Wall of Bees

I just learned that this week is National Pollinator Week, which I ironically started off with a post about sneaky beetle larvae that take advantage of bees visiting flowers.  So this seems like a good time to write about the Anthophora abrupta colony in Maryland that Noah Charney and I visited on our way back to Massachusetts from Tennessee a few weeks ago.  A. abrupta is a bee with no official common name, in the family (Apidae) that includes honey, bumble, and carpenter bees, along with some other less well known ones.  Rau (1929)* refers to it as “the turret-building bee,” and these turrets were what I wanted to see.  Along with mud-covered ground beetle eggs, water scavenger beetle egg cases (which I stumbled upon last spring), and the pupal cases of whirligig beetles (still looking), they were among the unillustrated phenomena described in our book that I most wanted to find and photograph.  They are typically found in sunny, vertical clay banks, and although the bees occur as far north as southern Ontario and Quebec, they are uncommon in the northern part of their range.  After asking around a bit last spring, I had determined that USDA bee researcher Sam Droege’s straw bale / adobe house was the place closest to home that I could count on seeing these bees and their turrets.

The back of Sam Droege's house, showing the Anthophora abrupta colony to the right of the door.

Noah and I arrived on June 2 and found the bees busily buzzing in and out of their burrows by the back door.  Just as I had read, their burrows ranged from being unadorned to having drooping mud turrets up to three inches long, some of these with a slot running along the top.  Based on this five-year-old photo of the colony, which shows a lot of turrets in the lower part of the colony, I believe the unadorned holes are mostly the old ones whose turrets have weathered away, as opposed to being burrows that simply never had turrets.  It is apparent in my photo above that the highest concentration of turrets is farthest from the lower right corner where the colony began.  However, the only burrows Noah and I saw the bees entering had no turrets, and we wondered if this  was related to the relatively dry recent weather.  Rau’s observations support this idea; he describes how the bees carry water to their burrows, use it to soften the earth as they excavate it, and then use this moistened excavated material to build the turrets.  He reports that in dry years the burrows had short turrets or none at all.

Anthophora abrupta burrows with turrets of varying lengths.

An example of a slotted turret.

An Anthophora abrupta returning to her burrow, her legs loaded up with pollen to provision her nest.

The purpose of the turrets isn’t entirely clear, but one possibility is that it protects the nests from parasites.  Noah and I saw several bee flies (Bombyliidae) hanging around the colony and appearing to lay eggs at the burrow entrances.  Bee fly larvae crawl into bee nests, where they slowly devour the bee larvae.  It does seem to me that the tip of a long turret would be a more difficult place for a fly to lay her eggs and have her larvae successfully make their way to the bee larvae.

A bee fly ovipositing at the entrance to an unturreted Anthophora abrupta burrow.

The 1979 Catalog of Hymenoptera lists several parasites of Anthophora abrupta nests but doesn’t mention any bee flies, and no one has yet offered an ID on the photos I submitted to BugGuide.net.  Some bee flies are highly host-specific, so it will be interesting to see what these turn out to be.**

In addition to providing homes for their various parasites when the nests are active, the bees’ old burrows may be reused by various cavity-nesting bees and wasps.  If you are not lucky enough to have a straw bale house with an Anthophora abrupta colony in it, there are various types of bee walls you can install to attract these pollinating insects.

Two old Anthophora abrupta burrows. The one on the right has been taken over by a grass-carrying wasp (Isodontia), which has provisioned its nest with paralyzed tree crickets; the one on the left is sealed with mud, which could indicate the nest of a mason bee (Osmia; provisioned with pollen and nectar), mason wasp (Eumeninae; provisioned with paralyzed caterpillars), spider wasp (Pompilidae; provisioned with one spider per cell), or keyhole wasp (Trypoxylon; many spiders per cell).

* Rau, Phil. 1929. The Biology and Behavior of Mining Bees, Anthophora abrupta and Entechnia taurea. Psyche 36(3):155-181.

** On 8/11/2011, Joel Kits identified this bee fly as Anthrax irroratus.  This species has previously been documented as parasitizing an unspecified Anthophora species, along with a number of other bees and wasps.

The species seems to be a nonspecific parasite whose hosts are various hymenopterous insects nesting in logs, banks, and open, flat areas. Females often are observed ovipositing in openings of tunnels in stumps and logs, and sometimes are attracted to the collector, flipping eggs at dark spots on the clothing.

Reference: Marston, Norman. 1971. Taxonomic Study of the Known Pupae of the Genus Anthrax (Diptera: Bombyliidae) in North and South America. Smithsonian Contributions to
Zoology, number 100, 18 pages.