Hi Ken:

Very cool observation! These are ereynetid mites, which aren't commonly phoretic on dung beetles (though they do hitch rides on bark beetles). There is a casual mention of ereynetids on "dung-inhabiting beetles and flies" in Krantz & Walter (2009) A Manual of Acarology (p. 245), but unfortunately without a clear reference to a supporting publication. Most likely your mites are predatory on small invertebrates in the dung and are using the beetle to get to a fresh pile of poo.

Cheers,
Heather

Of course! Great ID Heather! I should have thought of that.

Some additional general information. Ereynetidae is divided into four subfamilies; one lives in the nasal passages of frogs (Lawrencarinae), one in the respiratory system of slugs (Riccardoellinae), one in the nasal passages of birds and mammals (Speleognathinae), and one that is thought to be free-living (Ereynetidae). The last comprises three genera, one is associated with aquatic beetles (Hydrantes), one is only known from grass clippings in Ohio (Pseudotydeus), and then there's Ereynetes, which is known from soil and forest litter... and has also been collected on dung beetles.

The biology of most ereynetid groups is understudied and begs for more research. But just based upon this, it's easy to guess that your mites are in the genus Ereynetes.

Is it good enough guess for you to add a genus page to BG? ;)

Thanks!
It's surprising to me that even though I've seen thousands of these beetles, this is the first with mites and it's covered in them.