One of my (many) entomological fixations is invertebrates in urban settings, so I’ve been getting better acquainted with native pollinators in my own urban garden in Portland, Oregon. While artificial nest sites like bee blocks and stem bundles are popular among home gardeners, my explorations have illustrated how essential open, undisturbed soil is for ground-nesting bees. I’ve monitored pollinators regularly for over two years, and I see something new each year. In 2019, it was my first local sighting of ground-nesting Bumble Bee-Mimic Digger Bees (Anthophora bomboides), which recently took up residence in a nearby park. In mid-June, while doing dragonfly surveys there, I paused to check out the soil-packed rootwad of a large tree placed several years ago during a stream restoration project. This rootwad is taller than I am and it’s been a hotbed of activity for Western Yellowjackets (Vespula pensylvanica), which I enjoy watching as they go about their daily business.
Western Yellowjackets (Vespula pensylvanica) at nest entrance in rootwad, October 2017. Photo by C. A. Searles Mazzacano, all rights reserved.
This time I was greeted by a different sight—the rootwad was still a hotbed of activity, but each of the numerous nest holes was ornamented by a crooked little mud turret a few centimeters high, with a vertical slit along one side. Even more interesting was that the insects busily coming and going looked like smallish bumble bees…which in no way make nests like these. In a few seconds my brain clicked and I realized it was a different bee giving a great demonstration of its common name: Bumble-bee Mimic Digger Bees (Anthophora bomboides). This species isn’t uncommon, but it’s one I hadn’t observed before.
Anthophora are among my favorite native bees; they have robust furry little bodies in attractive shades of grey, cream, orange, yellow, or red, and they zip around at manic speeds. They are equally manic when it comes to mating. Anthophora are protandrous, which means that when adults complete their development and emerge in spring, males emerge earlier than females. Anthophora males go about the business of reproduction with unseemly enthusiasm; a male Anthophora may attempt to mate once every three seconds, and newly-emerged females may find themselves the center of attention of a few dozen males all frantically trying to mate at once.
Anthophora are among my favorite native bees; they have robust furry little bodies in attractive shades of grey, cream, orange, yellow, or red, and they zip around at manic speeds. They are equally manic when it comes to mating. Anthophora are protandrous, which means that when adults complete their development and emerge in spring, males emerge earlier than females. Anthophora males go about the business of reproduction with unseemly enthusiasm; a male Anthophora may attempt to mate once every three seconds, and newly-emerged females may find themselves the center of attention of a few dozen males all frantically trying to mate at once.
Anthophora bomboides (Bumblebee-mimic Digger Bee) nests in same rootwad, June 2019. Note female entering tunnel at upper right. Phtoos by C. A. Searles Mazzacano, all rights reserved.
The name Anthophora means “flower-bearer,” and these bees are super pollinators, for a variety of reasons. They are very good at shivering, which bees do to warm up their wing muscles, enabling them to fly and visit flowers when other bee species are too cold to be active (the flip side is that their furry bodies overheat more easily, so after a busy morning on a hot day they may retire to the shade). They are also one of the bee groups that does “buzz pollination,” whereby they hold onto a flower with their feet and vibrate their wing muscles to shake pollen out of the anthers (fun fact: non-native European Honey Bees (Apis mellifera can't buzz pollinate). Buzz pollination is critically important for plants such as tomatoes, peppers, eggplant, cranberry, and potato, whose anthers are akin to salt shakers, with small openings at the tip through which pollen must be shaken. Anthophora also have long tongues, which allow them to access nectar in deeper and more tubular flowers.
The tribe that contains this genus (Anthophorini) has over 700 species in seven genera worldwide. We have just two genera in the US (Anthophora and Habropoda), but identification to species is often (and rightly) described as “fiendishly difficult” (and when it comes to bees, a notoriously troublesome group to ID, that’s saying something). There are about 50 Anthophora species in the US, most of which are western. The genus contains both generalist and specialist species, and specialists can be important pollinators of wildflowers such as beardtongues (Penstemon) and lupines.
Like most of our native bees, almost all Anthophora nest in the ground, often in large aggregations, and several females may share the same nest entrance, though they maintain their own tunnels. Females use their forelegs and mandibles to loosen the soil and dig tunnels, shoveling loose dirt backwards; I’ve often been alerted to the presence of nesting females by puffs of dirt rising from the ground. Once a tunnel is complete, they excavate small chambers or cells, where a “loaf” of pollen and nectar is placed to provision the egg that is laid there. They line the nest with secretions from their Dufour’s gland, a specialized abdominal gland found in bees, wasps, and ants. In Anthophora, these secretions are rich in water-repellant triglycerides and hydrocarbons, to protect the nest and developing larvae from water and fungi. After nest provisioning and egg-laying is done, the tunnel opening is plugged with a circular bit of mud and the female flies away, dying not long afterwards.
