Posted: 31/07/2013 at 19:06
A few points to consider, whitefly reproduce incredibly quickly in a greenhouse and that means resistance to chemicals is prolific with whitefly.
The ideal way to get around this is to use two distinct chemical groups, ideally for the casual gardener a neonicitinoid (systemic) and a pyrethroid.
Spraying them alternately at two week basis starting with the nicitinoid would give you good control.
I would recommend Py as the Pyrethroid and something containing thiacloprid (there are lots of products) for the systemic. The idea is to knock out one that becomes resistant to the other.
Other methods to consider are biological controls, however they preclude the use of most chemicals, need to be sourced carefully and they are never 100% effective. Their big upside is they can potentially survive indefinitely (given a heated greenhouse and some care) and even whitefly can’t become resistant to being eaten!
The Bearded One