I use a spray but read the label before you buy one as some plants can,t tolerate it and if its an edible crop you have to look for an appropriate spray

They come from Rape Beetles. The farmers are told which crops to grow by DeFRA. We have had White Fly trouble for years around here. Dont use chemical sprays on food crops. Just leave them, and wash off in lots of water in the winter. And pray the boffins at DeFRA change their programme for next year.

bayer ultimate bug killer does the job