Mr. Spock says:
"It would probably destroy it.
Logically, one must assume that since there are no mustard bugs in space, in order to encounter a mustard bug then the Enterprise must be moving at warp 9 within the atmosphere of Earth, fairly close to the ground. Since the Enterprise was not built for such maneuvering with in the atmosphere of a class M planet, the stresses on the hull of the ship would be such that an encounter, such as you describe with a mustard bug, would be sufficient to literally blow the Enterprise apart...and Chief Engineer Montgomery Scott along with it. Logically speaking."
Live long and prosper.