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
References:
- Safer Brand Insect Killing Soap; http://www.saferbrand.com/store/outdoor-insect/5118
- University of Connecticut – Insecticidal Soap;
- Clemson Extension office – Insecticidal Soaps for Garden Pest Control; http://www.clemson.edu/extension/hgic/pests/pesticide/hgic2771.html
- Photo source; Ian Wongkar
Mr. Pavlis What soap would you recommend in eliminating aphids on Milkweed? But keeping it safe for caterpillars to eat it in the near future. How many days should I wait after applying it to the plant,? Before I put caterpillars on the plant to eat the plant. I raise Monarch’s
Spray with water.
Can I put diluted insecticidal soap on/in the soil?
It is a contact pesticide – so if the insect does not contact it – it does nothing. Not much point putting it on soil.
I was woundering if it should be left on or if I should wash it off like 30 minuets or so?
I don’t know if it makes a difference. Once it is dry it probably has limited effect until it rains and solubilizes it again.
I have a question. I had noticed some aphids on my plants (chilli, curry leaf, lily), and went ahead and put some dishwashing liquid foam off of a sponge on to the leaves. After about 5 minutes, I washed it off with a spray bottle with plain water. Will this lead to my leaves/plant burning or dying off? I was just beginning to get new growth on my plants since it’s the start of spring where I live.
Probably not.
Hi,
The link for reference #2 is dead. I think the new link is : http://www.ladybug.uconn.edu/FactSheets/insecticidal-soaps.php
Thanks – I updated the post.
When is the most effective time to apply the insecticide soap, in the morning or evening?
Interesting question. Since this is a contact poison, the best time probably depends on when the insect is there are easy to spray. Other than that I can’t think of any other reason why one would be better than the other, except that the presence of early morning due is probably not a good time.
It’s good to know that dish detergent shouldn’t be used. I have Safer’s soap and am hoping it doesn’t have an expiry date as I’ve had it for quite some time. Would it work on the tent caterpillar types? I had a whole nest of some kind of caterpillar in a new PG Hydrangea one year, on the unopened flower buds. I just picked them off, but it was a lot of work to do so.
Thanks for the info.
Safers soap should last a long time in the bottle. It might work on tent caterpillars, but you need to spray the caterpillars – not the nest. A better way to deal with tent caterpillars is to disturb the nest and step on any that fall to the ground.
This was very helpful.
After reading all the labels, I now find myself reaching for a spray bottle with plain water. Insects like aphids, young squash bug nymphs, etc. wash off so easily anyway. I tend to hand-pick caterpillars like cabbage loopers (using gloves for tomato hormworms) I do use a horticultural oil on scale, only. What are your thoughts on horticultural oils–used according to label directions?
Horticultural oils seem to work well. You need to be careful about when you apply them. Some are a heavier oil and are only to be used in early spring before leaf out. Others use a much lighter oil which can be sprayed at otehr times of the year.
We are having a tent caterpillar infestation here this year. I started by cutting off branches and burning the buggers, but found it as tiresome as I remember my father did. Then I sprayed them with olive oil, which worked instantly and easily, sending a stream of the spray high into the trees.
What damage did it do to the trees?
Very good, thank you.