Status Review of Three Formerly Common Species of Bumble Bee ...

Parallel decline of a cuckoo bee:B. terricola, as well as B. affinis, declines have likely caused a severe reduction inBombus ashtoni populations. Bombus ashtoni is bumble bee species that parasitizes otherbumble bees by entering nests and using the worker force of the host colony to raise theiryoung instead of the queen’s progeny. B. ashtoni exclusively parasitizes B. terricola andB. affinis (Plath 1934; Fisher 1984; Laverty and Harder 1988). B. ashtoni has not beenfound in any parts of its former range since 2000 (J. Ascher, personal communication,August 2008). Since parasitic bumble bees are dependent on their hosts for reproduction,declines in host numbers can have severe effects on parasitic bumble bee’s populations.Decline of B. affinis and B. terricola populations is the most likely cause of the possibleextinction of B. ashtoni. The absence of B. ashtoni since 2000 provides indirect evidenceof a decline of their hosts, B. affinis and B. terricola.SummaryBombus terricola was once common and widespread across 21 U.S. states and sevenCanadian provinces. Although some individuals were recently found in isolated parts ofits range between 2005 and 2008, the observations and data presented above suggest thatthis bumble bee has drastically declined in large parts of its range.V. THE WESTERN BUMBLE BEE, BOMBUS OCCIDENTALIS GREENEA. Species Description (nominate form)Queens and Workers:Bombus occidentalis queens and workers are similar in coloration. The queen is 17 to 19mm in length, 9 to 10 mm in breadth. The worker is 9 to 14 mm in length, 5 to 7 mm inbreadth. Their hair is entirely black on the head. Their hair is yellow on the front part ofthe thorax. The first through the basal section of the fourth abdominal segments haveblack hair. The apex of the fourth abdominal segment as well as segments five and six arewhitish. The hair on their legs is black.Males:The male is 13 to 17 mm in length, 6 to 8 mm in breadth. The hair on the head is paleyellowish on the front of the face. The top of the head has pale yellowish hairs medially,with some black hairs, especially laterally. The hair on the front of the thorax is paleyellowish. The hair on the first to third abdominal segments is black. The basal part of thefourth abdominal segment is black, with the remainder, as well as segments five to seven,whitish.While B. affinis and B. terricola exhibit some color variation throughout their ranges, B.occidentalis is the most chromatically variable North American species in the subgenusBombus. Franklin (1913) describes twelve female and twelve male variants of thisspecies in his revision of New World bumble bees. As a result, many variety names havebeen applied to this species. Major color variants of B. occidentalis females, pictured inFigure 6, include:17