Bombus variabilis — Variable cuckoo bumble bee

photo of Male <em>B. variabilis</em> dorsal
Male B. variabilis dorsal — USGS Bee Inventory and Monitoring Lab
The variable cuckoo bumble bee is considered one of the rarest bumble bees in North America and may possibly be extinct. Its status in Wisconsin is unknown. There are only three historic observation records, all from Waukesha County. It was last observed in 1930.
The variable cuckoo bumble bee is a nest parasite of the American bumble bee (Bombus pensylvanicus) (Colla et al. 2011 and Williams et al. 2014).
Cuckoo bumble bees (subgenus Psithyrus) do not have the ability to collect pollen and thus lay their eggs in the colonies of other species to raise their young (Hatfield et al. 2016). Only new, mated queens overwinter, emerging from diapause (a form of hibernation) in the spring. New queens kill or subdue the queen of a host colony and lay eggs, which the host colony raises. The resulting cuckoo bee offspring are all queens or males. New queens mate and enter diapause.

Status-Global/State:

Global: GU     Wisconsin: SNR    

image showing reference locations of body parts

Identification:

  • Female – Face black and vertex yellow or intermixed. Thorax with black spot or patch that points towards the abdomen and black on sides below the wingpads. Abdomen all black.
  • Male – Face black and vertex yellow or intermixed. Thorax with black spot or patch that points towards the abdomen and black on sides below the wingpads. Abdomen color pattern is variable between morphs, but all are black on T6-7. Some morphs are all black, some are mostly black with intermixed hair on the sides of T3-5, and some have yellow on T1, half of T2, T3 and sides of T4-5.
  • Other distinguishing features – Very short hair and wings dark brown.

Similar Wisconsin Species:

A similar bumble bee species in Wisconsin is the lemon cuckoo bumble bee (B. citrinus) (Williams et al. 2014).

Description of Habitat/Range:

Rangewide information on natural history and ecology are not well known (Colla et al. 2011).

Nectar Plants

Food plants include Asclepias (milkweeds), Aster, Cirsium (thistles), Echinacea (purple coneflowers), Eupatorium (Joe-pye weed), Helianthus (sunflowers), and Solidago (goldenrods), and Trifolium (clovers) (Williams et al. 2014 and Colla et al. 2011).

Flight Season:

Wisconsin's three observation records took place July and September. Range-wide information on natural history and ecology are not well known (Colla et al . 2011).

Literature Cited:

Colla, S., Richardson, L. and Williams, P. (2011) Bumble Bees of the Eastern United States. A product of the USDA Forest Service and the Pollinator Partnership with funding from the National Fish and Wildlife Foundation.

Hatfield, R., Jepsen, S., Thorp, R., Richardson, L., Colla, S. & Foltz Jordan, S. 2016. Bombus variabilis. The IUCN Red List of Threatened Species 2016: e.T21215168A21215249.

Williams, P.H., Thorp, R.W., Richardson, L.L. and Colla, S.R. (2014) The Bumble bees of North America: An Identification guide. Princeton University Press, Princeton.

photo of Male <em>B. variabilis</em> dorsal
Male B. variabilis dorsal — USGS Bee Inventory and Monitoring Lab
photo of Body diagram: female
Body diagram: female — Elaine Evans
photo of Body diagram: male
Body diagram: male — Elaine Evans
photo of Male <em>B. variabilis</em> side
Male B. variabilis side — USGS Bee Inventory and Monitoring Lab
photo of Male <em>B. variabilis</em> face
Male B. variabilis face — USGS Bee Inventory and Monitoring Lab
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