they don't do too much damage, but if your hive goes weak for a bit, they can destroy your colony. They bees fight them a lot. I wish I could have video captured the battles going on - maybe I will try to do that sometime.

https://en.wikipedia.org/wiki/Small_hive_beetle

This is the real problem for beekeepers - varroa mite:

https://en.wikipedia.org/wiki/Varroa_destructor