Greenfly Grubs?

[Northumbrian]

Tiny, white 2/3mm long threads have devastated the pansies in my window boxes. The soil appears to be wholly infested. Are these greenfly grubs? If so, or even if not, can anyone suggest a cure as I suspect the boxes will have to be disinfected. Thank you in advance.

[Hawkwalk]

No, not greenfly as these dont live in the soil and are not grubs. They are probably Sciarid fly larvae. You can treat against them but best to just change the compost and clean out the window boxes.