Marigolds planted to ward off unwanted insects & bugs

The single flowered marigolds (tagetes are the ones you need)
http://www.suttons.co.uk/Gardening/Flower+Seeds/All+Flower+Seeds/Tagetes+Lemon+Gem_135442.htm

give some protection in the enclosed surroundings of a greenhouse/polytunnel, but I don't think they make any difference on the plot - but they do look pretty 

I have found distributing a few nasturtiums across the garden to be reliable for the building up of ladybirds. There are some more attractive varieties on the market now. It's not long before you notice the ladybird larvae on the plants. They have a voracious appetite for aphids. Poached egg plants are beneficial for attracting hoverflies which gorge themselves on aphids as well. I think they work against whitefly as well, but i don't have any (that i've noticed) so can't attest to that.

Doesn't hurt to add a little colour to the garden, especially if it adds to the natural management of pests. Athough, a garden is best to have some pests or their predators won't hang about if there's an absence of food for their young. A matter of balance, but keep in mind it's very easy to distort the balance by the application of pesticides. Those little spray containers are just too tempting. Imitate nature for the best results. Natural predators will serve you well.

Companion planting is I believe what you are referring to. I too grow marigolds in the greenhouse to prevent whitefly, seems to work, I grow English marigold at the allotment, to feed the bees & hoverflies which in turn can the help pollinate my vegetables & fruit. I've also got daffodils planted for an early source of nectar.

I can send you plenty of English marigold seeds if you like & anyone else if you pm me your address