" (why do farmers not just spray soapy water???)"

Soaps require high application rates of both soap and water.  Up to 200 gal per acre of water is required to wet the crop adequately.  That is a lot of water.  Most chemicals only need 20 to 40 gal per acre of water.  Must add enough soap to overcome water hardness plus enough to kill the insect target.  Generally a 2% free soap solution is the minimum effective concentration.  Must be applied late in the day to run off from crop as soap is only active while the plant surface is still wet.  Narrow spectra of insects killed as it only works on soft bodied insects and kill rate is generally only 75% at best.  Needs reapplication in one to two weeks.  Causes phytotoxicity on quite a few plants.

Overall, very expensive to apply versus better insecticides due to all the water, requires so much soap it is expensive, requires too frequent reapplication to be economic on field crops and has too narrow a spectra of kill for most field uses.  Works great in some greenhouse and office/indoor applications.  I have used it in greenhouses and like the performance under those conditions on aphids.  Never had any luck at all using it on white flies in the green house.  Hydrogen cyanide is much better on white flies in the greenhouse.

Dick
" Any discovery made by the human mind can be explained in its essentials to the curious learner."  Professor Benjamin Schumacher talking about teaching quantum mechanics to non scientists.   "For every complex problem there is a solution which is simple, neat and wrong."  H. L. Mencken

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