What's killing my plants????

baaaa

This time last year I decided to plant my tomatoes early so I put them in the spare room under a light.All was going well,they were quite healthy until a little midget type insect showed up and destroyed them- within 24 hours they lost all their vigor and stopped drinking/using water etc and started wilting/dying...The insect is tiny and looks like a little brown triangle and when you catch it it leaves a rusty sheen on your hands(like a moths sheen),they fly fast and with purpose and don't buzz about aimlessly like fruit flies.I think they attacks them at their roots as there are no marks on the leaves.
So I tried again this year and the same thing happened again,grand healthy plants that suddenly lose their vigor overnight and stop using water and develop brittle shrunken leaves.
Anyone any ideas?

looksee

Is this the answer http://en.wikipedia.org/wiki/Whitefly though granted they are not brown. Greenfly and other aphids are the main cause of problems with tomatoes.

I like the nasturtium idea at the end!