Homemade insecticidal soap is an effective, economical and environmentally safe way to combat the small, soft bodied insects (aphids, mites, mealybugs) that attack houseplants and garden plants.
It will not, however, be effective on chewing insects such as caterpillars and beetles but a few large insects, including boxelder bugs and Japanese beetles, have been found to be susceptible.
Insecticidal soaps are considered selective insecticides because of their minimal adverse effects on other organisms. Ladybugs, green lacewings, pollinating bees and most other beneficial insects are not very susceptible to soap sprays, although, predatory mites, often important in the control of spider mites, are an exception as they are easily killed by soaps.
How the soap works is it penetrates the protective waxy coating surrounding the bug causing dehydration, and in turn killing the insect.
When choosing your soap brand, beware to use only brands that do not contain degreasers or antibacterial ingredients. These ingredients are VERY harmful to plants and will most likely burn or kill them. I prefer Fels-Naptha, but there are others that will work effectively (see below for examples).
To make your solution of insecticidal soap, you will need:
bar of Fels-Naptha (or other "real" soap brand like Ivory, Dr. Bronner's Pure Castile Soap)
To begin, make the concentrate. With a cheese grater, grate one quarter of the Fels-Naptha soap (or other brand soap) into the small bucket. Add 1 quart of very hot water and stir until the soap is dissolved. Put this mixture into a labeled bucket, or a mason large mason jar works well. When this solution cools, it will turn into a gel.
To use the soap spray, put 1 teaspoon of the concentrate per quart into a bottle sprayer.
This insecticidal soap is a contact poison, so spray it directly onto the insects you want to eliminate.
When we use this, we try to wash off the plant 2-3 hours after application to prevent any possible burning of leaves.
**Cautionary note...before widespread use of this spray, it is always a good practice to test on a small area or single plant and wait for 24 hours and check for any negative effects.
With the leftover bar of soap, you can make your own laundry detergent...more on that another day!