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Insecticidal Soap
- An effective alternative for garden insecticides -
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Summary: Insecticidal soap can be used as a safe and inexpensive alternative to garden insecticides in many pest management situations. Be cautious, however, because old, oxidized soap can damage your plants. A simple test is available to protect plants from this damage.
Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)
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What is insecticidal soap?
 Insecticidal soap is a very safe and effective alternative to conventional insecticides in many garden situations. It is used mainly for soft bodied pests like aphids, thrips, scale insect crawlers and spider mites. It is also a very effective leaf wash to remove aphid honeydew, sooty mold and other leaf debris.
All soaps are made from the "fatty-acids" of either plant or animal oils. Chemically, soaps are salts of the fatty acid portion of fats and oils. Insecticidal soap is simply a highly refined version of ordinary liquid soap. However, do not jump to the conclusion that you can substitute household soaps, or homemade soaps, for genuine insecticidal soap. Doing so will damage your plants and probably won't be very effective.
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Fresh 2% soap solution (left). The slight milky color is normal. If the solution turns cloudy, or a white precipitate forms (right), don't use it.
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Insecticidal soaps will not harm beneficial species and are generally very safe to use. Soap concentrates are less expensive in the long run, and easier to store, than "ready-to-use" packaging. However, always do a "jar test" to be certain that the concentrate has not spoiled.
Insecticidal soap "jar test"
As soap concentrate ages it oxidizes which causes it to turn dark brown. Old, oxidized soap has the potential to burn foliage. Solutions made from old soap concentrate often exhibit a white precipitate (see jar on right, above) when mixed with water and should be discarded. Old soap concentrate can be used as a cleaner like ordinary liquid soap just don't put it on your plants!
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Mix the soap concentrate as a 1-2% solution and pour into a jar. A 1-2% solution is about 1-2 oz. of concentrate in 1 gallon of water (always follow label instructions). The precipitate will form almost immediately if the concentrate has gone bad. Discard the solution if this happens and get new soap concentrate.
Using insecticial soap in your garden
Insecticidal soap is generally used as a 1-2% solution (1-2 oz. per gallon). Mix soap concentrate in a clean 1 gallon sprayer. Plan to treat in the morning of a warm day when the plants will be able to dry before evening. Spray plants with a coarse spray of plain water from the garden hose. Next, thoroughly spray plants with soap solution covering all surfaces. Finally, wait 30 minutes then wash soap off with water. The last step will prevent soap solution from damaging tender leaf tissue.
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Where to purchase insecticidal soap
Insecticidal soaps can be purchased online or at most garden supply stores; we hope you purchase them through 'Bugs!. For most gardeners the pint size will make a year's worth of working solution. At the end of the season use what's left of the concentrate to clean the inside of the greenhouse and your spray tanks.
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