  We present an analysis of four complete samples of radio-loud AGN (3CRR, 2Jy,
6CE and 7CE) using near- and mid-IR data taken by the Wide-Field Infrared
Survey Explorer (WISE). The combined sample consists of 79 quasars and 273
radio galaxies, and covers a redshift range 0.003<z<3.395. The dichotomy in the
mid-IR properties of low- and high-excitation radio galaxies (LERGs - HERGs) is
analysed for the first time using large complete samples. Our results
demonstrate that a division in the accretion modes of LERGs and HERGs clearly
stands out in the mid-IR-radio plane (L_(22 \mu m) = 5x10^(43) erg s^(-1)).
This means that WISE data can be effectively used to diagnose accretion modes
in radio-loud AGN. The mid-IR properties of all objects were analysed to test
the unification between quasars and radio galaxies, consistent with earlier
work and we argue that smooth torus models best reproduce the observation.
Quasars are found to have higher mid-IR luminosities than radio galaxies. We
also studied all the sources in the near-IR to gain insights into evolution of
AGN host galaxies. A relation found between the near-IR luminosity and
redshift, well-known in the near-IR, is apparent in the two near-IR WISE bands,
supporting the idea that radio sources are hosted by massive elliptical
galaxies that formed their stars at high redshifts and evolved passively
thereafter. Evaluation of the positions of the sample objects in WISE
colour-colour diagrams shows that widely used WISE colour cuts are not
completely reliable in selecting AGN.
