A few years ago I introduced some native knapweed in the garden. I planted them on the beds and they did well. However, the following year they didn't come back on the beds, they selfseeded in the cracks in the cement and now I have several large plants growing amongst pots from the cement. They attract many bees, but one in particular, enjoys this plant the most, the small mason bee Osmia leaiana. Males are beautiful golden bees with green eyes, while the females are dark, with an orange brush of hairs under they abdomen, which they use to collect pollen. They often dive head first into knapweed inflorescenced and their orange underside becomes quite visible.
Today I was very pleased to find out these bees are using my bee hotels, I hope to get some photos of the cells they are stocking soon.