Bumbling with Bombus: I’m a bad, bad, bee

Indiscriminate Cuckoo Bumblebee Queen

September was mostly a dry and sunny month in Edmonton with the Home Bug Garden dipping to freezing only twice and no lower than 1.3 C at the downtown airport. So, garden flowers, even the monkshoods that never quite manage to flower before being killed by frost, and a few of the insects that visit them (especially hover flies) have persisted unusually long this autumn. Outside the heat island of the City, though, we had hard frosts (-4) at the Moose Pasture by mid-September. Frost and dryness put end to most of the flowers and only a few asters were still blooming. Well, wild asters and the coneflower (Echinacea purpurea) we had planted on the edge of our small garden plot.

A usually too late bloomer Aconitum divergens

 We didn’t expect much on our mid-September trip except grasshoppers and dragonflies. So my wife didn’t bring her fancy camera, just the point-and-shoot Canon Powershot SX20 that she uses for landscape photos. That caught us up short, because our coneflower, was hosting a small swarm of bumblebees. Mostly these were queens of the Half-black Bumblebee (Bombus (Pyrobombus) vagans Smith, 1854) stocking up for the winter hibernation and a couple of drones hanging on for dear life. But one of the large black and yellow bees looked a bit different – and so it was, an Indiscriminate Cuckoo Bumblebee Bombus (Psithyrus) insularis (Smith, 1861).

Moose Pasture in mid-September 2011

 When bumblebee queens start their nests in the spring, mostly they just work hard to get their colonies growing. But sometimes a queen who has lost her nest or failed to start one will enter the colony of another and try to kill the resident queen and take over. This is called colony usurpation or queen supercedure. Presumably environmental factors such as spring weather and the availability of nest sites influence how often this happens, but usurpation can be common. As many as 20 dead queens have been found in a single nest (Kearns & Thomson 2001) indicating fierce competition for a site. Often a homeless queen tries to steal the nest of another of her species, but other Bombus species are also fair game. Some time in the distant past, one group of bumblebees gave up on raising their own workers and became obligate social parasites – invading nests, killing queens, and co-opting the resident workers to raise broods of cuckoo queens and drones. Compared to queens of other subgenera of Bombus, cuckoos in the subgenus Psithyrus are more heavily armoured, have larger mandibles, a longer curved sting, a more elaborate venom apparatus, more ovarioles, and lay smaller eggs (Fischer & Sampson 1992). So, Psithyrus species seem very well adapted to their nefarious activities.

Note yellow hairs on head, black on base of abdomen

 Our Bombus (Psithyrus) species has acquired the common name of ‘Indescriminate’ and the data that GA Hobbs gathered in the 1960s supports the name. Hobbs worked at what is now known as the Lethbridge Research Centre and most of his data is from the southern part of the Province. He found that what was then known as Psithyrus insularis was successful in usurping the nests of Bombus (Bombias) nevadensis, Bombus (Thoracobombus) fervidus (as B. (Fervidobombus) californicus), Bombus (Subterraneobombus) appositus, Bombus (Bombus) occidentalis and terricola, and many species of Bombus (Pyrobombus). For Pyrobombus, the most diverse and abundant Bombus in the Edmonton area, Hobbs (1967) found 97/191 (51%) of the nests harboured the social parasite. This seems very high, but is similar to what has been reported for European bumblebees parasitized by another species of Bombus (Psithyrus) (Erler & Lattorff 2010). If you think bumblebees are good, and you worry about their conservation, then this may be a bad, bad, bee.

Bombus (Psithyrus) insularis - Indiscriminate Cuckoo Bee

 References

Craig CH. 1953. Psithyrus insularis (Sm.) in a Nest of Bombus ternarius Say (Hymenoptera : Bombidae). Canadian Entomologist 85: 311-312.

Erler S & Lattorff HMG. 2010. The degree of parasitism of the bumblebee (Bombus terrestris) by cuckoo bumblebees (Bombus (Psithyrus) vestalis). Insectes Sociaux 210: 371-377. 30-100%, 33-50%

Fischer RM & Sampson BJ. 1992. Morphological specializations of the bumble bee social parasite Psithyrus ashtoni (Cresson) (Hymenoptera: Apidae). Canadian Entomologist 124: 69-77.

Hobbs GA. 1965. Ecology of Species of Bombus Latr. (Hymenoptera: Apidae) in Southern Alberta. II. Subgenus Bombias Robt. Canadian Entomologist 97: 120-128.

Hobbs GA. 1966. Ecology of Species of Bombus Latr. (Hymenoptera: Apidae) in Southern Alberta. IV. Subgenus Fervidobombus Skorikov. Canadian Entomologist 98: 33-39.

Hobbs GA. 1966. Ecology of Species of Bombus Latr. (Hymenoptera: Apidae) in Southern Alberta. V. Subgenus Subterraneobombus Vogt. Canadian Entomologist 98: 288-294.

Hobbs GA. 1967. Ecology of Species of Bombus Latr. (Hymenoptera: Apidae) in Southern Alberta. VI. Subgenus Pyrobombus. Canadian Entomologist 99: 1271-1292.

Hobbs GA. 1968. Ecology of Species of Bombus Latr. (Hymenoptera: Apidae) in Southern Alberta. VII. Subgenus Bombus. Canadian Entomologist 100: 156-164.

Kearns CA & Thomson D. 2001. The Natural History of Bumblebees. University of Colorado Press, Boulder.