Using Insecticidal Soap in Your Home, Garden or Landscape

Dan Gilrein, IPM Specialist Cornell Cooperative Extension

Insecticidal soaps are among the safest choices available for controlling pest problems in your garden or on your ornamental plants. They control many pests, including adelgids, aphids, mealybugs, whiteflies, mites and others on a wide variety of food and ornamental plants. If you are interested in using insecticidal soap or have ever had problems with it, here are a few guidelines you can follow for best results.

Insecticidal soap only works on contact, meaning the spray solution must coat the pest you are trying to control. Once the spray has dried, a moving insect will not be harmed by walking over the residue. Coverage, therefore, is extremely important. For example, spraying the upper leaf surface will leave whiteflies alive and healthy, since they usually feed under the leaves. The immature stages move little or not at all, and will not be killed by contact with the wet material.

Spray only when and where an infestation appears, and not as a preventive measure. Symptoms such as leaf or shoot distortion, sooty mold and holes in leaves require some further looking to actually see the cause and the extent of an insect infestation. Once you find the pest, treat only those plants or spots with the problem. Some pests, such as aphids, can sometimes be found on a single shoot and sometimes throughout a planting. Identify the pest and where it is before spraying!

Watch for phytotoxicity, an adverse plant reaction or injury from the soap treatment. Symptoms on foliage include yellow or brown spotting, "burned" tips, and/or yellow or brown scorching on the leaf edges. Soap spray may also cause marking on some pome (apple, pear, etc.) and stone fruit varieties. Phytotoxicity is perhaps the greatest concern most people have when using insecticidal soap.
However, by observing a few points you can decrease your chances for plant injury:

• Don’t treat plants under drought or other kinds of stress. Stressed plants may be especially intolerant of insecticidal soap solution. Make sure plants to be treated are well-watered and look healthy. Conifers, in particular, are more susceptible when under drought stress. Newly planted ornamentals, transplants, and unrooted or newly rooted cuttings all are experiencing a form of stress and should first have time to be well-established before being sprayed with insecticidal soap.
• Avoid treating sensitive plants. Some plants are known to be more easily injured than others by insecticidal soap. Following is a list of plants known to show phytotoxic reactions after treatment:
  • horse chestnut - Aesculus hippocastanum
  • mountain ash - Sorbus americana
  • Japanese maples - Acer palmatum
  • gardenia - Gardenia spp.
  • bleeding heart - Dicentra formosa
  • sweet pea - Lathyrus odoratus
  • maidenhair fern - Adiantum pedatum
  • crown of thorns - Euphorbia milii
  • lantana - Lantana spp
  • nasturtiums - Nasturtium spp
  • Easter lilies (during bud formation) - Lilium longiflorum
  • Certain varieties of azaleas, begonias, camellias, fuchsias, geraniums and impatiens may also be sensitive.
     
  • Test insecticidal soap first on a small part of palms, delicate ferns, ornamental ivies, and succulents before treating an entire plant or area. These may also be sensitive.
    • Rinse these plants with a clean water spray if they show signs of wilting within a few hours after treatment.

• Wait for new growth to harden off before treating. Tender, young foliage of evergreen trees or shrubs may be most sensitive. Fruit and nut trees in bloom also should not be sprayed. If ever in doubt, test a small part of a plant first. If the plant is sensitive, phytotoxic symptoms should appear after 48 hours.
• Apply when the temperature is below 90 degrees F and not in full sun. High temperatures and high relative humidity may increase plant stress and therefore sensitivity. The best time to apply insecticidal soap is in the early morning. The material works only while wet, and the slower drying conditions in early morning favor better control.

Compatibility with other pesticides and fertilizers.

Insecticidal soap is compatible with many other kinds of pesticides, but should not be mixed with rotenone-based insecticides, Manzate, Dithane, lime sulfur, copper sulfate or copper fungicides such as Bordeaux mixture. It should also not be combined with concentrated mineral fertilizers for spraying on foliage.

Compatibility with hard water.

Soft water is best for diluting to the proper strength. Soap combines with and is precipitated by certain minerals in hard water, especially calcium, iron and magnesium. Test for compatibility by allowing a quart of the prepared spray solution to stand for 15 minutes. A scum or "curd" of soap scale on the surface indicates the need for a water conditioner. The solution should normally be a light, milky color.

This publication contains pesticide recommendations. Changes in pesticide regulations occur constantly and human errors are still possible. Some materials mentioned may no longer be available, and some uses may no longer be legal.

Please read the label before applying any pesticide.

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