Yes, I was not explicit, Enstar is an IGR as you mention. It even says so on the label:
http://www.centralgrower.com/pdfs/EnstarIISpecLabel.pdf
Central Horticulture: Enstar II Insect Growth Regulator
Again I was not explicit, but one might deduce that I didn't use 4 liters of tank mix applied to a single white fly. A fly swatter would have been much quicker.
I used the 4 liters of tank mix to treat most all of the plants, and in particular the undersides of the leaves where whitefly eggs are most often located.