In reply to Rupert (W.Yorkshire) ) who wrote this in
, I, Marvo, say :

I dug up a tropaeolum ciliatum
that was withering and near death.
The roots were covered in small white thread like millipede type
things. They were about 1/2 inch long and could move fairly quickly.
They had a small head and many pairs of legs and the little beasts
had devoured a majority of the roots.
An ideas what this pest is??

Many legged millipede type things are often millipedes. Are you sure they
weren't? Were they semi-transparent (translucent) or coloured in any way?