I don't KNOW this, but I've read it in a few places: insecticidal soap has to be SOAP (properly so-called, soap from natural ingredients) and not detergent.
The reason given is usually that it is the
fatty acids (particularly long-chain fatty acids' potassium salts) in soap that kill the bugs.
For example:
https://en.wikipedia.org/wiki/...
Detergent might or might not have fatty acids, but usually not, since there are cheaper and more powerful synthetic surfactants.
I only know to look for the word "soap" in the brand name, then read the ingredients and look for familiar names.
