Subject: Citrus Trees

What do plants of the Citrus genus have in common?

I have potted tangerine trees that I put outside during the
summer, and no pests attack them. Is it because the
insects and fungi that normally attack them are not found
here? Or are they protected by terpenes?

Also what part of the leaf produces that lemon-like smell?
(My trees have no fruit). 

Re: Citrus Trees