She doesn't have baskets on her legs like a honeybee, so she's carrying pollen on her belly. Her nest is made of lilac leaves. By the time the asters bloom every year, the lilac's leaves are all serrated.
Notice how she's on the yellow-centered flower, surrounded by red-centered ones? Like honeysuckle, the color change is a signal that a flower has been successfully pollinated. The asters are just about at the end of their usefulness for the year; in another week or so all the centers will be red. Good thing, because the cool nighttime temps are creating a powdery mildew problem and I need to chop the plants down. Next year, I'll try the "Carrots Love Tomatoes" suggestion of spraying with chive tea - will keep you posted.