Insecticidal Soap – Use It Properly

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Robert Pavlis

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 controls aphids
Insecticidal soap controls aphids

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.

Growing Great Tomaotes, by Robert Pavlis

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.

YouTube video

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).

  1. SHAKE WELL. For best results use freshly mixed solution.
  2. DO NOT use on new transplants, newly rooted cuttings or plants stressed by drought. Avoid application when temperature exceeds 90° F.
  3. 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.

Microbe Science for Gardeners Book, by Robert Pavlis

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.

Dr. Bronner's Castile Soap
Dr. Bronner’s Castile Soap, note the added peppermint oils.

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

 

References:

  1. Safer Brand Insect Killing Soap; http://www.saferbrand.com/store/outdoor-insect/5118
  2. University of Connecticut – Insecticidal Soap; http://www.ladybug.uconn.edu/FactSheets/insecticidal-soaps.php
  3. Clemson Extension office – Insecticidal Soaps for Garden Pest Control; http://www.clemson.edu/extension/hgic/pests/pesticide/hgic2771.html
  4. Photo source; Ian Wongkar
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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

76 thoughts on “Insecticidal Soap – Use It Properly”

  1. I have a hydroponic system by Gardyn, have had infestation of aphids, used dawn and wateroil mixture for a month. Lightly spray all pods with water after an hour. Seems to be working. Found Natria insecticidal soap for vegetables, flowers and fruit. Is this safer and more effective/less work than what I have been doing? Plant leaves have bumps/scars, noticing sappy stuff again.

    Reply
  2. Appreciate the detailed article. Above says not to use on new transplants. How long after reporting is it safe to use? Thank you

    Reply
      • I used a two year old solution of insecticidal soap (Safer) that I had made up on five apple trees. We have had a cool moist spring/early summer, then started modest automated irrigation with the first 80 degree days of the year, a week later there is leaf and new stem curl on my 10 yr and 3 yr old apple trees and modest leaf curl on the two new transplants. The soil seemed moist. I have increased irrigation rate w/ improvement in the leaf curl. A very small percentage of the leaves are turning brown. What is your opinion of if this was a reaction to the spray or lack of proper watering? What should I do?

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  3. Unfortunately insecticidal soap isn’t nearly as cheap here.

    If I use potassium hydroxide and a long-chain-fatty-acid oil such as Grapeseed oil, to make soap, it should make an effective insecticidal soap right?

    Reply
  4. Hello will this damage aquatic plants like water lily.. I have flea beetles and the Bonide Insecticidal Super Soap is what I’m hoping to use..it says toxic to invertebrates on the label… Can I apply to the substrate and let it seep in to kill larvae ..and can I spray the water of the large planter pot it is in so it mixes with the water to kill them ?

    Reply
  5. Hi there, I have flies in my soil and I’m wondering if I can just spray the soil instead of my plant. The flies only seem interested in the soil

    Reply
  6. My dog likes to eat grasshoppers. Will she be affected by the safer insecticidal soap that the grasshoppers may have on them?

    Reply

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