The tribe that contains this genus (Anthophorini) has over 700 species in seven genera worldwide. We have just two genera in the US (Anthophora and Habropoda), but identification to species is often (and rightly) described as “fiendishly difficult” (and when it comes to bees, a notoriously troublesome group to ID, that’s saying something). There are about 50 Anthophora species in the US, most of which are western. The genus contains both generalist and specialist species, and specialists can be important pollinators of wildflowers such as beardtongues (Penstemon) and lupines.
Like most of our native bees, almost all Anthophora nest in the ground, often in large aggregations, and several females may share the same nest entrance, though they maintain their own tunnels. Females use their forelegs and mandibles to loosen the soil and dig tunnels, shoveling loose dirt backwards; I’ve often been alerted to the presence of nesting females by puffs of dirt rising from the ground. Once a tunnel is complete, they excavate small chambers or cells, where a “loaf” of pollen and nectar is placed to provision the egg that is laid there. They line the nest with secretions from their Dufour’s gland, a specialized abdominal gland found in bees, wasps, and ants. In Anthophora, these secretions are rich in water-repellant triglycerides and hydrocarbons, to protect the nest and developing larvae from water and fungi. After nest provisioning and egg-laying is done, the tunnel opening is plugged with a circular bit of mud and the female flies away, dying not long afterwards.
Female digger bee at the Oregon Coast, caught in the act of kicking excavated sand backwards out of her nest tunnel. Photo by C. A. Searles Mazzacano, all rights reserved.
Bumble-bee Mimic Digger Bees put their own twist on nest-building. They nest in banks and bluffs, generally not too far from a source of water, which they carry back to the nest in their crops to soften the soil for digging. My observations of the communal nest site in 2019 were almost exactly those of H.H. Nininger in 1920, who wrote, “Their burrows were placed close together and in an almost vertical position, and over the entrances of many of them were constructed very peculiar bent-over chimneys of clay…the warm sunshine kindled the vital spark in these insects to the greatest activity. They were scurrying in and out of their burrows…in what seemed to be feverish haste…” Nininger described the tunnels as 5-7 inches deep, with 2-5 oval egg chambers at the end. The “feverish haste” at my site included quite a few mis-identifications of the home tunnel, as many returning females shot into an opening and back out again just as quickly, occasionally with irritated help from the nest’s true owner. They use bits of wet soil from excavation to build their distinctive turrets, whose function isn’t completely known; hypotheses include deterring parasitism, preventing debris from entering the tunnel, serving as a sign for other females to nest, facilitating nest recognition, and protection from rain.
Bumble-bee Mimic Digger Bees are considered Batesian mimics (i.e., a palatable or harmless species assuming the appearance of a distasteful or dangerous one), as they are pretty docile even for a solitary bee; they may bite if handled roughly, but they aren’t big on stinging. They also provide habitat for an interesting array of other creatures. They have a strong association with nematode (roundworm) called Bursaphelenchus seani; in one study, the nematode was found in brood cells during all stages of larval and pupal development, and when adult bees emerged the juvenile nematodes moved into their reproductive tracts (which seems like a hell of a welcome to the world). Another paper described the nematode/bee relationship as phoretic, as they apparently hitch rides on adults so they can make their way to the protected nest cells made by female bees, where they feed on fungi (rather than on the nectar/pollen “bee bread”). Other studies note the presence in nests of a type of dermestid (carpet or skin beetle) in the genus Anthrenus, many of which are pollen-eaters and are known to scavenge the remains of pollen and dead bees in old nest cells, and a species of Medeus mite that feeds on pollen and fungi in host cells where young bees don’t develop successfully.
There are several take-home messages here. The first is that native pollinators can survive and even thrive in urban settings—not all species, but certainly some. The second follows from that—setting out bee blocks and stem bundles for native bees is an increasingly popular pastime and it’s great fun to see mason or wool-carding bees move into them, but the fact is that over 70% of native bee species in North America nest in the ground. Undisturbed soil is hard to come by in urban settings, and ground-nesting bees tend to do worse in urban areas than crevice- or wood-nesting bees. Third, it isn’t just a diversity of bees we protect when we conserve and sustain patches of healthy habitat, it’s also all the other species that may rely on, associate with, benefit from, or opportunistically encounter bees and their nests. So this winter, when many of us are planning next year’s garden or restoration projects (and possibly buying “bee houses” as gifts for gardening friends), consider the simple expedient of also protecting areas of undisturbed soil. Come next spring and summer, you’ll be rewarded by the appearance of small round holes, and seeing the “vital spark” of native solitary ground-nesting bees “scurrying in and out of their burrows.”
