Most kitchen herb and flower gardeners are aware of insecticidal soaps and how to use them. It’s virtually a staple in those hobbies. For those new to these activities, or even the small backyard gardeners unaware of their options, it usually becomes a godsend when they learn of it.
To be technical about it, insecticidal soaps aren’t exactly “organic,” since they aren’t an organic-sourced method of killing insects. At some times of the year, however–like now in the northern hemisphere–the pests can get pretty intense and a quick fix, non-toxic solution is needed.
Cheap is nice too, of course.
Home made insecticidal soap fits all of these criteria. It works great, is non-toxic, and it’s cheap to make. So let’s learn something about how these soaps work and then I’ll show you how to make your own. You can buy pre-mixed soap in the store, of course. If you do, buy the concentrated kind and dilute it yourself. The container is smaller (less waste) and it’s generally cheaper than buying the ready-to-go stuff.
How Insecticidal Soap Works
All soaps are an alkali of some kind that’s usually mixed with something that’s a fatty acid chain. Usually they are potassium-based or sodium-based. Potassium soaps are soft while sodium soaps are hard. Most often, natural soaps (non-synthetic mix) will be mixed with a fatty acid derived from a plant or animal.
The insecticidal soap you purchase in the store is most likely to be potassium mixed with coconut oil. This is a good, general-use soap for hand washing, shampooing, and so forth. And for controlling insects, of course.
Insecticidal soap works well against most soft-bodied insects like aphids and mites as well as most types of flies. It’s a contact-killer, so it only works when sprayed directly on them. It doesn’t leave a residue for long, so it’s a short-term affair. Most gardeners use it directly on the insects they see invading their gardens. Most of the time, once you get them knocked down, they’ll be mostly gone within a day or two anyway.
Most applied soap sprays for use as insecticide are in concentrations of less than 1% in the spray, which has about an 80% kill rate. This makes it pretty good and comparable to most commercial chemical sprays for short-term application. Most will last anywhere from 24-48 hours with 72 hours being the longest if conditions are right (shade, stable humidity, low air flow).
Nobody is entirely sure how soaps work to kill the bugs they do, but the likely phenomenon is a combination of asphyxiation and dehydration in the insect. The fatty acids are likely breaking down cell membrane integrity and causing cells to collapse, leading to respiration problems and loss of moisture. In some insects it may be acting to block cell metabolism, causing failure in metamorphosis.
However it works, it’s an effective kill that is non-toxic to most animals (pets, wildlife) and humans.
Making Your Own Insecticide From Soap
For this, you’ll need a spray bottle capable of spraying watery liquid, a bar of natural laundry or dish (non-detergent) soap, water, and a stove.
For the soap, I prefer natural Fels-Naptha laundry soap in the big bars (1/2 pound blocks), which you can get at most health food stores. They’re also available in smaller, standard-bar size (5-1/2 oz). The natural bar of Ivory (no scent) works well too. You can use other common dish washing and laundry soaps, but most of them have detergents in them that can cause phytotoxicity on your plants by dissolving the wax off the leaves.
To make the soap, put a pot on the stove with about a quart (four cups) of water in it. Begin that boiling while you cut up the soap bar. You’ll need about five or six tablespoons worth. From a 1/2-lb block of Fels-Naptha, that’s about an inch off the block. From an Ivory bar (standard size), that’s about half of it.
You can use a large knife and a butcher block to cut it, but that’s kind of pointless since you’re just going to grate it anyway. Get a cheese grater or use your hand-crank grater for salads/slaw and grate off what you need into a bowl or onto a block/plate.
When the water begins to boil, dump the soap grates in there and stir. Keep stirring it until the soap dissolves, then turn off the heat and stir until mostly cooled. I usually set the pan into a bowl or sink full of cold water to speed up the cooling process.
Pour the liquid soap into a jar, jug, or whatever’s handy. Label this container your “Soapicide Concentrate O Doom.”
Now take your spray bottle and mix it at about two tablespoons per quart. I usually take the water from my cooling bath (above), fill my sprayer with it (it’s about 1/2 a quart) and put a tablespoon of concentrate in there.
Then screw on the lid, shake a little, and spray in a mist directly onto plant leaves and infesting insects. I’ve seen this used in larger containers like atomizers and sprayed onto fruit trees, small garden plots, etc. as well as herb gardens and flowers.
Try to do it during the insects’ most active part of the day (usually early evening). Use progressively liberally and do it nightly or every other night until the insects seem to be gone.
This mix also works really well inside for concentrated house flies and, as a bonus, spot-cleaning. We use it in our house for quick wipe-down cleanings of bathrooms and tile and so forth. Make sure to label the spray bottle so you don’t mix it up with others.
Our “cat discipline” bottles, for instance (plain water) are small 8oz ones so we don’t mix them up and spray the wrong stuff on the cats.
This is useful stuff, so try it out and enjoy!