Insecticidal soap has been the organic gardener’s choice for insect control for some time. It works by washing off the wax cuticle that surrounds all soft-bodied insects. Once that cuticle is gone the insect dies of dehydration. There are several commercial insecticidal soaps on the market but thrifty gardeners sometimes like to make their own. The basic formula is 1-4 tablespoons of liquid soap for each gallon of water. Place in a spray bottle and go after those bad bugs.
Insecticidal soap kills by contact, not by poisoning the insect. This is good because it leaves no lasting harmful residue in your garden. But like other insecticides, it does not discriminate between good and bad bugs. A spray of this will kill a ladybug (good guy) as quickly as a thrip (bad guy). Keep this in mind and don’t spray for a preventative because you may be doing more harm than good.
Does insecticidal soap work as well as the commercial product? The answer is “yes and no.” Home-made soaps kill more insects but they also are more likely to harm plants. Like insects, plants also have a waxy film on the leaves that may be removed by the insecticidal soap. If this happens you will see leaf damage. Commercial insecticidal soaps are formulated to minimize potential damage to plants.
So what’s the bottom line? For my garden, I spend the few bucks and use commercially formulated insecticidal soap. If you want to make your own, test it first on part of a plant and watch for leaf damage during the first 24 hours after spraying. For more details take a look at the following link to a paper at Washington State University:
P.S. When making your own insecticidal soap do not use anti-bacterial soaps. They’re great for personal hygiene but they tend to cause more harm to plants than other liquid soaps.