What's this wasp doing?

[Cavedweller]

This afternoon I saw a HUGE cicada killer dragging the end of her abdomen all over my porch, then on a nearby tree.
I'm not real familiar with solitary wasps, was she laying some kind of pheromone trail? Or maybe she just had an itch XD

 

[EtienneN]

I would think pheromones, too.