and conservatories, are insecticidal soaps.

Insecticidal soaps are applied to “control” or “suppress” a variety of soft-bodied insect and mite pests including aphids, scales, thrips, mealybugs, whiteflies, and the twospotted spider mite (Tetranychus urticae). A soap is a substance derived from the synthesis of an alkali such as potassium (soft soap) or sodium (hard soap) hydroxide on a fat. Fats are typically a blend of particular fatty acid chain lengths. Soap is a general term for the salts of fatty acids, which are the primary components of the fats and oils present in animals and plants.

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 widely used as a liquid hand soap in public restrooms. It is now 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.

Soap in one of the oldest contact insecticides used in dealing with certain sucking (phloem-feeding) insect pests. Most soft-bodied insect and mite pests such as aphids, scale and mealybug crawlers, thrips, whiteflies, and many mites are susceptible to soap applications. Soaps are typically less active on beetles and other hard-bodied insects due to the insect’s thickened or hardened cuticle, which is resistance to penetration. However, this is not always the case since soaps have been demonstrated to kill certain 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 (if any) activity on insect or mite pests because soap residues degrade rapidly. As such, thorough coverage of all plant parts (leaves and stems) is essential in order to obtain sufficient mortality, prevent the development of additional generations, and thus avoid outbreaks of insect and mite pests. The effectiveness of insecticidal soaps on insect and mite pests may be associated with surface tension (forces on liquid droplet surfaces that prevent them from spreading over treated surfaces) and contact angle (angle at which a liquid interface contacts solid surface) of the spray solution. Additionally, it has been shown that insecticidal soaps are more active at higher temperatures (90°F or 32°C) and relative humidities (>85%).

The effect of insecticidal soaps on insect and mite pests may be associated with the rate (dosage) used. In general, an increase in the rate will typically lead to an increase in the mortality of insect and/or mite pests although it is important to make sure that these higher rates don’t result in plant injury or phytotoxicity. It should be noted that phytotoxicity will depend on plant size and age, and whether plants are herbaceous or woody. Spray applications of a 0.1% concentration of insecticidal soap led to a 72% and 79% removal of the cotton/melon aphid (Aphis gossypii) and the green citrus aphid (Aphis spiraecola) without any harm to Pyracantha spp. shrubs. The insecticidal soap sprays also reduced populations of both aphid species below that which could cause economic damage. Moreover, there may be differences in susceptibility to insecticidal soaps based on life stage (egg, nymph, larvae, or adult) with immatures (young) typically more susceptible than adults. For example, young bean aphids (Aphis fabae) are relatively easier to kill with insecticidal soaps than adults.

Although insecticidal soaps are comparatively less toxic, primarily due to minimal residues, to biological control agents or natural enemies than conventional pesticides in the chemical classes: organophosphate, carbamate, and pyrethroid; insecticidal soaps may be directly harmful to natural enemies. For example, ladybird beetle and green lacewing larvae, when present on treated plants, are killed by wet sprays. Even though applications of insecticidal soaps are “minimally” harmful to the predatory mite, Neoseiulus =Amblyseius cucumeris, sprays at a 4% rate are very toxic (90% mortality after 48 hours). Direct spray applications of insecticidal soap are extremely harmful (100% mortality) to the predatory mite, Phytoseiulus persimilis; however, there are no toxic effects 3 days after release. Overall, once the residues of insecticidal soap sprays have dissipated, they are less harmful to natural enemies.

The mode of action of insecticidal soaps is still not well-understood although there appears to be four ways by which insecticidal soaps kill insect and mite pests. First, soaps may penetrate through the fatty acids present in the insects’ outer covering (cuticle) thus dissolving or disrupting cell membrane integrity. This causes cells to leak and collapse, destroying respiratory functions, and resulting in dehydration and death of insect and/or mite pests. Second, soaps might act as insect growth regulators interfering with cellular metabolism and the production of growth hormones during metamorphosis (change in form). Third, it has been proposed that soaps block the spiracles (breathing pores) of insect pests, interfering with respiration. Finally, soaps may uncouple oxidative phosphorylation or inhibit the production of adenosine tri-phosphate (ATP).

There are a variety of fatty acids; however, only certain fatty acids have insecticidal properties. This is simply 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 (weed-killing) properties, so using materials that have short chain fatty acids will 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. In fact, most commercially-available insecticidal soaps contain potassium oleate (potassium salt of oleic acid), which is one of the most toxic soaps.

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