Every round that a creature is within 10' of the deathglow moth's abdomen, the creature must save vs. Death Magic or lose 1d3 points of Strength. If a character's Strength falls below zero, the character dies, and the death glow moth settles in to feed. Lost Strength can be recovered at the rate of one point per hour of rest. Feeding moths are easy to slay for their glow is extinguished. Their deadly glow dies when they do and cannot be harvested by intrepid hunters.
That works, but dying from dropping to 0 Str is weird (though similar things have popped up here and there). What do you think about switching to Con damage? As I mentioned above, the original text for the giant mosquitos had them damaging Str, but we changed that to Con damage as well.
INTELLIGENCE: Animal (1)
TREASURE: Incidental
Their coloration varies from a brown, bark-colored body and wings the color of tree moss to an almost albino species that matches the bark of birch trees.
Deathglow moths instinctively seek out a tree that matches its camouflage pattern on which to roost.