Here is how I implemented rot grubs in my DF campaign. I statted them as a trap, not a monster, because you don't fight them so much as detect/disarm them, or suffer the effects of them. They're far less of a monster than, say, leaping leeches.
Detect: Vision-based Per-2. This assumes a search! A quick glance while, say, searching for treasure is at -5, for a net Per-7.
Disarm: Burning the infested corpse, or using Cleansing (Magic, p. 94) will remove any rot grubs; they are easily killed (or harvested) after this . . . unless they find another victim to burrow in to.
Steal: Eww, but yes. Takes some creativity and a Animal Handling-5 roll (unless the character is a worminger with the appropriate Animal Handling expertise.
Effect: Usually 1d rot grubs will burrow into a target; each causes 1 HP of injury immediately as it furiously burrows into the flesh of the victim. Each inflicts another 1 HP of injury every 3d seconds, and requires a HT roll at -1 per rot grub in the victim. On a failure, the rot grubs have reached the vitals and inflict x3 damage from this point on. On a critical failure, one has reached the heart, killing the victim. Success means damage as before; critical success means the grubs take twice as long before they inflict injury again. Application of flame to the burrowing holes before the second round of damage kills the rot grub but also inflicts the usual fire damage for the source. Each burrowing hole must be cleaned separately. Cleansing will remove all rot grubs immediately.
Enjoy, if that's the sort of maggoty thing you like to enjoy.