I live in New Mexico. My yard is all rock with no grass at all. I have two trees in my yard they only like one of the trees. The bark is about to turn red from the amount of them. They're crawling on the rocks every morning and they're not mites as I already looked that up. They also have partners that are just a little bigger that look like sunflower seeds with legs and they fly too. The red bugs are small, maybe 4 millimeters in length, and they all seem to disappear as the sun come up.
Sounds like it could be Bark Beetles or Kissing Bugs. Bark beetles usually bore into the tree and are much larger than you described so I'm leaning toward kissing bugs. The treatment for bark beetles should be more "on the tree" but if you are sure they're living under the rocks, follow the treatment program I listed below. If it turns out to be bark beetles, this will work on them too.
But if these are Kissing Bugs, I'd treat a lot more than just around the tree! Also known as conenose or assassin bugs, they get active in the spring and at this time it's common to see adults and young. They are especially active in Arizona and this is where we get the most reports of problems with them invading homes. Kissing bugs are nasty! They are usually brown with red or orange spots on their sides and can appear red from a distance. Their bodies resemble sunflower seeds too. They like to live in the ground around trees and rocks and usually close to if not with some small animal like a ground squirrel, rat or gopher – even prairie dogs.
Kissing bugs are nasty because they feed on blood. They got their name "kissing bug" because they like to feed on humans and prefer to do so through our lips! This is usually done during the night, when we're sleeping, and their bite can leave irritated marks and swelling after they feed.
Kissing bugs typically live in groups and like to stay out of sight during the day. They'll crawl under rocks and mulch and lay in hiding when the sun is out. They're attracted to porch lights and many times will nest on homes. Once this starts it's quite common for the house to get infested with this invasive pest.
At this point I suggest you consider some yard treatments. Two good products to apply to the rocks would be BIFEN GRANULES and some BIFEN CONCENTRATE. If you don't have a good sprayer, go with the CYONARA. Apply the granules first and then spray over the top with either liquid. I'd also spray the tree just to make sure you don't miss any of them. A good treatment throughout the entire yard should knock down this nest and hopefully they haven't moved inside the home just yet. Do a good inspection around the perimeter of the home to make sure I would also spray the outside of the home just to be sure – especially if you typically burn a porch or other night light – because once they get inside they can be a pain to control.
Here is a more detailed report on this pest posted by the University of Arizona:
Here are some links to the products mentioned above for kissing bug control:www.bugspray.com