They are most likely the solitary bees, such as the "colletids" and "andrenids" that often emerge in the spring each year. Even with the cooler weather we've experienced, rain that has softened the soil (for digging purposes) and open/exposed areas warm more quickly, contributing to this burst of activity. The bees begin foraging for food and seeking out new nesting sites. This activity can continue through June. They dig vertical tunnels in the soil, on which they make small side chambers where they store pollen for their offspring.
These bees are "solitary," which means there is not a true colony. A lot of the "swarming" that you see is males and females. The bees frequently make small mounds in the soil, often where the soil is loose, and vegetation may be sparse. You frequently see clusters of these nests, but they are all made by individual queens that do the work without the help of workers, as occurs in a honeybee nest. Although the damage can be unsightly with large numbers of mounds in the yard, it is mostly a cosmetic issue. The bigger problem is usually that people walking by panic because they assume that these are swarms of honey bees (or a similar bee) that will likely attack them if they venture too close to 'the nest'. Turf-nesting bees can sting, but rarely do so, since they are not "social" and you do not have a large number of worker bees that are trying to protect a nest. There is no "mass attack" as might occasionally occur with a close encounter of the yellow jacket kind. These bees are beneficial and should be left alone if possible.
If the bees try to nest in a sandbox, a simple solution is to cover it during the day, but it will take a few weeks for you to deter most of the bees that show up over time. While the tarp approach isn't always successful, wider areas can be saturated with plain or soapy water that will bring the bees out. Since soap will work as an insecticide to some extent, it may kill some of the bees in the process, but still consider this preferable over the use of conventional insecticides. The waterlogged soils will hopefully deter the bees, but again, we're looking at activity that can take place over a few weeks.
If you have further questions about insects in the landscape, do not hesitate to call N.C. Cooperative Extension, Franklin County center at 919-496-3344.
