People often ask me how to find ground-nesting bee nests. In reality I see nests in the ground all the time but most are very tiny and aggregations are fairly infrequently encountered. Here is a time-series of a Lasioglossum sp. female at the entrance of her nest, likely only 2-3 mm in diameter. She quickly saw off the more brightly colored male.
I observed a couple of two-spotted sunflower bees, Melissodes bimaculatus, foraging for pollen on this Floriani Red Flint corn in Boulder, CO. While it may seem strange, I observe bees foraging on corn and other grass for pollen fairly frequently. These could provide important resources if other host plants are scarce or may be used to supplement pollen provisions. These two bees returned repeatedly and filled their scopa up each time. You can easily see the yellow corn pollen packed onto their hind legs.
I am a Postdoctoral Associate at CU-Boulder and have lead a number of projects studying patterns of native bee diversity in Colorado. Please enjoy your visit and contact me if you have any questions.