I am thinking they are soil mites that are parasitic. They latch onto the crevices of a larvae's skin and start to feed. I am sure of this because I saw a small brown hole on one of my larvae after cleaning a cluster of tan mites off. After this I proceeded to squish the mites that were visible. I accomplished this with a soft children's paint brush to gently flick them off, and then pummeled them with the hard end of it. Also, I do not think the shell of an adult beetle can be penetrated by a small mite. In excess, however, they may even be able to block the beetles mouth preventing it from feeding. The mites may just irritate them as well. I first got this infestation problem when I failed to heat treat my substrate enough when I used organic soil as the base. Sometimes the wood that you collect from the outside contains the mites inside. This is especially true if the logs are on the ground. For this problem, I crush up all of my wood inside a ceramic bowl. I then put a plate on top and do 3 minutes in cycles in the microwave. Be careful when doing these cycles and make sure the bowl and wood cools down enough. I say this because I had bowls explode on me before, and the wood even catch an ember and ignite a flame. Once the wood is finished, it is time to heat the soil. When doing this, I use a large ziplock storage container. I put the soil in, and heat it for increments of 6 minutes for maybe 4 cycles letting it cool in between for 10 minutes after each cycle is complete. By this time the soil is extremely dry. After it is cool I then rehydrate it with water and mix it. I then get the wood flakes and mix it into the soil. Since then, I have encountered little to no mites and if I did, the problem was taken care of immediately with the microwave. With an all wood substrate that has been fermented I would guess that you can do the same. However, I think this can kill helpful gut microbes that larva use to break down food and dry out the substrate. It may even bring mold because inside the container the dirt can hold more moisture. After this I write down my results inside a book and see what I can improve on to keep the mites away.
