Regular readers of this blog may have noticed I often recommend insecticidal soap for controlling insect pests found on plants. Why? Because this product is safe for humans, inexpensive, ecological and effective against many insects. Plus I don’t have to give elaborate explanations on how to use it: it’s pretty basic.
On the down side, though, for some reason, the term seems confusing. I regularly get questions from people that want to know what I mean by “insecticidal soap”, as if “insecticidal” and “soap” and didn’t belong together. No, I’m not referring to a homemade soap-based product (although that’s possible; more on that later), but the commercial product, widely available under the name insecticidal soap. You’ll find it in any garden center, probably under more than one trademark.
What Is It?
Insecticidal soap is an organic product made of potassium salts of fatty acids and is derived from plant oils. In other words, it is just what it says it is, a soap. In fact, it’s a pure soap, with no other ingredients. Manufacturers specifically use long chain fatty acids to make insecticidal soaps because short chain fatty acids, such as those used in dishwashing liquids, tend to be toxic to plants and indeed are often used as herbicides.
How Soap Kills Pests
Insecticidal soap acts by contact. It is effective only when it is wet and has no residual effect. In other words, it loses all utility once it dries out. Therefore you can’t use it to prevent insects and mites, it must touch and coat living pests to be effective.
Soap kills insects in several ways:
- It penetrates the arthropod’s cuticle then damages cell membranes, causing the contents of the cell to leak out, leading to dehydration.
- It melts the protective wax that covers certain insects (mealybugs, scale insects, certain aphids, etc.) and this leads to dehydration from evaporation.
- It blocks the pores through which insects breathe, leading to asphyxiation.
The pest treated usually dies very rapidly, within minutes of treatment.
Essentially Harmless for Humans and Animals
Insecticidal soap can be an irritant, but is not considered toxic to birds or mammals. After all, it is only soap! If you want to wash with insecticidal soap, go for it! Just rinse well afterwards, as it is more concentrated than most hand soaps.
Do keep it out of the reach of children, though. They can become ill if they swallow it in large quantities. Of course, this is also true of the hand soaps that we frequently leave out for kids to handle… but that’s another story!
Insects Sensitive to Insecticidal Soap
Insecticidal soap is most effective on soft-bodied arthropods, a group that includes most plant pests:
- Sawfly larvae;
- Spider mites;
- Young caterpillars.
Insects Resistant to Insecticidal Soap
Normally, adult pollinating and predatory insects are relatively unaffected by insecticidal soap. This category includes:
On the other hand, insecticidal soap will readily kill beneficial (predatory) mites and should not be used if you use them to treat your plants. It does sometimes kill the soft-bodied larvae of certain beneficial insects (up to 15% of ladybug and lacewing larvae may die after a treatment with insecticidal soap, for example). Always use insecticidal soap with discretion, as you would any other pesticide.
Also, insecticidal soap can be toxic to aquatic animals. Avoid its use near bodies of water.
Curiously, insecticidal soap is also useful in treating certain plant diseases (especially powdery mildew) and, at high doses, will also kill algae and mosses, although specialized herbicidal soaps are usually used for that purpose.
Applying Insecticidal Soap
Always dilute insecticidal soap according to the directions listed on the label (attention: different brands may have different rates). Just use plain tap water in preparing the solution, except if your water is exceptionally hard (soap tends to precipitate in very hard water), in which case it’s best to use rainwater.
In the “do as I say, not as I do” category, insecticidal soap can be irritating to the eyes and to sensitive skin. It is therefore wise to wear safety glasses, long clothing and gloves when applying soap spray.
Spray evenly over all plant surfaces, including (and even especially) under the leaves and at leaf axils, when pests tend to congregate. It’s most effective indoors and on cool, gray but dry mornings and evenings with little wind. It is less effective on hot, sunny days or windy days: such conditions can cause the soap to dry out too quickly. And on rainy days, it may be washed off too rapidly to be effective.
Generally, the treatment should be repeated, because if even one pest escapes, the infestation can start all over. Repeat every 7 days. If you have no success after 3 treatments, try something else, as soap buildup on plant tissues can cause damage.
The fatty acids used in insecticidal soap are specifically chosen to be effective against insect pests while being the least toxic possible to plants. Still, some plants may be damaged. The product label will normally indicate which plants are susceptible. In fact, under extreme conditions, such as temperatures over 90˚F (32˚C) or when the plant is severely dehydrated, any plant can theoretically be damaged.
If in doubt about the sensitivity of a given plant, do a test on one leaf and wait 24 to 48 hours. If there is no negative reaction (yellow or browning of the leaf margin, yellow or brown spots, etc.), you can proceed with the treatment.
Plants Sensitive to Insecticidal Soap
Avoid treating the following plants with insecticidal soap:
- Bleeding heart (Lamprocapnos spectabilis)
- Horse chestnut (Aesculus spp.)
- Japanese maple (Acer palmatum)
- Mountain ash (Sorbus aucuparia)
- Sweet pea (Lathyrus odoratum)
Plants Slightly Sensitive to Insecticidal Soap
Don’t spray these plants on hot days or when they are suffering from drought.
- Cherry (Prunus spp.)
- Conifers (new growth only)
- Crown of thorns (Euphorbia milii)
- Easter lily (Lilium longiflorum)
- Gardenia (Gardenia jasminoides)
- Hawthorn (Crataegus spp.)
- Lantana (Lantana camara)
- Maidenhair fern (Adiantum spp.)
- Nasturtium (Tropaeolum spp.)
- Plum (Prunus spp.)
- Purslane (Portulaca spp)
- Tomato (Solanum lycopersicum) – some varieties
Homemade Insecticidal Soap
Most people making what they believe to be their own insecticidal soap are wasting their time. Typically they use dishwashing liquid as a basic ingredient, assuming this product is a soap, but in fact, most modern dishwashing liquids contain no soap at all, but rather detergents, quite a different product… and one that is not very effective against insects. A dishwater liquid spray is little more effective against pests than just spraying them with water.
Of course, many people claim their grandmothers used dishwater on their bug-infested plants and this worked for them. Maybe, but they washed their dishes with soap, not detergents.
Also, detergents can be harmful to plants: most are made up largely of the phytotoxic short chain fatty acids. Also, dishwater liquids contain dyes, fragrances, stabilizers, antibacterial products and a good dose of various other chemicals, many of which can also be harmful to plants.
The strange thing is that people who treat their plants with dishwashing liquid solutions often do so under the impression it is more “natural” than insecticidal soap! Nothing could be further from the truth: the list of chemical products going into the average dishwashing liquid is staggeringly long; insecticidal soaps contain only one product and it’s organic: soap!
That said, you can make a relatively effective insecticidal soap (always test a leaf or two first, though) from pure soaps like Castile soap, black soap or natural soap.
You have two choices when you buy insecticidal soap: a ready-to-use spray bottle or a concentrate. Although both may appear to be about equal in price, the concentrate contains much more product, usually enough for years of use for most gardeners… and all you have to do is add water. You can recycle a spray bottle from any cleaning product and use it to spray the diluted product on your plants: just rinse it well before use.
Insecticidal soap: so simple to use, so useful and yet so often misunderstood!