Two tablespoons of ordinary liquid dish soap or detergent per gallon of water easily kills small, soft-bodied insects; stronger solutions are needed for larger insects. Insecticidal soap is especially effective against aphids, spider mites, thrips, mealybugs, and whiteflies. Hard water mixed with soap is not very effective: try soft or rainwater instead.
Insecticidal soap can damage plants.
High concentrations can scorch leaves, especially at higher temperatures. Seedlings, recent transplants and fresh cuttings are especially damage-prone, and should not be treated with soap solutions. Euphorbias and many other succulents, and Brassicas seem most affected; Begonia, bleeding heart, Impatiens, Rhododendron, maple and mountain ash are among other sensitive plants. Repeat sprayings are most damaging, and conditioned water may also increase damage.
Spot-treat a small area and observe for 24 hours prior to treating the entire plant, and test on a few plants prior to using it on many.
Commercial offerings, such as Safer's insecticide, are specially formulated, but at least some test data suggests that homebrew is equally effective when used in an appropriate manner. Ivory liquid dishwashing detergent has been studied and found quite effective in this role, at a 1-2% concentration; similar products should also work well. Safer's, however, appears less hard on plants.
Harsh detergents can do significant damage; beware of regular dishwashing products. Even if damage is not grossly apparently, yields of fruits and vegetables may be reduced.
Finally, soap solutions may be combined with insecticidal oils for greater efficacy. Again, test these solutions on a small area of one plant, first, and use with caution.
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