We have cabbage whitefly on the Brussels sprouts at the allotment!
Here’s what the RHS has to say about what it is and how to treat it:
Cabbage whitefly is a sap-feeding insect that can infest cabbage and all other types of leafy brassica.
The symptoms to look out for are white-winged insects about 1.5mm long which fly up in clouds from the underside of brassica leaves when disturbed, flat, oval, whitish-green scale-like nymphs are attached to the lower leaf surface and black or greenish-grey sooty moulds can develop on the upper leaf surface on the sticky honeydew excreted by this pest.
Whitefly causes little damage so there isn’t too much to worry about but if you want to treat it an organic pesticide can be sprayed onto the lower leaf surfaces include, try Vitax Organic 2 in 1 Pest and Disease Control or Scotts Bug Clear for Fruit & Veg. Or for chemical control you can use Bayer Sprayday Greenfly Killer All-In-One or Westland Plant Rescue Fruit & Vegetable Bug Killer.