Last week, I came across one of the largest collections of soil "mining bees" that I've ever seen in Ohio. The "colony" was located in a picnic area and numerous males were making their low-level flights in search of females. The sparse turfgrass coupled with early-evening lighting made conditions perfect for taking pictures.
Unfortunately, the colony's location made me worry for the safety of the busy bees. Misplaced fears can make these important native pollinators targets of misinformed insecticide applications.
There are multiple species of bees representing a number of families that create individual (solitary) burrows in the soil; however, there is a high probability that burrowing bees found in Ohio belong to the genus Andrena (family Andrenidae) with over 100 species living in our state. Many of these species have prominent banding on their abdomens which is even visible in flight.
These native pollinators are typically 3/16 - 3/4" long, depending on the species, and have banded abdomens. Females dig individual burrows several inches deep into the soil. They prefer to nest in well-drained soil that is lightly exposed to sunlight. This includes areas in landscaping with sparse vegetation such as openings created by weakened turfgrass.
Each burrow consists of a hole about the diameter of a wooden pencil surrounded by a mound of loose, excavated soil particles. The loose soil particles can disappear after a heavy rainfall leaving only the hole. The size, shape, and color of the soil particles may cause the mounds to be mistaken for those produced by ants or even earthworms.
Mining bees are considered solitary bees with no social structure. However, large numbers of females often locate their burrows in close proximity to one another giving the appearance of an organized colony. The collective nesting behavior may be associated with maximizing the chances for males to find and mate with females.
The females become receptive to mating after they provision their burrows with wads of pollen mixed with nectar to nourish their larvae. You can observe receptive females peering from their burrows. If you keep watching, you will observe one or more males clamoring around burrow entrances intent on getting acquainted with a female … at which point you should look away.
Mated females deposit multiple eggs in their burrows and the resulting larvae feed and develop on the pollen / nectar banquet provided by the females. Winter is spent as pupae in the burrows with adults emerging in the spring to start a new round of bees.
Occasionally, you may find conical-shaped holes dug within the mining bee colonies. The holes are evidence that skunks or raccoons were searching for a bee morsel meal.
Mining bees are important polylectic plant pollinators meaning they gather pollen from many different plants. They are particularly important for pollinating spring-blooming food crops including apples, cherries, and blueberries.
Unfortunately, the low-level flight plans by the males may be frightening to the unenlightened. While the females are busily digging and provisioning their burrows, the pugnacious males cruise menacingly back and forth just above the soil chasing other males or possibly predators. It's all a rouse because the males lack stingers (= ovipositors).
However, the collective buzzing sound made by the males can be intimidating to uninformed gardeners or landscape managers. Indeed, the family name Andrenidae is derived from the Greek anthrene which originally referred to any buzzing insect.
Solitary soil burrowing bees, including mining bees, are not aggressive. You almost need to try to get stung to be stung. Even then, stings from these bees don't pack much of a punch; their small stingers can't penetrate far into the skin.
Of course, large numbers of bees buzzing around at knee-height may trigger fear in the uninitiated prompting ill-advised efforts to eliminate these beneficial insects including applications of insecticides. This practice should be strongly discouraged. Instead, cultivation practices aimed at thickening turfgrass will convince the bees to burrow elsewhere.
Fortunately, fear of these highly beneficial insects may be calmed through education. For example, I've taken pictures of signs placed over colonies in Spring Grove Cemetery and Arboretum by Dalton Westerbeck who is an Ohio State Beekeepers Association, Master Beekeeper. The signs are a proactive educational approach to calming fears by explaining to visitors exactly what is happening with these beneficial bees. Included on the sign is a list of plants that will support these helpful pollinators; a sign of good environmental stewardship!