The majority of bee species nest below ground with around 70% excavating their own nests. Indeed, nesting in the ground is considered the ancestral nesting behavior of all bees. Although there may be some “ground rules” regarding how we refer to these solitary bees, I’m never certain if we should call them, “ground-nesting,” “soil mining,” or something else.
Solitary soil mining, ground-nesting, soil-digging 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.
As noted above, bees nesting in the soil is common across several hymenopteran families. Small pollinator bees creating individual (solitary) soil burrows include members of the Andrenidae and Halictidae families with multiple species found in Ohio. Members of the Colletidae family are also found in our state and include so-called cellophane or plasterer bees based on the interesting practice of the females lining their soil burrows with a cellophane-like substance. Even the Apidae family which includes honey bees (Apis mellifera) has some members that burrow into the soil; the so-called “digger bees.”
Soil mining bees are considered solitary bees with no social structure. However, large numbers of females, sometimes numbering in the hundreds, commonly locate their burrows near one another giving the appearance of an organized colony. These collections are sometimes called “colonies;” however, they should more accurately be called “aggregations.”
The driving forces behind the collective nesting of soil mining bees are poorly understood. It may be as simple as maximizing the chances for males to find and mate with females, or perhaps the bees just like being near one another. Speculation has also focused on shared protection from predators or parasitoids. However, I’ve commonly knelt within large aggregations to take pictures, sometimes right on top of the burrows, and I’ve never experienced any sort of concerted effort to drive me off.
Various studies have implicated site conditions such as soil texture, compaction, soil moisture, soil surface features, or the proximity to resources such as flowers. Although one explanation does not fit all, a common shared feature between bee nesting aggregations is bare soil or soil covered by sparse vegetation that’s exposed to several hours of sunlight.
It's important to keep two things in mind with solitary soil mining soil bees. First, as with all Hymenoptera, only the females have stingers (= ovipositors). However, the vast majority of the bees buzzing around collective nest sites are stingless males; it’s all a rouse.
Second, speaking from personal experience, female soil mining bees are not the least bit aggressive. As noted above, I’ve knelt for hours within buzzing bee communal nesting sites to take pictures and videos and I’ve never been stung or even harassed by the bees.
In fact, the females are extremely shy creatures. Rather than buzzing forth from their burrows to attack in the style of ground-nesting yellowjackets (Vespula spp.), female solitary soil-nesting 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.
These native pollinators are typically 3/16 - 3/4" long, depending on the species, and may have banded abdomens. Some sport brightly-colored thoraxes like the so-called 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 that is 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 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 intent on getting acquainted with a female which commonly leads 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.
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 stingless males cruise back and forth just above the soil chasing other males or possibly predators.
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; thickening the turfgrass 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 Spring Grove Cemetery and Arboretum. 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!