Supporting literature
Brooks, R.W. 1983. Systematics and Bionomics of the Anthophora: the Bomboides group and species groups of the new world (Hymenoptera: Apoidea, Anthophoridae). University of California Publications in Entomology, Volume 98.
Cane, J.H. 1981. Dufour’s gland secretion in the cell lining of bees (Hymenoptera: Apoidea). Journal of Chemical Ecology 7: 403-410.
Giblin, R.M. and H.K. Kaya. 1983. Field observations on the association of Anthophora bomboides standfordiana (Hymenoptera: Anthophoridae) with the nematode Bursaphelenchus seani (Aphelenchida: Aphelenchoididae). Annals of the Entomological Society of America 76(2): 228-231.
Nininger, H.H. 1920. Notes on the life-history of Anthophora stanfordiana, Psyche 27(6): 135-137.
Norden BB, Batra SWT, Fales HM, Hefetz A, Shaw JC (1980) Anthophora bees; unusual glycerides from maternal Dufour’s glands serve as larval food and cell lining. Science 207: 1095–1097.
O’Connor, B.M. 1996. Two new mites (Acari: Acaridae) associated with long-tongued bees (Hymenoptera: Apidae) in North America. Journal of the Kansas Entomological Society 69(4) suppl.: 15-34.
Bumble-bee Mimic Digger Bees are considered Batesian mimics (i.e., a palatable or harmless species assuming the appearance of a distasteful or dangerous one), as they are pretty docile even for a solitary bee; they may bite if handled roughly, but they aren’t big on stinging. They also provide habitat for an interesting array of other creatures. They have a strong association with nematode (roundworm) called Bursaphelenchus seani; in one study, the nematode was found in brood cells during all stages of larval and pupal development, and when adult bees emerged the juvenile nematodes moved into their reproductive tracts (which seems like a hell of a welcome to the world). Another paper described the nematode/bee relationship as phoretic, as they apparently hitch rides on adults so they can make their way to the protected nest cells made by female bees, where they feed on fungi (rather than on the nectar/pollen “bee bread”). Other studies note the presence in nests of a type of dermestid (carpet or skin beetle) in the genus Anthrenus, many of which are pollen-eaters and are known to scavenge the remains of pollen and dead bees in old nest cells, and a species of Medeus mite that feeds on pollen and fungi in host cells where young bees don’t develop successfully.
There are several take-home messages here. The first is that native pollinators can survive and even thrive in urban settings—not all species, but certainly some. The second follows from that—setting out bee blocks and stem bundles for native bees is an increasingly popular pastime and it’s great fun to see mason or wool-carding bees move into them, but the fact is that over 70% of native bee species in North America nest in the ground. Undisturbed soil is hard to come by in urban settings, and ground-nesting bees tend to do worse in urban areas than crevice- or wood-nesting bees. Third, it isn’t just a diversity of bees we protect when we conserve and sustain patches of healthy habitat, it’s also all the other species that may rely on, associate with, benefit from, or opportunistically encounter bees and their nests. So this winter, when many of us are planning next year’s garden or restoration projects (and possibly buying “bee houses” as gifts for gardening friends), consider the simple expedient of also protecting areas of undisturbed soil. Come next spring and summer, you’ll be rewarded by the appearance of small round holes, and seeing the “vital spark” of native solitary ground-nesting bees “scurrying in and out of their burrows.”
Supporting literature
Brooks, R.W. 1983. Systematics and Bionomics of the Anthophora: the Bomboides group and species groups of the new world (Hymenoptera: Apoidea, Anthophoridae). University of California Publications in Entomology, Volume 98.
Cane, J.H. 1981. Dufour’s gland secretion in the cell lining of bees (Hymenoptera: Apoidea). Journal of Chemical Ecology 7: 403-410.
Giblin, R.M. and H.K. Kaya. 1983. Field observations on the association of Anthophora bomboides standfordiana (Hymenoptera: Anthophoridae) with the nematode Bursaphelenchus seani (Aphelenchida: Aphelenchoididae). Annals of the Entomological Society of America 76(2): 228-231.
Nininger, H.H. 1920. Notes on the life-history of Anthophora stanfordiana, Psyche 27(6): 135-137.
Norden BB, Batra SWT, Fales HM, Hefetz A, Shaw JC (1980) Anthophora bees; unusual glycerides from maternal Dufour’s glands serve as larval food and cell lining. Science 207: 1095–1097.
O’Connor, B.M. 1996. Two new mites (Acari: Acaridae) associated with long-tongued bees (Hymenoptera: Apidae) in North America. Journal of the Kansas Entomological Society 69(4) suppl.: 15-34.
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