Answer: Insecticidal soap is a very effective contact spray against soft-bodied insects such as aphids. It works by dissolving the outer skin. Spraying plants to keep aphids away doens't work because once it dries, it loses effectiveness. Many plants are sensitive to Insecticidal soap sprays and the recommendation is to rinse it off the plant 20 minutes after application. In that length of time the insects will have been killed and since there is little residual effect, the product can be rinsed off. So, to answer your question, rinsing it off after application is not defeating the purpose since it's a contact spray. While there is some residual effect, for best results it should be sprayed directly on aphids. If aphids have found your herb plants, you can achieve nearly the same results by simply hosing the plants off with a strong stream of water from the hose. Hope this answers all your questions!
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