  Here we describe the least distributive lattice congruence $\eta$ on an
idempotent semiring in general and characterize the varieties $D^\bullet,
L^\bullet$ and $R^\bullet$ of all idempotent semirings such that
$\eta=\mathcal{D}^\bullet, \mathcal{L}^\bullet$ and $\mathcal{R}^\bullet$,
respectively. If $S \in D^\bullet [L^\bullet, R^\bullet]$, then the
multiplicative reduct $(S, \cdot)$ is a [left, right] normal band. Every
semiring $S \in D^\bullet$ is a spined product of a semiring in $L^\bullet$ and
a semiring in $R^\bullet$ with respect to a distributive lattice.
