As most of my plants are grown for shows, we use peat free to comply with the RHS guidelines, but it seems as if most of the plugs I get seem to be in peat based compost. What you say Palustris would make sense as most of the grubs were in the plug and not the surrounding compost. I was hoping to move these plants up a pot size, but they will have to stay in 4" for another month or two. So lights and heat needed now (more expense) to help them to catch up. Plenty of time, not needed until mid june. As luck would have it they were isolated from the rest of the nursery apart from the suspected culprit plants.