The grounds of the Environmental Center have been covered with little mounds of dirt this spring: the sign of a healthy ground bee population. There are many species of ground-nesting bees (in fact, most of the world's bees nest in the ground), but the bees in the video below are plasterer bees (aka polyester bees). Each mound holds a single female bee. The mound is the entrance to a deep tunnel where she will lay an egg, deposit a supply of liquid food, and then seal it off with a "plastic" lid. The egg will hatch next spring, feed on the food cache, then dig its way out to repeat the cycle. In the video below you'll notice that when they're not out foraging the bees sit atop their mounds, presumably guarding the holes over which they've labored. In a seeming game of hide-and-seek, they quickly retreat into the safety of the ground at the slightest movement or shadow, then slowly climb back out to keep an eye on things.
Watch where you're walking!
This great piece from the Washington Post provides a good overview of plasterer bees, including the following image:
For more information about ground and other bees visit the Xerces Society's informative page on native bee biology.