Insecticidal soap is used to control many plant insect pests. Soap has been used for more than 200 years as an insect control. Because insecticidal soap works on direct contact with pests via the disruption of cell membranes when the insect is penetrated with fatty acids, the insect's cells leak their contents causing the insect to dehydrate and die. Insecticidal soap is sprayed on plants until the entire plant is saturated because the insecticidal properties of the soap occurs when the solution is wet. Soaps have a low

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

ian toxicity and are therefore considered safe to be used around children and pets, and may be used in

organic farming Organic farming, also known as ecological farming or biological farming,Labelling, article 30 o''Regulation (EU) 2018/848 of the European Parliament and of the Council of 30 May 2018 on organic production and labelling of organic products and re ...

Composition

Insecticidal soap's active ingredient is most often a potassium salt of fatty acids. Insecticidal soap should be based on long-chain

fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, fr ...

s (10–18 carbon atoms), because shorter-chain fatty acids tend to be damaging for the plant (

phytotoxicity Phytotoxicity describes any adverse effects on plant growth, physiology, or metabolism caused by a chemical substance, such as high levels of fertilizers, herbicides, heavy metals, or nanoparticles. General phytotoxic effects include altered plan ...

). Short (8-carbon) fatty-acid chains occur for example in

coconut oil frameless , right , alt = A cracked coconut and a bottle of coconut oil Coconut oil (or coconut butter) is an edible oil derived from the wick, meat, and milk of the coconut palm fruit. Coconut oil is a white solid fat; in warmer climates duri ...

and soaps based thereon. "'Green Soap' is a potassium/coconut oil soap ....

hat A hat is a head covering which is worn for various reasons, including protection against weather conditions, ceremonial reasons such as university graduation, religious reasons, safety, or as a fashion accessory. Hats which incorporate mecha ...

has also been shown to be effective, as an unlabeled insecticide, in controlling soft-bodied insects including aphids". Recommended concentrations of insecticidal soap are typically in the range 1–2 percent soap mixed with water. One manufacturer recommends a concentration of 0.06% to 0.25% (pure soap equivalent) for most agricultural applications.; another one recommends concentrations of 0.5 to 1% pure soap equivalent. In the

European Union The European Union (EU) is a supranational political and economic union of member states that are located primarily in Europe. The union has a total area of and an estimated total population of about 447million. The EU has often been des ...

, fatty acid potassium salts are registered and allowed as insecticide at a 2% concentration. Insecticidal soap is most effective if it is dissolved in soft water, since the fatty acids in soap tend to precipitate in

hard water Hard water is water that has high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of limestone, chalk or gypsum, which are largely made up of calcium and magnesium carbonates, bicarbo ...

, thereby reducing the effectivity. Insecticidal soap is sold commercially for aphid control. Labels on these products may not always use the word soap, but they will list "potassium salts of

s" or "potassium

laurate Lauric acid, systematically dodecanoic acid, is a saturated fatty acid with a 12-carbon atom chain, thus having many properties of medium-chain fatty acids. It is a bright white, powdery solid with a faint odor of bay oil or soap. The salts and ...

" as the active ingredient. Certain types of household soaps (not synthetic

detergent A detergent is a surfactant or a mixture of surfactants with cleansing properties when in dilute solutions. There are a large variety of detergents, a common family being the alkylbenzene sulfonates, which are soap-like compounds that are more ...

s, ) are also suitable, but it may be difficult to tell the composition and water content from the label. Potassium-based soaps are typically soft or liquid.

Mechanism of action

The mechanism of action is not exactly understood. Possible mechanisms are: * Soap, which enters via the insect's

trachea The trachea, also known as the windpipe, is a Cartilage, cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs. The trachea extends ...

, may disrupt

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

s, resulting in the cell contents leaking from the damaged cells (

cytolysis Cytolysis, or osmotic lysis, occurs when a cell bursts due to an osmotic imbalance that has caused excess water to diffuse into the cell. Water can enter the cell by diffusion through the cell membrane or through selective membrane channels ...

). * Soap may dissolve the wax layer on the cuticle ("skin"), which leads to water loss by evaporation. * Soap may block breathing openings or trachea, which leads to suffocation. * Soap may interfere with growth hormones. * Soap may affect insect

metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...

Affected organisms

Insecticidal soap works best on soft-bodied insects and

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

s such as

aphids Aphids are small sap-sucking insects and members of the superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white woolly aphids. A t ...

, adelgids,

mealybugs Mealybugs are insects in the family Pseudococcidae, unarmored scale insects found in moist, warm habitats. Many species are considered pests as they feed on plant juices of greenhouse plants, house plants and subtropical trees and also act as a ...

, spider mites,

thrips Thrips ( order Thysanoptera) are minute (mostly long or less), slender insects with fringed wings and unique asymmetrical mouthparts. Different thrips species feed mostly on plants by puncturing and sucking up the contents, although a few are ...

