Aphids

I just squish them. If there's too many to squish by hand I would use a soapy spray. When the weather gets cold they don't reproduce very much so if you can get them now you'll reduce numbers in spring. Remove older, lower leaves and check the growing tips since that's where they like to hang out. I've got lots of brassica whitefly in the greenhouse at the moment, in colder weather they get dozy and don't fly away as quickly. The yellow sticky traps work well for whitefly but have also inadvertently killed hoverflies, parasitic wasps and spiders so I don't like using them anymore.