We’ve covered beetles before at a variety of sizes (from Small to Huge) and CRs (1/3, 4, 4, and 8). So where do scarab beetles and stalk beetles fit in? (Note that these are Inner Sea Bestiary monsters (courtesy of Jason Nelson), so I have no stat blocks to link you to.) At CR 6 and 2 respectively, they fill out the gaps in terms of combat ability. But the real reason to give them a look is their special abilities. The corpse-eating scarab beetles are a nasty surprise for lightly armored PCs (and resistant to most tomb-dwelling undead to boot). And the bleed-biting, property-damage-causing stalk beetles are as deadly to things as they are to people.
After much investigation (and a difficult Perception check), a party rogue spots a loose stone that will allow him entrance into a crypt—provided he shucks off his armor to squeeze into the tight space. But the loose stone is a lure—the crawlspace drops the would-be thief into a den of gnawing scarab beetles and chained ghasts.
Adventurers may turn up their nose at textiles, but for merchants they can mean big money—and that means brutal competition. When silk imports from the Lung Province disrupt the native linen and wool industries, the local spinners, carders, and weavers strike back, releasing stalk beetles into the caravan district to devour the imported fabric. But when the beetles spread to their own warehouses, the locals regret their actions and recruit hometown heroes to undo their mistake (ideally without revealing their own culpability).
When the daring General Yves Vigneault returns from his blistering Southern Campaign, he brings crop-devouring stalk beetles with him. His plan is to unleash them upon the fields and windmills of neighboring Uleem, destroying in one fell swoop the heretic nation’s food supply and its only means of keeping out the sea. Unless adventurers intervene, whatever the beetles don’t finish, the rising waters and attendant dyke drakes (Advanced river drakes) will.
—Inner Sea Bestiary 5