What is insecticidal soap?
Insecticidal soap – also known as soft soap or horticultural soap – is a special kind of soap that’s designed for use specifically on plants. It’s typically made using a potassium salt from fatty acids (the fatty acids can be animal- or plant-based). This creates a gentle foam that works by damaging the cell membranes of soft-bodied insects such as aphids, whiteflies, spider mites, and mealybugs, causing them to dehydrate and die. Be aware it does not discriminate and can harm beneficial insects as well as the ‘pests’ you want to control. Considered non-toxic, it can also help control certain types of leaf fungi, such as sooty mould and honeydew. Some manufacturers also claim insecticidal soap can prevent insect attacks by leaving an unpenetrable film over each leaf.
Insecticidal soap is not the same as hand soap, which is a detergent. Detergents are designed to cut through grease and oil, and could potentially damage plant leaves by harming their waxy coating.
How to use insecticidal soap
To minimise potential damage to beneficial insects, inspect plants first to make sure you are targeting pests such as aphids, and not the species that eat them, such a ladybird and hoverfly larvae. Avoid spraying plants in strong sunlight which can cause scorching to foliage and flowers.
Is insecticidal soap pet and wildlife safe?
While insecticidal soap doesn’t contain toxic chemicals and is unlikely to harm pets as soap is unpalatable, it can have harmful effects on wildlife as soap spray kills both unwanted and beneficial insects. Also, insects like aphids are a valuable source of food for birds and other species such as wasps, hoverflies, lacewings and ladybirds, so think twice before using insecticidal soap on outdoor plants.
How to make homemade insecticidal soap
Making homemade sprays using household products carries the risk of harm to your plants as these are all detergents. The safest approach when using insecticidal soap for plants is to buy horticultural soap as this is suitable for use on plants, usually in a ready-to-use spray formulation.
If you want to make your own insecticidal soap, choose a simple eco-friendly product such as mild washing up liquid, although bear in mind this is still a detergent and could damage your plants. As a guide, use half a teaspoon of washing up liquid diluted in a litre of water, and apply using a clean hand pressure sprayer. Try it on just one or two plants first and wait several days to observe the results before further use.
Advice on buying insecticidal soap
- Insecticidal soap works by dehydrating soft-bodied insects such as aphids and mealybugs, but could also harm beneficial insects such as ladybird and lacewing larvae, so think carefully about whether this is the product for you
Where to buy insecticidal soap online
Frequently asked questions
What is a good alternative to insecticidal soap?
For plants growing outside, the best alternative to insecticidal soap is to establish a natural balance in your garden by encouraging wildlife and beneficial insects. House sparrows, chiffchaffs, ladybirds, lacewings, hoverflies and wasps are just some of the species that rely on aphids for food, so leaving them to find and control aphids naturally is the best alternative action. If necessary, you can tackle small infestations by squashing them between finger and thumb or dislodging them with a jet of water from a hose (this may also affect insect predators such as ladybird larvae). Indoors or in a conservatory or greenhouse, you could use a biological control to target a specific pest. This is safe for pets, children, and wildlife.
Can you use insecticidal soap on your barbecue?
Insecticidal soap won’t have the same ability to cut through grease and fat in the way that detergents can. There’s no reason not to use a stronger cleaning product on your barbecue.