, jumping plant lice,

scale insect Scale insects are small insects of the order Hemiptera, suborder Sternorrhyncha. Of dramatically variable appearance and extreme sexual dimorphism, they comprise the infraorder Coccomorpha which is considered a more convenient grouping than the ...

whiteflies Whiteflies are Hemipterans that typically feed on the undersides of plant leaves. They comprise the family Aleyrodidae, the only family in the superfamily Aleyrodoidea. More than 1550 species have been described. Description and taxonomy The ...

, and

sawfly Sawflies are the insects of the suborder Symphyta within the order Hymenoptera, alongside ants, bees, and wasps. The common name comes from the saw-like appearance of the ovipositor, which the females use to cut into the plants where they lay ...

larvae. It can also be used for

caterpillar Caterpillars ( ) are the larval stage of members of the order Lepidoptera (the insect order comprising butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawflies (suborder Sym ...

s and

leafhopper A leafhopper is the common name for any species from the family Cicadellidae. These minute insects, colloquially known as hoppers, are plant feeders that suck plant sap from grass, shrubs, or trees. Their hind legs are modified for jumping, and a ...

s, but these large-bodied insects can be more difficult to control with soaps alone. Many pollinators and predatory insects such as

lady beetles Coccinellidae () is a widespread family of small beetles ranging in size from . They are commonly known as ladybugs in North America and ladybirds in Great Britain. Some entomologists prefer the names ladybird beetles or lady beetles as they ...

bumblebee A bumblebee (or bumble bee, bumble-bee, or humble-bee) is any of over 250 species in the genus ''Bombus'', part of Apidae, one of the bee families. This genus is the only extant group in the tribe Bombini, though a few extinct related gener ...

s, and

hoverflies Hover flies, also called flower flies or syrphid flies, make up the insect family Syrphidae. As their common name suggests, they are often seen hovering or nectaring at flowers; the adults of many species feed mainly on nectar and pollen, while ...

are relatively unaffected. However, soap will kill predatory mites that may help control spider mites. Also, the soft-bodied aphid-eating larvae of lady beetles,

lacewing The insect order Neuroptera, or net-winged insects, includes the lacewings, mantidflies, antlions, and their relatives. The order consists of some 6,000 species. Neuroptera can be grouped together with the Megaloptera and Raphidioptera in t ...

, and hoverflies may be affected negatively. According to one study a single soap application killed about 15% of lacewing and lady-beetle larvae, and about 65% of predatory mites ('' Amblyseius andersoni''). Green peach aphids are difficult to control since they reproduce quickly (one adult female can deposit up to four

nymphs A nymph ( grc, νύμφη, nýmphē, el, script=Latn, nímfi, label=Modern Greek; , ) in ancient Greek folklore is a minor female nature deity. Different from Greek goddesses, nymphs are generally regarded as personifications of nature, are typ ...

per day) because they tend to reside under the leaves and in leaf axils ("leaf armpits"), where they may not be wetted by a soap spray. Manufacturers indeed state that their insecticidal soaps are only suitable for controlling green peach aphids if used in combination with another insecticide, whereas the same soaps can control other aphids on their own. Among green peach aphids that are in contact with a 2% soap solution, around 95% of the adults and 98% of

die within 48 hours. At 0.75% concentration, the mortality rates are reduced to 75% and 90%, respectively. Since 2011, insecticidal soap has also been approved in the United States for use against

powdery mildew Powdery mildew is a fungal disease that affects a wide range of plants. Powdery mildew diseases are caused by many different species of ascomycete fungi in the order Erysiphales. Powdery mildew is one of the easier plant diseases to identify, as ...

. In the European pesticide registration, its use as an insecticide is listed for aphids, white fly, and spider mites. It may not be used against

algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...

and

moss Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta (, ) '' sensu stricto''. Bryophyta (''sensu lato'', Schimp. 1879) may also refer to the parent group bryophytes, which comprise liverworts, mosses, and hor ...

Use

Insecticidal soap solution will only kill pests on contact; it has no residual action against aphids that arrive after it has dried. Therefore, the infested plants must be thoroughly wetted. Repeated applications may be necessary to adequately control high populations of pests. Soap spray may damage plants, especially at higher concentrations or at temperatures above 32 °C (90 °F). Plant injury may not be apparent until two days after application. Some plant species are particularly sensitive to soap sprays. Highly sensitive plants include:

horse chestnut The genus ''Aesculus'' ( or ), with species called buckeye and horse chestnut, comprises 13–19 species of flowering plants in the family Sapindaceae. They are trees and shrubs native to the temperate Northern Hemisphere, with six species n ...

, Japanese maple (''Acer''), '' Sorbus aucuparia'' (mountain ash), Cherimoya fruit, ''

Lamprocapnos ''Lamprocapnos spectabilis'', bleeding heart, fallopian buds or Asian bleeding-heart, is a species of flowering plant belonging to the fumitory subfamily ( fumarioideae) of the poppy family Papaveraceae, and is native to Siberia, northern China ...

'' (bleeding heart), and

sweet pea The sweet pea, ''Lathyrus odoratus'', is a flowering plant in the genus ''Lathyrus'' in the family (biology), family Fabaceae (legumes), native plant, native to Sicily, southern Italy and the Aegean Islands. It is an annual plant, annual climbi ...

