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Insecticidal soap is a product which is applied to plants to kill insects such as aphids and mites. Many garden supply stores and nurseries carry insecticidal soaps which have been specifically formulated for use on plants, and these products are generally recommended over homemade versions, for reasons which will be discussed below. Applications of insecticidal soap can be used to control ongoing infestations, and to deter insects so that they do not get a chance to damage plants.
When soap has the right formulation, it acts as a contact killer for insects by penetrating their bodies and disrupting their internal functions. Classically, insecticidal soap is applied as a diluted spray so that full coverage of a plant is ensured, and it may need to be applied every few weeks to continue providing protection. The solution can be used on both indoor and outdoor plants, and it usually eliminates a wide variety of unwanted insect visitors.
While soaps can kill insects, they can also kill plants, and this is why some gardeners recommend using commercial products rather than making one's own. While detergent and liquid soap can be prepared in a 2% solution and sprayed on plants to kill insects, these soaps may not always work, and, more importantly, they could hurt or kill the plants. Homemade insecticidal soap also usually needs to be applied more frequently, which increases the risk of damage to plants.
People who want to make insecticidal soaps at home should try to use liquid soap, rather than detergent, as soap formulations tend to be more gentle. The soap should be free of perfumes and dyes which could hurt the plants, and it should be diluted with water to make a two percent solution. People with hard water will want to add water softeners, as the minerals in the hard water can interfere with the action of the soap. Once prepared, the soap mixture can be sprayed on plants, or stored for use in the future.
Gardeners who want to use insecticidal soap products formulated to be safe for plants can usually choose between conventional and organic products at nurseries and garden stores, and the staff may have specific recommendations for particular insects or plants. It is also important to take note of the directions on the label, because some products are sold in a concentrate which needs to be diluted before use, while others are ready for use.