What's the material (acoustic insulation?) which is under the rear seat bottom and spare tire?

I'm hoping one of you can tell me what material is used under the rear seat bottom and under the spare tire. This material looks like some sort of tape/paste which is firm, but yields to sharp pressure. In my car this material has become cracked and torn. Worse, it's now become home to what I believe are Black Carpet Beetles. (You can check out photos here: Creepy Crawlies in the Back | PriusChat)

While these bugs have been (inadvertently) provided some Cheerios, cheese crackers and similar snack spillage to eat, it looks like they're chowing down on the insulation material itself. What the heck is this stuff?!

In controlling the bugs, I've removed some sizeable chunks of the stuff. Should I try to replace it? If so, with what? How?

Thanks in advance for the information.