HEy Timo,
I think they use the dust to protect against mites and other parasites. Osage has a natural insecticide in it, as does juniper cedar, and other woodss. Dave Cheney wrote an article about Rose City Archery (they make port orford cedar shafts) and in the article he stated that the honeybees would collect the sawdust left over from sanding shafts, and they would coat their bodies with it. They'd also take it back to the hive to share with everyone, as it would help eliminate mites that could devastate the hive. Of course I could be wrong, but I won't admit it!!