Soap has been used since the 1700's as an all-purpose insecticide. Soap is safe, and easily breaks down in the environment. It can be used on food and non-food plants, even on the day of harvest. When sprayed on insects, some soaps break down the insects' protective coatings, and causes them to dehydrate and die.
All soaps are long-chain fatty acids, but not all soaps have insecticidal properties. Insecticidal soaps are specifically formulated to have high insect-killing properties, while being safe for most plant species. Dishwashing soap, on the other hand, contains perfumes and dyes, and in concentrated form can be harmful to plants.
Insects must come into direct contact with the spray droplets for the insecticidal soap to be effective. Good coverage is essential. Spray the soap solution directly onto the insects when they are first sighted on buds, stems, bark and leaves. Be sure to wet both sides of the leaves and growing points of the plants. Spraying in the evening or early morning hours is best so the spray droplets do not dry out. Spraying during the heat of the day, or on a day with high humidity, can cause damage to the foliage.
Spray a small amount of the plant first to test that the affected plants are not damaged before a full-scale application is made. Then apply the solution weekly for two-three weeks. For best results, follow the label directions and do not exceed the recommended rates.
Insecticidal soaps will go a long way in controlling a variety of insects, including aphids, spider mites, whiteflies, thrips, soft scale, mealybugs, earwigs, rose or pear slugs and young caterpillars. But don't use insecticidal soap when beneficial insects, like ladybugs, praying mantis, bees and wasps are present, as the soap may harm them also.
Soaps are being developed to control fungal diseases and weeds. The fungicidal soap contains sulphur, and has proven effective in controlling powdery mildew and black spot on roses. Look for these soap products at your local garden center.
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