Insecticidal soaps can be a valuable tool for the pest manager. Soaps control many targeted pests with fewer potential adverse effects to the user, beneficial insects and the environment as compared with traditional pesticides. To be most effective, it's important to understand how insecticidal soaps "work," to know their mode of action, and to recognize both their benefits and limitations.

The Way Insecticidal Soap Works or “Mode of Action”
Insecticidal soaps work only on direct contact with the pests. The most common soaps are made of the potassium salts of fatty acids. The fatty acids disrupt the structure and permeability of the insects' cell membranes. The cell contents are able to leak from the damaged cells, and the insect quickly dies. There is no residual insecticidal activity once the spray application has dried.

Benefits of Insecticidal Soap:

• Insecticidal Soap Kills Soft-bodied Insects.
  Insecticidal soaps are most effective on soft-bodied pests such as aphids, adelgids, lace bugs, leafhoppers, mealy bugs, thrips, sawfly larvae, spider mites and whiteflies. Soap has a limited effect on non-target beneficial insects such as ladybird beetle larvae, parasitic wasps and honey bees, but it can be quite disruptive to soft-bodied predators, such as syrphid fly larvae and beneficial predatory mites. Once the spray has dried, however, beneficial insects can safely re-enter the treated area.
  
• Safe for Mammals and Environment.
  Soaps have low mammalian toxicity. However, they can be mildly irritating to the skin or eyes. Insecticidal soaps are biodegradable, do not
  persist in the environment, and they do not contain any organic solvents. It is less likely that resistance to insecticidal soaps will develop as quickly as to the more traditional pesticides. Resistance within the insect tends to develop more quickly with materials that have a very specific mode of action. A material that affects the insect's nervous system in a very specific way has a greater chance of developing resistance in a shorter period of time. Insecticidal soaps can be used in rotation with other pesticides with more specific modes of action to help slow the development of resistance. It is also possible to tank-mix soaps with a reduced rate of another pesticide. However, mixtures with foliar nutrients or pesticides containing metallic ions, such as zinc or iron, may be physically incompatible or phytotoxic.

Limitations in the Use of Insecticidal Soap:

• Insect Must Come In Contact With Wet Insecticidal Soap.
  As mentioned earlier, once an insecticidal soap spray has dried, there is no residual activity because soaps work only on contact. Therefore, if an insect has not been coated with the spray, it will not be affected by walking over or ingesting plant material that has been treated with soap. Insecticidal soaps should be applied when conditions favor slow drying to maximize effectiveness, e.g., in the early morning hours with dew coverage or in the early evening. Avoid treating with soaps on hot sunny afternoons which promote rapid drying of the material. Thorough coverage is vital for the soap to be effective. Spray thoroughly, but not beyond the point of runoff. Repeat applications may also be needed as determined by follow up scouting or monitoring.
  
• Know The Target Insect and Its Life Cycle.
  As with any contact insecticide, familiarity with the biology and life cycle of the targeted pest will lead to more effective management. For example, insecticidal soaps are useful in controlling azalea lace bug nymphs but will have no effect against lace bug eggs. In addition, all stages of the lace bug are found on the undersides of leaves. Spraying only on the upper surfaces will have no effect, as the treatment will not come in contact with the targeted pest. A monitoring program to detect when the lace bug nymphs hatch from the eggs will determine the best time for treatment.
  
• Insecticidal Soap Can Burn Some Plants.
  Insecticidal soaps may cause phytotoxicity (toxic to the plant) symptoms, such as yellow or brown spotting on the leaves, burned tips or leaf scorch on certain plants. Horse chestnut, mountain ash, Japanese maple, sweet gum, jade plant, lantana, gardenia, bleeding heart and crown of thorns are known to be sensitive to soap. Certain cultivars of azalea, poinsettia, begonia, Impatiens, ferns, palms and succulents may also be sensitive. When uncertain, spot treat a portion of the cultivar, and wait at least 24 hours to see if any phytotoxic symptoms develop before treating an entire group of plants. Plants under drought stress, young transplants, un-rooted cuttings and plants with soft young growth are more likely to develop phytotoxic symptoms and should not be treated with soap. Soap mixed in hard water may be less effective and more toxic to the treated plants. A precipitate may be formed when the metal ions (e.g., Calcium, iron or magnesium) found in hard water bind to the fatty acids in the soap.
  
  Soaps are effective tools in an integrated approach toward pest management if they are used properly with an understanding of their limitations and benefits.
  

References:
Miller, R.D. 1989. The Use of Horticultural Oils and Insecticidal Soaps for Control of Insect Pests of Amenity Plants. Journal of Arboriculture 15(11)257-262.
Gills, S.and Raup, M.1990.Use of Insecticidal Soap and Neem for the Control of Azalea Lacebug.University of Maryland Cooperative Extension Service.Mycogen Corporation.1991.Technical fact sheets.Mycogen Corporation, 5451 Oberlin Drive, San Diego, California 92121.

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