Insecticidal soap can be an effective against many of the small, soft-bodied insects and mites that attack landscape shrubs, trees, flowers, vegetables, and houseplants. They may be used on fruits and vegetables until the day of harvest and are one of the few alternatives available to homeowners for control of spider mites. However, these products must be used carefully because of the potential for plant tissue burn – phytotoxicity.
Knowing a few key points will allow you to get the most from these products.
Recognize target pests
Develop your identification skills. Some pests are very vulnerable to insecticidal soaps; others are not. Many sap feeders, such as aphids, scale crawlers, plant bugs, mealybugs, thrips, and spider mites usually can be controlled by insecticidal soap. However, caterpillars, sawfly larvae, and many beetles are not affected, for a variety of reasons.
How insecticidal soaps work
Insecticidal soaps are contact insecticides. The exact impact is unknown but it appears that soaps penetrate and disrupt the pest’s exoskeleton, its internal cell membranes, or block the spiracles- openings through which arthropods breathe. Consequently, pests must be thoroughly wetted to be killed. The spray needs to be applied to runoff to upper and lower leaf surfaces. There is no residual effect after the spray has dried. It is best to treat early or late in the day when the spray droplets will evaporate most slowly.
Pest control afforded by insecticidal soap should be apparent within two days of application. Something in the range of 50% to 60% control would be normal because not all individuals are hit by the spray, some may hatch from eggs present when the treatment was applied, or some may arrive after treatment.
Check treated foliage two or three days after an application to evaluate control. Several treatments may be needed to significantly reduce heavy infestations. Follow label directions on re-treatment intervals; overtreatment may injure the plants.
Product use tips
Read the product label carefully; look for warnings of plants that can be damaged by insecticidal soap. Some of the plants listed on labels as being sensitive to soap sprays include: Azaleas, Begonias, Camellias, cherries, ferns, Fuschia, Impatiens, jade plants, hawthorn, palms, sweet pea, plums, Portulaca, and some tomato varieties.
Application of insecticidal soap to stressed plants, young, tender seedlings, or when temperatures are high may result in injury, even to plants that normally tolerate the spray. When in doubt, treat a small area and check in a day or two to for phytotoxicity (leaf burn or distortion). Don’t spray during the hottest part of the day or when temperatures are above 90F.
Substituting other soaps and detergents
Insecticidal soaps are potassium salts of fatty acids that have been designed and formulated for use on plants and have been evaluated for potential to injure plants. Some liquid hand soaps and dishwashing detergents are used successfully for insect control but there is a greater chance of phytotoxicity when using these products.