Dickinson compares death to a wood-boring beetle—specifically the kind that burrows into trees and kills them from inside. This isn't poetic decoration; it's precise agricultural knowledge. Tree borers were a real threat in 19th-century New England, and farmers had actual strategies: bait traps, dig them out with knives, or as a last resort, girdle the tree to control how it dies.
The metaphor makes death small and invasive rather than grand. Not a reaper or a king, but vermin. Something competent to kill but also potentially stupid enough to fall for bait. The poem walks through the stages of fighting an infestation: detection, treatment, surgical removal, and finally acceptance.
What's crucial is the shift at "Then, if it have burrowed / Out of reach of skill." The poem moves from active combat to grim surrender. "Ring the tree and leave it" means girdle it yourself—kill the patient to beat the disease to it. The final line's resignation—"'T is the vermin's will"—is devastating because it grants agency to something she just called an insect.