Whitefly don't over winter in the soil but on leaves and plants outside. They lay their eggs on leaves so anything you bring in with leaves on it may have the eggs on it. This includes any houseplants that you have had outside. I would give any plant a good spray with insecticidal soap or the horticultural oil or neem oil befiore you bring them in. Make sure you get every part of the leaf, undersides too.

Watch very carefully in the spring and at the first sign of them get up the sticky traps. Leave any that hit it on there if they were ready to mate they will attract others. Then spray the plants every 3 days with neem oil or insecticidal soap. You could grow a couple of sacrifical plants before you start the others and get rid of them before you start what you want to keep.

Inspect the undersides of leaves for eggs they are yellow at first and then turn black. You will have to be persistant to get rid of them

Do not put your tubers in plastic bags. They will rot. You can store them in paper bags with the top taped closed.

Thank you, Oilpainter. You provided very good information and directions. I could easily remove the leaves of all the plants I bring indoors because they are all dormant. I no longer put any of my houseplants outdoors in summer, and they do not get infected with whitefly. The problem is knowing when they are around in the winter when I start my sowing indoors. The only way I know they are around is when I see them on my seedlings. However, I will do a few 'sacrificial' plants for them. They seem to like soft-stemmed plants like tomatoes - don't do too much damange to the others. My yellow traps were not too successful last year, but I'll keep on tryng.

One more question: I trade seeds with other gardeners. Is there is chance that I could spread whitefly through seeds?
Thank you,
Northerner.