Tanya, actually, roaches prefer plant material as food. But they'll also eat decaying organic matter, and have been known to eat bones, glue, sandpaper, ink, wallpaper, even the shielding on electric cable. So I'm sure your dendrobium roots must have been like caviar to them!
Mix up some insecticide and pour it through the pot a few times, catching the runoff and pouring it through again. They might have left egg casings in the medium that would be hard to differentiate from a piece of bark, and you want to get those before they hatch out.