. Other sensitive plants are, for example: ''

Portulaca ''Portulaca'' (, is the type genus of the flowering plant family Portulacaceae, with over 100 species, found in the tropics and warm temperate regions. They are known as the purslanes. Common purslane (''Portulaca oleracea'') is widely consume ...

'', some tomato varieties, '' Crataegus'' (hawthorn), cherries,

plum A plum is a fruit of some species in ''Prunus'' subg. ''Prunus'.'' Dried plums are called prunes. History Plums may have been one of the first fruits domesticated by humans. Three of the most abundantly cultivated species are not found i ...

, ''

Adiantum ''Adiantum'' (), the maidenhair fern, is a genus of about 250 species of ferns in the subfamily Vittarioideae of the family Pteridaceae, though some researchers place it in its own family, Adiantaceae. The genus name comes from Greek, meaning "un ...

'' (maidenhair fern), ''

Euphorbia milii ''Euphorbia milii'', the crown of thorns, Christ plant, or Christ thorn, is a species of flowering plant in the spurge family Euphorbiaceae, native to Madagascar. The species name commemorates Baron Milius, once Governor of Réunion, who introdu ...

'' (crown of thorns), ''

Lantana camara ''Lantana camara'' (common lantana) is a species of flowering plant within the verbena family ( Verbenaceae), native to the American tropics. It is a very adaptable species, which can inhabit a wide variety of ecosystems; once it has been introdu ...

'', ''

Tropaeolum ''Tropaeolum'' , commonly known as nasturtium (; literally "nose-twister" or "nose-tweaker"), is a genus of roughly 80 species of annual and perennial herbaceous flowering plants. It was named by Carl Linnaeus in his book ''Species Plantarum'' ...

'' (nasturtium), '' Gardenia jasminoides'', ''

Lilium longiflorum ''Lilium longiflorum'' (Japanese: テッポウユリ, ''Teppōyuri''), often called the Easter lily, is a plant endemic to both Taiwan and Ryukyu Islands (Japan). '' Lilium formosanum'', a closely related species from Taiwan, has been treated as ...

'' (Easter lily).

Conifer Conifers are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single ...

s under (drought) stress or with tender new growth are sensitive as well. Damage may occur as yellow or brown spotting on the leaves, burned tips, or leaf scorch. Plants under

drought A drought is defined as drier than normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosenfeld, J. Tierney, an ...

stress, young transplants, unrooted

cutting Cutting is the separation or opening of a physical object, into two or more portions, through the application of an acutely directed force. Implements commonly used for wikt:cut, cutting are the knife and saw, or in medicine and science the scal ...

s and plants with soft young growth tend to be more sensitive. Sensitivity may be tested on a small portion of a plant or plot before a full-scale application. One manufacturer recommends that applications are done with 7- to 14-day intervals, with a maximum of three applications, as repeated applications may aggravate phytotoxicity. In addition, water conditioning agents can increase phytotoxicity. Thanks to its low mammalian toxicity, application of insecticidal soap is typically allowed up to the day of harvest.

References

{{reflist, refs= "Soaps" and Detergents: Should They Be Used on Roses?
R.A. Cloyd, American Rose Society (2018).
M.L. Flint, University of California Davis, July 2013.
W.S. Cranshaw, University of Colorado. March 2008.
J. D. Ubl, Clemson University, July 2009. Insecticidal Soaps - Modes of Action
(video). L.L. Keeley (20 Aug 2013). Toxicity of biopesticides to green apple aphid, predatory insects and mite in an apple-tree orchard
L. Raudonis et al., Zemdirbyste-Agriculture, vol. 97, No. 1 (2010), p. 49–54. Pesticide product label for Super insecticidal soap concentrate
US Environmental protection agency, July 2011. According to the ''Super insecticidal soap concentrate label'' above: 0.5 pint of a 25% concentrate per 25 gallons = 0.06%. Toxicity effects of an insecticidal soap on the green peach aphid
{{webarchive, url=https://web.archive.org/web/20150402192822/https://www.mysciencework.com/publication/file/1970822/toxicity-effects-of-an-insecticidal-soap-on-the-green-peach-aphid-homoptera-aphididae , date=2015-04-02 . E. Tremblay et al., Phytoprotection 90: 35-39 (2009). Pesticide product label for M-Pede insecticide miticide fungicide
US Environmental protection agency, July 2011. Fatty acids C7 to C20
{{webarchive, url=https://web.archive.org/web/20150419172351/http://ec.europa.eu/sanco_pesticides/public/index.cfm?event=activesubstance.detail&language=NL&selectedID=1322 , date=2015-04-19 . EU-database voor pesticides. Review report for the active substance Fatty Acids C7 to C20
{{webarchive, url=https://web.archive.org/web/20150419172318/http://ec.europa.eu/sanco_pesticides/public/index.cfm?event=activesubstance.ViewReview&id=746 , date=2015-04-19 . Europese Commissie (2013). Insecticides Sustainable agriculture

Composition

Mechanism of action

Affected organisms

Use

See also

References