Bugs destroying willow tree

I moved into a new property 3 weeks ago. It has a beautiful willow tree in the garden. On closer inspection I noticed lots of sawdust at the base, lots of creepy crawlies and the inside of the tree looks as though it’s being hollowed out. There are a large amount of red ants nearby but not on or in the tree that I noticed. What could be causing this and what can I do?