Is soap good to kill insects in the garden?
Yes it is, in fact, gardeners have been using soaps as an insecticide since the early 1800’s. But there are some qualifications. An insect wears its skeleton on the outside – called an exoskeleton. This is the hard shell we see on many insects like beetles, grasshoppers and cockroaches. Soap will breakdown this exoskeleton, causing desiccation and thus killing the insect. But soap has no residual affect and therefore must be sprayed directly on the pest. Soaps will not kill insects that land on plants after they have been sprayed.
The other problem with soaps is that some plants are very sensitive to them and could exhibit some burning on the leaves - called phytotoxicity. This is especially true if you are mixing your own concoction. Use only a mild dishwashing soap with no additives. Purchased products are well refined and would have fewer problems with phytotoxicity.
Here is a University of Hawai`i recipe for a good oil and soap mixture: Make a concentrate of one tablespoon dishwashing liquid and one cup of vegetable oil. When ready to use, shake well and mix 1 – 2 ½ teaspoons of the this mixture into one cup of water. Spray plants thoroughly, every 5 – 7 days as needed. Don’t spray in the heat of the day.