How would you have known to hyphenate manges-tu here? I had not encountered hyphenation when forming a question up to this point.
Good point, although I think I came across the hyphenated form before this. In principle, you hyphenate if the verb comes before the subject.
Both forms are perfectly acceptable though, you could easily say "Tu manges du mais?". Having said that, this form is used more often, particularly while speaking.