  We investigate the behavior of a single particle hopping on a three
dimensional cubic optical lattice in the presence of a Mott insulator of bosons
in the same lattice. We calculate the critical interaction strength between the
impurity and background bosons, beyond which there is bound state (polaron)
formation. We give exact results in the limit of a perfect Mott insulator,
where polaron formation is equivalent to impurity localization. We calculate
the effects of lattice anisotropy, higher impurity bands, and fluctuations of
the Mott insulator on the localization threshold. We argue that our results can
be checked experimentally by RF spectroscopy of impurity particles.
