Midsummer Prairie - But Where Are the Bees and Butterflies?

Yesterday I took a long, leisurely walk through prairie and oak savanna habitats in Carleton College's Cowling Arboretum. It was a day that seemed to presage autumn, with moderate temperatures and a good breeze pushing clouds that occasionally looked stormy, though we got no rain.

Compass plant is the tallest flower on the scene, routinely reaching
 5-6' or more.

Lush mix of grasses and flowering plants

I think this is hoary vervain (Verbena
 stricta).  I'm sure someone will let me know if it's not.

Purple prairie clover (Dalea purpurea) and grasses blowing in the wind

But where were the bees and butterflies? Granted, it was a windy day, which probably accounts for a good part of the quiet, but at least low down among the thick stems I would have expected to see the landscape busy with insect activity -- but I barely saw any.

In fact, I've seen very few butterflies or bees at all this year. At home, my flowering thyme, bee balm and Joe Pye weed should be humming with bees, but I've seen only a couple here and there, and a couple of butterflies. There are many factors at play, including our cold spring, but something certainly doesn't feel right.