In spring, white caterpillar tents appear in the trees, and the speaker links them to his father's trouble: a large, non-cancerous tumor on his lower spine. The speaker imagines it as a white knot swollen with "caterpillars of pain." It has made sitting or lying down nearly impossible for weeks, so his father has had to keep standing and walking. After surgery, the father takes his first steps without it. The speaker's repeated "Be gone" works like a spell, which fits the title, and the tumor becomes a "sexless flower" and an "empty tent."
The second stanza moves into the woods, where the father will soon walk again, now to heal rather than just ease the pain. A tanager eats caterpillars and gnats while singing a hoarse song, which the speaker hears as echoing his father's voice. The mood is relieved and tender, though the final image of "mouthfuls of wings" and "spines" keeps a little of the sting.