  Let $D$ be an irreducible plane curve. In this article, we first introduce a
notion of a quadratic residue curve mod $D$, and study quadratic residue concis
$C$ mod an irreducible quartic curve $Q$. As an application, we study a
dihedral cover of $P^2$ with branch locus $C+Q$ and give two examples of
Zariski pairs as by-products.
