There are several species of solitary bees most of which nest in the ground. You can tell solitary bees from yellow jackets in the ground by the entrance hole to their nests. Yellow jackets will generally have one hole that they use to come and go from. Solitary bees nest sites will have many holes (they look like worm holes). Some are gregarious nesters with 100s, sometimes 1000s, nesting in the same area of ground.
One of the most common solitary bee in our area is the Mason bee. The Mason bee is a solitary bee that can have large populations nesting together. They are all good pollinators and should be left alone whenever possible. We have never seen a solitary bee that is aggressive. Following are some of the most common solitary bees we see in our area.