Ah, hymentoptera. Never change. Fascinating that some of them manage to make reproductive biology even odder than the trolls do!

The Mother Grub being a form of troll is the most sensible explanation. The only reason I raised the possibility of her not being a troll is that Kanaya's virgin mother grub is considered a "lusus", and is shown as blank white like other, more normal lusii. Maybe stylisation is lying to us, and the lusii aren't actually white?