Rose tosses her flask and manages to predict where the swarm goes, tossing it directly into the path of the beetles, the flask breaks open, and acid splashes out, dousing them in its liquid. They don't seem to flinch however, which makes sense, seeing as they excrete the stuff.

Meanwhile Casper takes an almighty blow at them as they move, and while he squashes several of them, it's just not enough to make a difference.