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Insecticidal Soap

- An effective alternative for garden insecticides -

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.)

Insecticidal soap

Insecticidal soap is a very safe and effective alternative to conventional insecticides for many garden and landscape pests. It is used mainly for soft-bodied pests like aphids, thrips, scale insect crawlers and spider mites but 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 ordinary dish soaps, or homemade soaps, for genuine insecticidal soap. Doing so will damage your plants and probably won't be very effective.

insectcidal soap "jar" test

Fresh 2% soap solution (jar on left). The slight milky color is normal. If the solution turns cloudy, or a white precipitate forms (jar on right), don't use it.


Insecticidal soaps will not usually harm beneficial species such as predators and parasites and are generally very safe for the applicator. Soap concentrates are less expensive in the long run, and easier to store, than ready-to-use packaging. However, always do what I call 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 plant 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!


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). A white precipitate will form almost immediately if the concentrate has gone bad, discard the solution if this happens and replace with new soap concentrate.

Using insecticidal 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 quart 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 (see Using Insecticidal Soap To Control Aphids and Using Insecticidal Soap To Control Spider Mites for more information).

Sources

Insecticidal soaps can be purchased online (DoMyOwnPestControl) or at most garden supply stores; of course 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 pesticide sprayers.

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Jack DeAngelis, PhD,  , email:  [email protected]

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