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.
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.
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.”
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.