Insecticidal soap

Structural formulas of the potassium salts of individual fatty acids (examples)

Potassium oleate, the potassium salt of oleic acid .

Potassium palmitate, the potassium salt of palmitic acid .

Potassium stearate, the potassium salt of stearic acid .

Insecticidal soap is based on the potassium salts of fatty acids and is used to combat many plant pests. Since insecticide soap only works if it comes into direct contact with the pests, it is sprayed onto plants in such a way that the entire plant is wetted. Soaps have low mammalian toxicity and are therefore considered safe to use around children and pets, and can be used in organic farming .

composition

Insecticide soap should be based on long-chain fatty acids (10-18 carbon atoms), as short-chain fatty acids are harmful to the plant ( phytotoxicity ). Short (8-carbon) fatty acid chains come e.g. B. in coconut oil (less than 10%) and soaps based on this oil. The recommended concentrations are typically in the range of 1–2%. 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. Fatty acid potassium salts are registered in the European Union and allowed as an insecticide in a concentration of 2%.

Insecticidal soap is most effective when dissolved in soft water, as the fatty acids in the soap tend to form water -insoluble calcium soaps in hard water that precipitate. This reduces the effectiveness.

Insecticidal soap is sold commercially for aphid control. The word soap is not always used on the labels of these products, but the active ingredient is "potassium salts of fatty acids " or "potassium lauric acid ". Certain types of household soaps (not synthetic detergents ) are also suitable, but it can be difficult to tell the composition and water content on the label. Potassium soaps are typically soft or liquid.

Mode of action

The mechanism of action is not well understood. Possible mechanisms are:

• Soap entering through the insect's trachea can disrupt cell membranes , causing cell contents to leak out of the damaged cells ( cytolysis ).
• Soap can dissolve the wax layer on the arthropod cuticle ("skin"), causing water loss through evaporation.
• Soap can block your breathing holes or windpipe, causing choking.
• Soap can interfere with growth hormones.
• Soap can disrupt the metabolism of insects.

Affected organisms

Insecticide soap works best in weichkörperigen insects and arthropods such as aphids , Adelgidae , mealybugs , spider mites , thrips , psyllid , scale insects , greenhouse Motte scale insects and larvae of sawflies . It can also be used on caterpillars and dwarf leafhoppers, but these large-bodied insects can be more difficult to control with soaps alone. Many pollinators and predatory insects such as ladybirds , bumblebees and hoverflies are relatively unaffected. However, soap kills predatory mites, which can help control spider mites. The soft-bodied aphid-eaters of ladybirds, netwings and hoverflies can also be negatively affected. According to one study, a single application of soap killed around 15% of lacewing and ladybird larvae and around 65% of predatory mites ( Amblyseius andersoni ).

The green peach aphid is difficult to control as it reproduces quickly (an adult female can shed up to four nymphs per day) because they tend to hide under leaves and in leaf axils where a soap spray cannot reach them can. Manufacturers claim that their insecticidal soaps are only suitable for controlling green peach aphids when combined with another insecticide, while the same soaps can control other aphids on their own. Of the green peach lice that come into contact with a 2% soap solution, about 95% of adults and 98% of nymphs die within 48 hours. At a concentration of 0.75%, the mortality rates are reduced to 75% and 90%, respectively.

Since 2011, insecticidal soap has also been used in the USA against powdery mildew . Their use as an insecticide for aphids, greenhouse whiteflies and spider mites is listed in the EU plant protection product approval. In different concentrations it can also be used against algae and moss .

application

Insecticidal soap solution only kills pests on contact; it has no residual action against aphids that reach the plant after the soap has dried. Therefore, the infected plants must be thoroughly moistened. Repeated applications may be necessary to adequately control high populations of pests.

Soap spray can damage plants, especially at higher concentrations or temperatures above 32 ° C (90 ° F). Plant injuries can appear only two days after application. Some types of plants are particularly sensitive to soap sprays. The highly sensitive plants include: horse chestnut , Japanese maple , rowanberry , cherimoya , bleeding heart , and fragrant flat pea . Other sensitive plants are e.g. For example: purslane , some types of tomatoes, hawthorns , cherries , plums , maidenhair ferns , Christ thorn (Euphorbia) , changing florets , nasturtiums , gardenia , Easter lily . Also conifers under (dry) stress or with tender new growth are sensitive.

Damage can be caused by yellow or brown spots on the leaves, burnt tips or leaf burns. Plants under drought stress, young plants, unrooted cuttings and plants with tender young growth tend to be more sensitive. Sensitivity can be tested on a small part of a plant or terrain prior to large-scale application.

One manufacturer recommends that applications be performed every 7 to 14 days with a maximum of three applications, as repeated applications can exacerbate phytotoxicity. In addition, water treatment agents can increase phytotoxicity.

Due to its low toxicity to mammals, the use of insecticidal soap is usually allowed up to the day before harvest.

Individual evidence

1. ↑ ^{a b c d e} “Soaps” and Detergents: Should They Be Used on Roses? RA Cloyd, American Rose Society (2018).
2. ^ ^{A b c} Insect Control: Soaps and Detergents . WS Cranshaw, University of Colorado. March 2008.
3. ↑ ^{a b} Aphids: Integrated Pest Management for Home Gardeners and Landscape Professionals . ML Flint, University of California Davis, July 2013.
4. ↑ ^{a b c} Insecticidal Soaps for Garden Pest Control . JD Ubl, Clemson University, July 2009.
5. ↑ ^{a b c d e f g h i} Pesticide product label for Super insecticidal soap concentrate . US Environmental Protection Agency, July 2011.
6. ↑ According to the Super insecticidal soap concentrate label above: 0.5 pint of a 25% concentrate per 25 gallons = 0.06%.
7. ↑ ^{a b c d} Pesticide product label for M-Pede insecticide miticide fungicide . US Environmental Protection Agency, July 2011.
8. ↑ Fatty acids C7 to C20 - EU database voor pesticides ( Memento from April 19, 2015 in the Internet Archive ).
9. ↑ ^{a b c} Review report for the active substance Fatty Acids C7 to C20 - Europese Commissie (2013) ( Memento from April 19, 2015 in the Internet Archive ).
10. ^ Insecticidal Soaps - Modes of Action (video). LL Keeley (Aug 20, 2013).
11. ↑ ^{a b} 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), pp. 49-54.
12. ↑ ^{a b} Toxicity effects of an insecticidal soap on the green peach aphid. In: E. Tremblay et al., Phytoprotection 90: 35–39 (2009) ( Memento from April 2, 2015 in the Internet Archive )