Noticed this yellowish "stuff" over the last couple of months in one corner of our pergola. Will not wash away with a garden hose and cannot be removed with a broom. Wood is 100% cedar (posts are 8"x8") that is covered with multiple coats of weatherproof semi-transparent stain. Still, there have been the occasional bees that have borrowed into the underside some of the joists, and a trellis that my wife has put nearby, but nothing excessive that I've noticed, and I'm out there almost every day. There is one hole about 5 feet away from where this substance has accumulated. I'm all about leaving bees alone, but if this is a sign of a real problem, I have no issue with killing them all quickly.