I recently came across one of the largest colonies of soil mining bees I’ve ever seen in Ohio. The image below shows the extent of the collection of these important plant pollinators found in Spring Grove Cemetery and Arboretum (SGCA).
Bees nesting in the ground may seem unusual. After all, our non-native European honey bee (Apis mellifera, family Apidae) nest above the ground, and in the eyes of the general public, they are the “poster child” for bees.
However, bees nesting above ground is the exception rather than the rule. The vast majority of bee species nest below ground with around 70% digging subterranean burrows. Indeed, nesting in the ground is considered the ancestral nesting behavior of all bees.
The bees I’m sighting (and citing) are relatively small bees that excavate individual soil burrows. They are tagged with a range of common names including “ground-nesting bees,” “burrowing bees,” “soil burrowing bees,” and “mining bees.” I may be breaking some ground rules, but I call them “soil mining bees,” although I remain uncertain whether or not it's “soil-mining.”
Bee Together
Another term commonly applied to soil mining bees is “solitary.” However, large numbers of females, sometimes numbering in the hundreds, commonly locate their burrows near one another in colonies.
Calling an aggregation of soil mining bees a “colony” is appropriate in biological terms because the word “colony” is applied to a collection of animals of the same species living together for some mutual benefit. Although a social structure may exist, it’s not a requirement. This is true of soil mining bees; they don’t appear to have any obvious social structure.
The driving forces behind the collective nesting habits of soil mining bees are poorly understood. The mutual benefit may involve maximizing local resources, enhancing successful mating, mounting a stronger defense, etc. Of course, environmental conditions such as soil drainage or exposure to sunlight may also influence nesting site selection.
Enhancing successful mating seems a commonsensical explanation. A large collection of females in one location makes it easier for males to locate mates.
It’s also speculated that as with other animal colonies, the aggregation of soil mining bees provides shared protection from predators or parasitoids. On the surface, this makes sense. As with all Hymenoptera, female soil mining bees have modified ovipositors (= stingers) that can be used for defense.
However, speaking from personal experience, female soil mining bees are not aggressive. I’ve knelt for hours within communal nesting sites to take pictures and videos and I’ve never been stung or even harassed by the female bees although the sites were buzzing with activity.
In fact, the females are extremely shy creatures. Rather than buzzing forth from their burrows to attack in the stinging style of ground-nesting yellowjackets (Vespula spp.), female solitary soil mining bees will withdraw back into their burrows to await the departure of shadowy figures looming over their chambers. It took a considerable amount of patience to capture the accompanying images of females peering out of their burrows.
I’ve been buzzed by male soil mining bees as they cruise low to the ground in search of receptive females; however, such aggressive behavior is extremely rare. Of course, the males lack ovipositors (= stingers), so it’s a ruse.
Site conditions may also influence the selection of certain sites by female soil mining bees. A common feature in the collective nesting sites is bare soil or soil covered by sparse vegetation. Important enticing conditions may also other soil-related features such as soil texture, compaction, and soil moisture as well as proximity to resources such as flowers.
Indeed, some have speculated that soil mining bee colonies may develop through individual females simply finding a common site to their liking. It may be generationally maintained by daughters liking what their mothers, grandmothers, great-grandmothers, etc., etc. selected.
Of course, the collective nesting habits of “solitary” soil mining bees may be driven by one or all of the above as well as factors not yet discovered. The bottom line is that soil mining bee colonies may range in size from only a few square feet to hundreds of square feet. However, despite the size of these collections of important pollinators, they do not represent a threat to homeowners or their pets.
Bee Familiar
Our native soil mining bees are typically 3/16 - 3/4" long, depending on the species. The bees range in color and markings. Some have obvious black and white bands on their abdomens while others have black abdomens. Some are very colorful particularly when their pollen baskets (corbiculae) are ladened with grains of larval goodies as with the Bicolored Striped Sweat Bee (Agapostemon virescens, Family Halictidae) pictured below.
Females dig individual burrows several inches deep into the soil. As noted above, they prefer to nest in soil covered by 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 soil particles can provide clues to the underlying soil texture. For example, the large colony highlighted in this Alert transected an area of sandy soil as reflected by the particles in the soil mounds.
Something that I noticed this year that I’ve never observed in the past is the appearance of horizontal “runways” in the mounds. They appear to be produced as the female bees shove soil particles out of their burrows. I’m uncertain if this feature is common to certain species, or if it is associated with soil texture. It may also be a common feature as long as mounds aren’t subjected to heavy rainfall while the females are excavating their burrows.
The loose soil particles can disappear after heavy rainfall which leaves 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.
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. The males are intent on getting acquainted with a female which may lead to a mating scrum … 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.
Soil mining bees have been on the wing in southern and central Ohio for a few weeks. However, their activity has been rising and falling with our roller coaster temperatures.
The rise in awareness of the importance of pollinators has, by extension, elevated the awareness of the importance of protecting pollinators. However, actions may not always be guided by awareness. Unfortunately, the response to solitary soil mining bees may be influenced by NIMBY (not in my backyard) attitudes.
The misperception that the bees represent a serious stinging threat may drive homeowners, school administrators, park managers, and others to consider taking drastic, ill-advised measures. Misplaced fears can make these important native pollinators targets of misinformed insecticide applications.
Bee Nice
Soil 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. Of course, 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.
Large numbers of bees buzzing around at knee height may trigger fear in the uninformed prompting ill-advised efforts to eliminate these beneficial insects including applications of insecticides. This practice should be strongly discouraged.
Instead, long-term management plans should focus on changing the environment using plant cultural practices aimed at making the location less attractive to the bees. For example, soil mining bees prefer to burrow in thin turf. Applying proper turfgrass management techniques to thicken the turf canopy will convince the bees to burrow elsewhere.
Another effective approach is to use education to calm the fears of these highly beneficial insects. I've taken several pictures in the past of proactive signs placed over colonies in SGCA. The signs explain to visitors exactly what is happening with these beneficial bees. Included on some of the signs is a list of plants that will support these helpful pollinators; a sign of good environmental stewardship!
Selected References (By Published Date)
Michener, C.D., 1964. Evolution of the nests of bees. American Zoologist, pp.227-239.
Cane, J.H., 1991. Soils of ground-nesting bees (Hymenoptera: Apoidea): texture, moisture, cell depth and climate. Journal of the Kansas Entomological Society, pp.406-413.
Paxton, R.J., 2005. Male mating behaviour and mating systems of bees: an overview. Apidologie, 36(2), pp.145-156.
Klein, A.M., Vaissière, B.E., Cane, J.H., Steffan-Dewenter, I., Cunningham, S.A., Kremen, C. and Tscharntke, T., 2007. Importance of pollinators in changing landscapes for world crops. Proceedings of the royal society B: biological sciences, 274(1608), pp.303-313.
Wilson, J.S. and Messinger Carril, O.J., 2016. The bees in your backyard: a guide to North America's bees. Princeton University Press.
Harmon-Threatt, A., 2020. Influence of nesting characteristics on health of wild bee communities. Annual Review of Entomology, 65, pp.39-56.
Antoine, C.M. and Forrest, J.R., 2021. Nesting habitat of ground‐nesting bees: a review. Ecological Entomology, 46(2), pp.143-159.
Szczepko-Morawiec, K., Wiśniowski, B., Motyka, E., Celary, W. and Kruk, A., 2024. Ecological amplitude and indication potential of mining bees (Andrena spp.): a case study from the post-agricultural area of the Kampinos National Park (Poland). Scientific Reports, 14(1), p.9738.
