What's wrong with my onions?

Question

I harvested my onions yesterday and there were three or four that pulled up incredibly easy, as if they had no grip on the soil. The roots were grey and very thin, unlike the other onions' which were thick and white and healthy, and there were a few spots of whitish mould on the bases. There were also some tiny mite-type insects and baby millipedes crawling on the bases.

Is this white-rot? Insect root damage? Or something else?

Answer 1

It sounds as if your onions could be affected by a fungal infection or by thrips.  Below, the first link will show pictures of onion plants affected by various diseases and disorders.

To see what a thrip looks like and read about the damage it does, try the second link.  

Whatever it is, I would avoid planting onions in that area again.

Happy Gardening!