There are many species of bees that create individual (solitary) burrows in the soil. Several hymenopteran families are represented including Andrenidae (Mining Bees), Apidae (Tribe Anthophorini (Digger Bees)), and Colletidae which are called cellophane bees owing to the waterproof plastic-like material they use to line their soil burrows. They all have one thing in common: they are important pollinators.
All of these bees may be found in Ohio and a correct identification requires taking a close look at wing veins and other body features including certain structures on their heads. However, if you come across a collection of bees making individual soil burrows, there is a high probability they belong to the genus Andrena (family Andrenidae) with over 450 species occurring in the U.S. and over 100 species found in Ohio.
These native pollinators are typically 3/16 - 3/4" long, depending on the species, and many 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. I was surprised last week to find an active, healthy mining bee colony in exposed soil at the junction of two forest trails.
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.
Females become receptive to mating after they provision their burrows with wads of pollen mixed with nectar to nourish their larvae. You can easily observe these females peering from their burrows. If you keep watching, you will also observe one or more males clamoring around the burrow entrance intent on getting acquainted with the female.
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.
The Fear Factor
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.
Unfortunately, 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.
Solitary soil burrowing bees, including mining bees, are not aggressive. You almost need to try to get stung, to get stung. Even then, stings from these bees don't pack much of a punch; their small stingers can't penetrate far into the skin.
More importantly, mining bees are significant pollinators of spring blooming food crops including apples, cherries, and blueberries. Quoting from a study published in 2006 in the Journal of the New York Entomological Society*: "Of the 31 native bee species collected [in New York apple orchards], 14 species belong to eight subgenera of Andrena."
Replacing Fear with Enlightenment
Fortunately, fear of these highly beneficial insects may be calmed through education. I came across a great example yesterday at Spring Grove Cemetery and Arboretum. Dalton Westerbeck, ISA Certified Arborist and Ohio State Beekeepers Association, Master Beekeeper, is finding and marking all of the mining bee colonies in the Grove this spring so he can document their population densities and seasonal development.
One of Dalton's mining bee colonies is located within the dripline of the Ohio State Champion Katsura Tree (Cercidiphyllum japonicum); a high profile site. In a proactive educational approach to calming fears, Dalton had placed a sign over the bee colony 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!
Spring Grove's dedication to pollinators is also evidenced by their practice of allowing Spring Beauty wildflowers (Claytonia virginica) to sprout from turfgrass. This spring ephemeral does not last long and will eventually disappear. In the meantime, the white carpets buzz with mining bees and other pollinators.