Clethra alnifolia (Summer Sweet) is a plant very dear to me. I first saw it growing some years ago in a friend's garden. He's since moved away and the new owners have cut the thing down. I'm glad to say I saw it blooming in it's glory.
My friend was on vacation and I was looking after his garden. I didn't know what the plant was at the time. I entered the garden immediately smelling something incredibly sweet. It was a wonderful smell and I looked around at what on earth it could be. I finally homed in on in more than 30' away from the garden gate as the strange shrub by his garage. By then my jaw had dropped at the extreme array of pollinators working the shrub. Many of these insects I'd never seen before and several to this day I've never seen since. Enormous black wasps that are bulky like a Cicada Killer, on par with that of a Hummingbird flew over the thing. They were not Hummingbird moths... those I could recognize on the plant. A huge array of Scoliid Wasps flocked around the plant among tiny metallic green and gold sweat bees. To my surprise Honey Bees and Bumblebees weren't that common at all.
To this day I've never seen such a diversity on any other plant. I've since planted 6 Clethra alnifolia's (Summer Sweet) and not once seen the cloud of insects I saw that day. Pictured is a red/pink flowering cultivar called "Ruby Spice." The true species has all white flowers. Only 3 of the plants are a size just shy of what my friend had in his yard. But planted in mass should have the same effect right? It's hard to believe red flowers would dull it that much.
Well at any rate, I probably caught a plant in it's most ideal conditions at the peak of blooming. At least the ants seem to like it too.