Whitefly, this year, is on every green thing!! If you take the trouble to spray, DO spray UNDER each leaf - where they are. Very time comsuming.
"Brassica whitefly can be a problem in some areas depending on the weather. Although winter brassica crops (Brussels sprouts, broccoli, cabbage etc) can usually cope with this pest, the very warm autumn this year has allowed the pest to build up to dramatic numbers, rather than showing a decline as is more common at this time of year.
Where infestations are severe, a couple of sprays of insecticidal soap may be necessary. Use a good quality sprayer – preferably one with a lance - and try to spray in the cool of the morning when the whitefly are less active. Pick off any yellowing lower leaves before you spray, and remember to spray under the leaves too."