There's no smell, so quite consistent with what you're suggesting
The more I looked at it the more it looked like insect damage.
You still need to get it looked at to see how far the damage goes and if the insects are still active.
But after that you can just replace the bad stuff but don't forget to use Cuprinol on all cut surfaces and the new wood, AFIK there is no easy way to kill an active infestation yourself other than cutting out all the infested wood.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!