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 A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

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 organic agriculture or ecological farming or biological farming,Labelling, article 30 o''Regulation (EU) 2018/848 of the European Parliament and of the Council of 30 May 2024 on organic production and labelling of ...

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, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an ...

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 Coconut oil (or coconut fat) is an edible oil derived from the kernels, meat, and milk of the coconut palm fruit. Coconut oil is a white solid fat below around , and a clear thin liquid oil at higher temperatures. Unrefined varieties have a disti ...

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

hat A hat is a Headgear, 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 incorpor ...

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 union, supranational political union, political and economic union of Member state of the European Union, member states that are Geography of the European Union, located primarily in Europe. The u ...

, 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 a 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, bic ...

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

detergent A detergent is a surfactant or a mixture of surfactants with Cleanliness, cleansing properties when in Concentration, dilute Solution (chemistry), solutions. There are a large variety of detergents. A common family is the alkylbenzene sulfonate ...

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 (: tracheae or tracheas), also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all animals' lungs. The trachea extends from ...

, may disrupt

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

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 ...

). * Dissolving the wax layer on the

cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...

("skin"), which leads to water loss by evaporation. * Blocking breathing openings or trachea, which leads to suffocation. * Interfering with growth hormones. * Affecting insect

metabolism Metabolism (, from ''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 cellular processes; the co ...

Affected organisms

Insecticidal soap works best on soft-bodied insects and other

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

s such as

aphids Aphids are small sap-sucking insects in the Taxonomic rank, family Aphididae. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white Eriosomatinae, woolly ...

, adelgids,

mealybugs Mealybugs are insects in the family Pseudococcidae, unarmored scale insects found in moist, warm habitats. Of the more than 2,000 described species, many are considered pests as they feed on plant juices of greenhouse plants, house plants and su ...

, spider mites,

thrips Thrips (Order (biology) , order Thysanoptera) are minute (mostly long or less), slender insects with fringed wings and unique asymmetrical mouthparts. Entomologists have species description , described approximately 7,700 species. They fly on ...

, jumping plant lice,

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

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 A ...

, and

sawfly Sawflies are wasp-like insects that are in 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 plant ...

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 ...

s and

leafhopper Leafhopper is the common name for any species from the family (biology), family Cicadellidae: based on the type genus ''Cicadella''. These minute insects, colloquially known as hoppers, are plant feeders that suck plant sap from grass, shrubs, or ...

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

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 taxon, extant group in the tribe Bombini, though a few extinct r ...

s, and

hoverflies Hoverflies, also called flower flies or syrphids, make up the insect family (biology), family Syrphidae. As their common name suggests, they are often seen Hover (behaviour), hovering or nectaring at flowers; the adults of many species feed main ...

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 Hemerobiiformia are a suborder of insects in the order Neuroptera that include most of the lacewings, antlions and their allies. The phylogeny of the Neuroptera was explored in 2014 using mitochondrial DNA sequences. The results indicate 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 (; ; sometimes spelled nymphe) is a minor female nature deity in ancient Greek folklore. Distinct from other Greek goddesses, nymphs are generally regarded as personifications of nature; they are typically tied to a specific place, land ...

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

. 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 ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

and

moss Mosses are small, non-vascular plant, non-vascular flowerless plants in the taxonomic phylum, division Bryophyta (, ) ''sensu stricto''. Bryophyta (''sensu lato'', Wilhelm Philippe Schimper, Schimp. 1879) may also refer to the parent group bryo ...

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, Japanese maple (''Acer''), ''

Sorbus aucuparia ''Sorbus aucuparia'', commonly called rowan (, also ) and mountain-ash, is a species of deciduous tree or shrub in the rose family. The tree has a slender trunk with smooth bark, a loose and roundish crown, and its leaves are pinnate in pairs o ...

'' (mountain ash), cherimoya fruit, '' Lamprocapnos'' (bleeding heart), and

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

. Other sensitive plants include: '' Portulaca'', some tomato varieties, ''

Crataegus ''Crataegus'' (), commonly called hawthorn, quickthorn, thornapple, Voss, E. G. 1985. ''Michigan Flora: A guide to the identification and occurrence of the native and naturalized seed-plants of the state. Part II: Dicots (Saururaceae–Cornacea ...

'' (hawthorn),

cherries A cherry is the fruit of many plants of the genus ''Prunus'', and is a fleshy drupe (stone fruit). Commercial cherries are obtained from cultivars of several species, such as the sweet ''Prunus avium'' and the sour ''Prunus cerasus''. The name ...

plum A plum is a fruit of some species in Prunus subg. Prunus, ''Prunus'' subg. ''Prunus'.'' Dried plums are often called prunes, though in the United States they may be labeled as 'dried plums', especially during the 21st century. Plums are ...

, '' Adiantum'' (maidenhair fern), ''

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

'' (crown of thorns), ''

Lantana camara ''Lantana camara'' (common lantana) is a species of flowering plant in 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 introduced i ...

'', '' Tropaeolum'' (nasturtium), ''

Gardenia jasminoides ''Gardenia jasminoides'', commonly known as gardenia and cape jasmine, is an evergreen flowering plant in the coffee family Rubiaceae. It is native to the subtropical and northern tropical parts of the Far East. Wild plants range from 30 centime ...

'', ''

Lilium longiflorum ''Lilium longiflorum'', often called the Easter lily, is a species of plant endemic to both Taiwan and Ryukyu Islands (Japan). '' Lilium formosanum'', a closely related species from Taiwan, has been treated as a variety of Easter lily in the past ...

'' (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 sin ...

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 a period of 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, ...

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 sca ...

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