Insecticidal soap

One of the least expensive means of pest control available, insecticidal soaps, are a big drawcard for the environmentally-conscious. However, knowing how to use them properly is crucial, says Paul Donovan.

Insecticidal soaps are effective against small, soft-bodied species, such as the black bean aphids (Aphis fabae) pictured here.
Photo: Paul Donovan

Treating aphid, spider mite and other pest infestations with ‘insecticidal soap’ doesn’t mean reaching for the dishwashing liquid or laundry detergent, diluting it with water, and then spraying it on the infested plant. This will not kill worrisome pests; in fact, it is more likely to kill the plant. ‘Soap’ can loosely be defined as a mixture of ingredients, the primary aim of which is to dissolve the grease on plates, pots and pans, and rid your body of dirt.

Only those with the correct potassium/salts/fatty acid mix have insecticidal properties. These include products such as Concern and Safer’s insecticidal soap, which are used in the same way as conventional insecticides, and generally applied with a sprayer. These are contact insecticides, which means they work when they ‘touch’ the pest, usually disrupting the permeability of the cell membranes. The cells then begin to disintegrate, killing the insect quite quickly.

Alternatively, the soap may strip away the skin’s protective coating, reducing its ability to retain fluid, so that the insect dies from dehydration. Therefore, insecticidal soaps are far more effective when used during infestation and are a waste if used as a precautionary measure. By the time the pests arrive, any residual soap won’t be strong enough to serve as a contact insecticide.

A Misnomer
There is a mistaken belief that insecticidal soaps are effective across a wide range of insect types. The truth is, they work best on small, soft-bodied species such as aphids, mealy bugs, greenfly, scale insects and spider mites. (A word of caution here: insecticidal soaps kill not only spider mites, but natural predatory mites that help keep the spider mite numbers in check).

That said, soaps can be used to ‘assist’ in controlling larger pests, such as caterpillars, but they will take longer to work, due to the increased body mass of the pest being targeted.Several applications may be required and this can cause undue stress to the plant and may be less cost-effective than other methods of control.

Effective against Smaller pests
Insecticidal soaps have been shown to be between 60% and 70% effective against small pests. Their biggest appeal is that many predatory insects, such as ladybirds, are not affected, (although their larvae are). Furthermore, important pollinators such as bees are also not affected.

Beware of leaf scorching
You need to take care when choosing and using an insecticidal soap, as some may trigger phytotoxicity in the plant. This is when the soap is toxic to the plant and ends up killing it. The soap causes the leaves to develop brown or yellow patches, or curl up at the end and fall off – a condition known as ‘leaf scorching’.

The label on the soap should tell you which varieties of plants not to use it on, but these warnings seldom cover all types. If you are unsure whether the plant will be sensitive to the soap, test it on a leaf first and wait 24 hours. If the leaf shows no signs of distress, you can be quite sure the soap is safe to use on the whole plant.

It’s also important to limit the number of times the plant is sprayed. A large number of treatments, especially over a short period of time, can greatly increase the risk of phytotoxicity, even in resistant strains. You should also avoid using insecticidal soaps on plants that are already under stress, such as those that have just undergone replanting or are in the early stages of growth. These too are susceptible to phytotoxicity.

Importance of soap concentration
Insecticidal soaps must be mixed according to the directions on the label – this is usually a 2% solution. Mix the soap too strong and you risk phytotoxicity; any weaker and its effectiveness is reduced. There’s also some evidence to show that mixing with hard water can bring about the same effect as if you were to mix the soap at a higher concentration. Heavy metals in the water bind to the soap particles, making it less effective and more toxic. In other words, the effectiveness of the soap has more to do with using the correct concentration and water type than the actual amount you spray on the pests.

Minimum Environmental impact
Insecticidal soaps have minimal impact on the environment, with the added bonus of low toxicity to the end-user. However, always wear protective gloves when spraying insecticidal soap, as it may cause mild irritation to the skin. Protective glasses are also a good idea because, should the substance enter the eyes, it has a similar effect as suds from a bar of bath soap. In either case, flushing with water will bring about immediate relief.

As one of the least expensive means of pest control at our disposal, insecticidal soaps are a big draw to the environmentally conscious. However, to get the most out of them, it is wise to know how to use them properly and understand that, as effective as they are, they do have their limitations.

Paul Donovan, who has a farming background, is a biologist and has worked with reptiles and insects in zoological collections in the UK for more than 30 years. He is currently based in Botswana, where he promotes the protection and use of biological control agents and advises farmers on the best ways to control crop pests. Contact him at
[email protected].