These likely belong to a type of native bee called a mining bee (or one of the related bee families that are also native). A wide variety of, and the majority of, our locally native bees are ground-nesting. Although they take advantage of good habitat by nesting near each other in sometimes large colonies, they do not share nests and do not defend them the way a truly social bee or wasp does (like yellowjackets or honey bees). Therefore, they almost never sting any person or pet (and are not motivated to do so) and simply go about their busy lives gathering nectar and pollen to stash in their burrow for the baby bees that will live there for the summer after the adults finish their task and die out. Activity can be quite frantic for a few weeks, but then the adult bees disappear as they die off and the holes will naturally fill in by rainwater and other means; the next generation of bee adults will not appear until next spring.

These are valued pollinators and their nesting habitat is disappearing due to development, so we encourage gardeners to let them nest undisturbed wherever possible. They do not harm plants or cause dieback, and are merely taking advantage of soil that is each to reach due to sparse vegetation. (Therefore, the thinning of the grass came first, and the bees chose the site because of that, rather than causing vigorous grass to decline.)

If the site is still suitable for turfgrass -- that is, if it gets at least 5-6 or more hours of direct summer sun, drains well, and isn't in soil that's too wet or dry -- you can try some of the steps described in our Lawn Care and Maintenance pages to rehabilitate it. Of those actions, having a laboratory soil test performed is a good first step to know if any lime or particular fertilizer amendments would benefit the grass. Then, if you are growing a cool-season lawn type (like tall fescue, the most widely-grown lawn type in Maryland), you can overseed in late summer or early autumn to boost the density of the lawn. While you can't do much about the sandier, easy-to-dig nature of the soil the bees appreciate, by increasing the lawn density and vigor, later generations of bees will find the area less suitable for their needs and may nest elsewhere.

If the site is not as suitable for turfgrass any more, such as being too shady or too prone to drying out (as sandier soils can do during drought), then you could convert the area to a groundcover planting with or without mulch between plants, and this will similarly discourage the bees from wanting to nest there, though it's still possible a few will make do and work with those altered conditions. Anything that makes the soil surface less accessible will tend to be less appealing to these kinds of ground-nesting bees.

Miri