"Who breaks a butterfly upon a wheel?" is a quotation from Alexander Pope's "Epistle to Dr Arbuthnot" of January 1735.
It alludes to "breaking on the wheel", a form of torture in which victims had their long bones broken by an iron bar while tied to a Catherine wheel.[1] The quotation is used to suggest someone is "[employing] superabundant effort in the accomplishment of a small matter".[2]
The quotation is sometimes misquoted with "on" in place of "upon".