Insecticidal soap can be used to get rid of certain insect pests on plants, and it is one of the most non-toxic pesticides available. However, it is still a pesticide and needs to be treated as one. Many people use insecticidal soaps incorrectly, or for the wrong type of insect. In this post I will review how it works, which insects it controls and how to use it correctly. I’ll then review the problems with home made insecticidal soap.
Insecticidal Soap – What Is It?
Insecticidal soap, like Safer® Brand Insect Killing Soap is a true soap. It is made by reacting potassium hydroxide with long chain fatty acids. Fatty acids are made from fats.
These soaps have been specially formulated to be effective insecticides while at the same time doing very little damage to most plants.
Organic insecticidal soap controls soft body insects such as aphids, mealybugs, spider mites, and whitefly. It also controls arthropods such as earwigs, spiders, millipedes, mites, flies, and ants. It can also be used to control caterpillars and leafhoppers, but these large insects are difficult to control with this product. It does kill soft-bodied larvae of lady beetles and lacewings – beneficial insects.
This is a general pesticide that will kill both pests and beneficials.
How Does It Work?
Insecticidal soap is a contact poison. It must come into contact with the pest in order to affect it. The soap is water based and dries fairly quickly. Once dry it has no effect on pests.
Scientists don’t fully understood how they work, but the latest information suggests that the fatty acids disrupt the insects outer cell membranes. Once disrupted, contents of the cells leak out and the insect dies. For this to work well the whole body of the insect needs to be covered.
Dry soap will not disrupt the insect membrane, so it has no effect.
The dry soap will decompose fairly quickly into harmless compounds.
How do You Apply Insecticidal Soap?
If you can’t see the pest – don’t bother spraying. A pest that flies away will not be harmed.
Here are some simple instructions for Safer Brand Insect Killing Soap . For more detailed instructions see the link below (ref 1).
- SHAKE WELL. For best results use freshly mixed solution.
- DO NOT use on new transplants, newly rooted cuttings or plants stressed by drought. Avoid application when temperature exceeds 90° F.
- Apply when insects or signs of their damage appear. Thoroughly wet all surfaces of infested foliage and branches.
Note the comment “Thoroughly wet all surfaces of infested foliage and branches”. This does not mean you should cover the whole plant. Just spray the infected areas. Remember, you need to spray the pest to be effective – spraying leaves does not work.
If you are mixing your own diluted solutions from concentrate, use soft water, like distilled water. Hard water makes the product less effective.
Keep in mind that this is a pesticide and it can harm plants. The detailed instructions for Safers says it will harm; seedlings, new transplants, cuttings, Euphorbias, delicate ferns, bleeding hearts, azaleas, sweet peas to mention a few. See the full instructions for a complete list.
Home Made Insecticidal Soap
There are many recipes on the internet for home made, DIY insecticidal soap. My post, Dish Soap Can Damage your Plants discusses the problem with these in detail. Home made solutions using soaps found in the home are chemically not the correct type of soap and are more likely to be phytotoxic (poisonous) to your plants – they might even kill them.
Dish soap is not even a soap – it is a detergent. You can’t make insecticidal soap using a detergent because they are very toxic to plants.
Is Dr. Bronner’s Castile Soaps Safe?
Castile soaps, of which Dr. Bronner’s is a popular brand, are made from vegetable oils (particularly olive, palm, and coconut) using potassium hydroxide. So they are potassium based soaps and therefore many people claim that they are the same as insecticidal soap. Most castile soaps are mixed with fragrances and essential oils for a wide range of uses but not as insecticides. This soap maybe safer than other sodium-based soap, but nobody really knows what the additives will do to plants. Since it is not labeled as a pesticide it should not be used.
Insecticidal Soap Products
Clemson University Extension office (ref 3) provides this list of available products:
- Bonide Insecticidal Soap Concentrate;
- Safer Insect Killing Soap Concentrate;
- Schultz Garden Safe Insecticidal Soap Insect Killer Concentrate;
- Natural Guard Insecticidal Soap Concentrate;
- Espoma Earth-tone Insecticidal Soap Concentrate;
- Concern Rose & Flower Insect Killer II
- Raid Earth Options Insecticidal Soap
- Lilly Miller Worry Free Insecticidal Soap
- Whitney Farms Insecticidal Soap
- Bayer Advanced Natria Insecticidal Soap
- Safer Brand Insect Killing Soap; http://www.saferbrand.com/store/outdoor-insect/5118
- University of Connecticut – Insecticidal Soap; http://www.ladybug.uconn.edu/FactSheets/insecticidal-soaps.php
- Clemson Extension office – Insecticidal Soaps for Garden Pest Control; http://www.clemson.edu/extension/hgic/pests/pesticide/hgic2771.html
- Photo source; Ian Wongkar