I am trying to identify a ?pest in my allotment. They are white approx 2-5mm long and they look a bit like a worm but with a bigger head?? There are everywhere, they attach to any roots, or anything underground. My soon was digging a hole and filling it with water and there were hundreds of them in the "soup". How can I get rid of them. Previous years I just wash them off but their numbers seem to be on the increase.