Mary Fisher writes

--
I'd like to know too. We haven't a conservatory and this year, for some
reason, there haven't been any whitefly in the greenhouse but the brassica
patch is wick with them.

I'd also like to know what harm they do to the crops (as opposed to being
irritating).

Indoors, they can get thick enough to severely damage leaves and weaken
the plant.

If it's just one or two plants to be brought in I'd suggest wave the
plants around vigorously outside until all the adults are dispersed,
pick off all leaves with nymphs (look like small green plates on the
underside of the leaf) and also any leaves with tiny black 'prickles'
sticking downwards as these I suspect are the eggs. The bring the plants
inside and watch like a awk for any sign of reinfestation.
--
Kay