I recently reattempted the brutal Bulldog Road climb. The experience became even more painful since I was stung many times by bees at the beginning of the climb. I was initially stung twice near a tree; I moved away and used a sting relief wipe (from my trusty REI Backpacker First Aid Kit) on the affected areas. When I moved quickly toward the tree to recover a fallen object I was instantly stung again twice. This was so remarkably efficient I had the feeling that a nontrivial system was at work against me. Conversations with friends revealed that in fact bees emit alert pheromones, and I found a paper from the Entomological Society of America showing that different combinations of pheromones may determine different behaviors.
Bee behavior is a fascinating subject, and recently some other folks hiking nearby have also learned about it the hard way, while somebody else made it out unscathed.