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Re: CULT: soap as insecticide

  • Subject: Re: [iris-talk] CULT: soap as insecticide
  • From: Bill Shear <wshear@hsc.edu>
  • Date: Tue, 04 Dec 2001 08:38:39 -0500

On 12/3/01 5:24 PM, "vince lewonski" <vincelewonski@yahoo.com> wrote:

> The following link is about using soap as an insecticide. I am suspicious,
> though I do remember reading about using plain water to kill aphids -
> apparently they attach to the plant via their probiscus (probiscii???) and are
> killed when the water blasts them loose, leaving the probiscus behind. The
> same principle may be at work when blasting critters with soap. Or maybe their
> spiracles get clogged. Has anyone tried this?

This from ENTOMOLOGY AND PEST MANAGEMENT, by L. D. Pedigo (1999):

"Various kinds of soaps have been used by homeowners and gardeners for many
years.  Records of using soap sprays to control insects date back to the
late 1800s and continue into the early 1900s.  Spraying with soap declined
with  the advent of new effective insecticides in the mid-1940s.  However,
there has been a renewed interest in soaps with the recent growing desire
for alternatives to synthetic insecticides."

"The active ingredients in insecticidal soaps are fatty acids, which
probably affect the insect central nervous system and remove protective
waxes on the surface of the insect cuticle.  Commercial soap products, such
as Safer Soap, usually comprise potassium salts of fatty acids.  They are
used against aphids, spider mites, mealybugs and whiteflies on garden
vegetables, shrubs, trees, and house plants.  Soap sprays are effective only
when the liquid contacts the insect and have little or no residual effect.
Therefore, repeated applications at short intervals are usually necessary to
achieve desired levels of pest suppression."

My own experience with Safer has been very positive.  It quickly knocks down
aphids and whiteflies, and with several sprayings over a week-long period
will eliminate mealybugs and many types of scale.  Murphy's Oil Soap is less
expensive and evidently just as good.  Others like Ivory Diswashing Liquid.
Note that you have to use soap, not detergent.  Detergents have some
insecticidal activity, but not nearly as much as soaps.  The activity of
detergents is due to the lowering of the surface tension of water, which
allows water to get into the breathing tubes of the insects and drown them.
Obviously this is effective only if lots of detergent and lots of water are

Soaps have very low phytotoxicity (they don't readily harm plants) and are
rapidly degraded in the soil.

Bill Shear
Department of Biology
Hampden-Sydney College
Hampden-Sydney VA 23943
FAX (434)223-6374
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"A man thinks as well through his legs and arms as his brain.  We exaggerate
the importance and exclusiveness of the headquarters.  Do you suppose they
were a race of consumptives and dyspeptics who invented Grecian mythology
and poetry?  The poet's words are, "You would almost say the body thought!"
I quite say it.  I trust we have a good body then."  --Henry David Thoreau,
Journals, Dec. 31, 1860.

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