Today I visited John Starnes' blog: Rosegasms

And I found his link to an article he wrote for the St. Petersburg Times.
In his article he provides a formula or recipe for homemade insecticidal soap. Note that the insecticidal soap recipe is the recipe that adds vegetable oil.

I haven't tried it; I just found it. Please report if it works as well for you as the commercially available products.

My thanks in advance to Mr. Starnes.

Direct link to the St. Petersburg Times article below:

Here is a link that might be useful: Wash away your garden worries by JOHN A. STARNES JR.