Some mites can be seen on the substrate in the third photograph, and on the second photograph a group of mites can be seen on the grub in about the 11 o’clock position at the base of the abdominal caecum.  The two lots of mites seem to be of different sizes, and they may be two different species.  University of Alberta acarologist Dr Heather Proctor comments:   Cool photos! By zooming in I was able to see that the cluster of mites at 11 o’clock are phoretic deutonymphal Astigmata. There’s not enough detail to say what family, but the most likely are Acaridae or Histiostomatidae. The two mites on the substrate in photo 3 (plus two that you can see on the beetle itself) look like nymphal Mesostigmata. What they are doing on the beetle is a good question! They are not the typical uropodine mesostigs that would use the adult beetle as a phoretic host; rather, they look more gracile, possibly Parasitidae or Macrochelidae (which also are frequently phoretic on insects).  Sorry I can’t identify them any more finely than Astigmata and Mesostigmata based on the photos.

 

Probably  Polyphylla crinita (Col.: Scarabaeidae)   Ian Cruickshank

Probably  Polyphylla crinita (Col.: Scarabaeidae)   Ian Cruickshank

Probably  Polyphylla crinita (Col.: Scarabaeidae)   Ian Cruickshank