A large Scoliid Wasp.
Also known as a Mammoth Wasp. And for good reason. This one is big as you can probably judge by comparing it to the shot of the bee and the Cosmos flower I have included..
Apologies for the lack of quality of the photos. I was so surprised to see this fly by me that I hardly had time to gather my wits and was unable to get a decent photograph before it flew away.
First time I have ever encountered this species, and what a treat!
They are solitary parasitoids of scarab beetle larvae. Female scoliids burrow into the ground in search of these larvae and then use their sting to paralyze them. They will sometimes excavate a chamber and move the paralyzed beetle larva into it before depositing an egg. Scoliid wasps act as important biocontrol agents, as many of the beetles they parasitize are pests, including the Japanese beetle. Male scoliids patrol territories, ready to mate with females emerging from the ground. Adult wasps may be minor pollinators of some plants and can he found on many wildflowers in the late summer.