Whitefly

You can swat them away but they'll return. You can hang sticky traps - like old-fashioned fly paper - nearby and it will deal with a number of of them. You can spray with something like pyrethrum but, again, they will be back. They're one of the most dificult pests to eradicate. The only upside really is that they don't do a lot of damage unless they're in serious numbers.

There's also a tiny wasp predator called Encarsia Formosa which you can buy.  The problem with using predators like these is the cost (they don't come cheap) and there is always a minimum temperature you need to maintain, which is rather tricky in an unheated greenhouse at the moment if you are in the UK!

I plant french marigolds, whether they work or not...

Ladybirds?  You can get ladybird attractant, adult ladybirds and ladybird larvae on t'interweb, together with ladybird 'houses' so they can overwinter.  They're supposed to be good at getting rid of greenfly, whitefly, aphids and red spider mites.  I don't like using pesticides in my garden in case they damage the bee population.