These really are tiny, but the trouble is there are normally hordes of them.
They can cause the same problems as Aphids - curling leaves, sooty mould and all sorts of other fungi.
Thank you Amy Hengst for this photo
Having a tremendous breeding rate, from egg to adult in three weeks, even if you did try and control with sprays - which only effect adults - you would be at it continuously.
Plus a few would always escape to start all over again. Nice and comfy in your nice warm and cosy greenhouse.
The Encarsia wasp, or to address it formally, Encarsia formosa
These are even smaller than the whitefly, and their tiny larvae feed on their slightly bigger larvae.
Don’t ask the details please !(However, these are tropical wasps so not suitable for outdoor cabbage white fly.)
They will take about 4 weeks to start having a significant effect, but the effects will rapidly increase, particularly as new generations of the wasps come along. ( Although you may like to top up your wasp pupae after two weeks to ensure they are continuously on the job ).