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Linda Naeve, Reiman Gardens, (515) 294-2710,
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,

Wash Away Houseplant Pests with Insecticidal Soap

By Linda Naeve
Extension Coordinator
Reiman Gardens

Most gardeners don't put their love of plants on hold when the ground freezes. They fulfill their need to garden by bringing special plants, such as coleus, geraniums and hibiscus, indoors to keep them through the winter. Unfortunately, they often bring in unwanted hitchhikers along with the plants. This week's Reiman's Pick, insecticidal soap, is a safe and easy way to keep some houseplant pests from becoming permanent houseguests.

Indoor conditions are nearly perfect for several insect pests. Warm, dry conditions and the lack of predators provide an environment where populations of small, soft-bodied insects, such as aphids, whiteflies, mealybugs and scales, can grow quickly. They may not be noticeable one week, but you may find your houseplants covered with scales, aphids or mealybugs the next.

Insecticidal soaps are an excellent defense against an invasion of soft-bodied insects. Commercial insecticidal soaps, available at garden centers, are specifically formulated to have high insect-killing properties, while being safe for people and most plants. They also are considered selective insecticides because they have minimal adverse effects on other organisms. Soap sprays do not harm most beneficial insects, except predatory mites.

Insecticidal soaps contain a potassium salt of a fatty acid. These soaps work against soft-bodied insects by washing away the protective coating on the surface of the insect and by disrupting normal membrane functions inside the insect. To be effective, the insect must come in direct contact with the spray.

Although commercial insecticidal soaps are readily available and easy to use, they also can be expensive. You can make your own inexpensive insecticidal soap with a common household product - dish detergent. Dish detergents, such as Ivory, Palmolive and Sunlight, make effective pesticides. However, the right concentration is important for it to be effective and prevent damage to the plants. Apply the detergent, diluted with water to a concentration of about 2 to 3 percent (or about 1 tablespoon per quart of water), using a spray bottle.

An insecticidal soap, whether purchased or homemade, is safe and effective if mixed and used properly. You need to have thorough coverage of the plant for the soap to kill insect pests, so be sure to spray the undersides of leaves and other hiding places, such as the intersection of the stems and branches. Insects that aren't completely wetted won't be controlled. Insecticidal soap has a short activity period. It may be necessary to spray every week for a few weeks to control certain pests, such as spider mites and immature scales, or if you have a severe infestation of aphids or mealybugs.

Frequent spraying, however, increases the risk of damaging the leaves. Insecticidal soaps have caused some phytotoxicity, or burning on the leaves, on some plants when applied often or at a high concentration. To reduce the potential for damage, do not exceed a 2 to 3 percent solution and wash or rinse the plants off within a couple of hours after spraying.

Inspect your houseplants carefully every time you water. Insect pests are much easier to control if caught and controlled before populations have a chance to build up and damage your plants.

Organic gardeners endorse the use of insecticidal soap for the control of certain pests. Another form of non-chemical control, biological control, is used at Reiman Gardens. The staff at Reiman Gardens employs biological control methods to avoid the use of chemical insecticides in the Conservatory and Christina Reiman Butterfly Wing exhibits.


Editors: A color photo, suitable for publication, is available at right. Click on the thumbnail photo to go to the fullsized photo. The picture's fullsize photo is 328K.

Caption: Insecticidal soap.

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