Goldenrod soldier beetle (Chauliognathus pennsylvanicus) is the most common soldier beetle in the Midwest. Adults can be seen from June to early October but their numbers peak in August to mid-September. They are found on goldenrods and other flowers in prairies, meadows, abandoned fields, gardens, parks, and roadsides. The long, yellowish-brown wings with black marks give this species a distinctive appearance.
http://www.minnesotaseasons.com/Insects/goldenrod_soldier_beetle.html
I saw an abundance of them the other day on the goldenrod right next to Fort Snelling. I guess that fits in with their peak population being in late August-early September!