Soil “mining bees” have been on the wing in southern and central Ohio for a few weeks but their activity has been rising and falling with our roller coaster temperatures. Still, we’re receiving reports of homeowners, school administrators, park managers, and others feeling besieged and opting to consider taking drastic, ill-advised measures. Misplaced fears can make these important native pollinators targets of misinformed insecticide applications.
Many species of small pollinator bees create individual (solitary) burrows in the soil. These 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 often locate their burrows near 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. We commonly refer to these collections which may involve hundreds of burrows as “colonies” although this is not entirely accurate.
It's important to keep two things in mind with soil mining bees. First, as with all Hymenoptera, only the females have stingers (= ovipositors). However, the vast majority of the soil mining bees buzzing around the so-called colonies are stingless males; it’s all a rouse.
Second, speaking from personal experience, female soil mining bees are not the least bit aggressive. I have knelt for hours within buzzing bee colonies to take pictures and videos. 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 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 image below of a female peering out from her burrow.
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. 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.
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 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 are a list of plants that will support these helpful pollinators; a sign of good environmental stewardship!