Companion planting ... will any variety work ?

My particular experience of nasturtiums is that they attract blackfly like almost nothing else. I wouldn't plant them withtin a mile of any veg I had.

As far as marigolds go, aim for the simple single flowered ones, these attract the insects which may help you.

Tagetes in the greenhouse does seem to work as a whitefly deterrent (I say "seem to" because there is no way of knowing whether I would have had them if I didn't grow the marigolds.  )

I wouldn't risk using alliums planted near or amongst carrots to confuse and therefore deter carrot fly - I still think you need luck for it to work 100%, and given the havoc they can wreak I wouldn't like to bet on me being luckier than the flies!

I've never really understood the sacrificial nasturtium idea - blackfly can and do migrate from one plant to another, so providing a large breeding colony of them near your beans or whatever doesn't seem like a great idea to me.

Ordinary tagetes from a garden centre (or grown from seed) do keep off whitefly in the greenhouse. One year, my plants failed to grow and I had whitefly for the first time.

I think it is he smell of the tagetes foliage that deters whitefly.

I just grew normal french marigolds alongside the tomatoes last year and had no whitefly.

I know that greenhouse whitefly and the whitefly on cabbages and other brassicas are different, but I do wonder if the same will work for them. I always seem to be plagued by the snowstorm by the end of the season.

I'm going to give it a go this year to find out. They will at least look pretty and the bees like them 

If anyone does already know the answer of course .........