In a book of Alexander McCall Smith I found this phrase:
No historical novelists need apply.
Why isn't it this with a to for the infinitive?
No historical novelist need to apply?
It's a polite idiom used to tell them not to apply, because they are not wanted.
The phrase is often seen when a job vacancy is re-advertised after a suitable person was not found.
Previous applicants need not apply.
A variant is "need not apply," and as the previous poster said, it's an idiomatic expression that doesn't follow precisely the rules of English.
You should keep in mind that signs that people make are notoriously bad examples of proper English.