Bombus

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	Psithyrus annotated checklist

Back to tree	Number of species in equal-area (611,000 km²) grid cells with an equal-interval blue scale.

26 species

B. citrinus female visiting Trifolium.

Psithyrus
ecology and behaviour

HABITAT: Mountain meadows, forest edges, and grassland.

FOOD-PLANTS: Short to medium tongue-length bumblebees visiting shallow to medium flowers. Females lack pollen-collecting corbiculae on the hind legs (see photo left).

NESTING BEHAVIOUR: There is no worker caste and all species are obligate social parasites ('cuckoos') in colonies of the other social Bombus species (reviewed by Alford, 1975; Fisher, 1987). The degree of host specificity varies among species.

MATE-SEARCHING BEHAVIOUR: Males patrol circuits of scent marks.

Subgenus PSITHYRUS Lepeletier
Psithyrus Lepeletier, 1832:373, type-species Apis rupestris Fabricius (= Bombus rupestris (Fabricius)) by subsequent designation of Curtis, 1833:pl. 468
Apathus Newman, 1835:404, replacement name for Psithyrus Lepeletier, incorrectly stated to be a junior homonym of Psithyros Hübner, [1819]:132 (= Macroglossum Scopoli, 1777:414)
Psithyrus (Laboriopsithyrus) Frison, 1927:69, type-species Bombus laboriosus Fabricius (= Emphoropsis laboriosus (Fabricius) in the sense of Frison (= Bombus citrinus (Smith), a misidentification, see Milliron, 1960:99) by original fixation
Psithyrus (Ashtonipsithyrus) Frison, 1927:69, type-species Apathus ashtoni Cresson (= Bombus ashtoni (Cresson)) by original designation
Psithyrus (Fernaldaepsithyrus) Frison, 1927:70, type-species Psithyrus fernaldae Franklin (= Bombus fernaldae (Franklin)) by original designation
Psithyrus (Eopsithyrus) Popov, 1931:134, type-species Apathus tibetanus Morawitz (= Bombus tibetanus (Morawitz)) by original designation
Psithyrus (Metapsithyrus) Popov, 1931:135, type-species Apis campestris Panzer (= Bombus campestris (Panzer)) by original designation
Psithyrus (Allopsithyrus) Popov, 1931:136, type-species Apis barbutella Kirby (= Bombus barbutellus (Kirby)) by original designation
Psithyrus (Ceratopsithyrus) Pittioni, 1949:270, type-species Psithyrus klapperichi Pittioni (= Bombus cornutus (Frison)) by original designation
Psithyrus (Citrinopsithyrus) Thorp in Thorp et al., 1983:50, type-species Apathus citrinus Smith (= Bombus citrinus (Smith)) by original designation
Bombus (Psithyrus) Williams, 1991:44
[Psithyrus (Fernaldepsithyrus) Amiet, 1996:86, incorrect subsequent spelling]

TAXONOMIC STATUS: It has long been considered useful to regard Psithyrus as a separate genus in recognition of the distinctive behaviour of the species, as social parasites in colonies of the remaining Bombini, and in recognition of their distinctive morphology. However, most recent studies have agreed that, although Psithyrus is itself very likely to be monophyletic, the remaining bumble bees without Psithyrus are not (Plowright & Stephen, 1973; Obrecht & Scholl, 1981; Ito, 1985; Williams, 1985b, 1991, 1995; Pamilo et al., 1987).

Williams (1991 [pdf]) recommended recognising a single genus Bombus for all bumble bees, to include Psithyrus as a subgenus. This was a return to an emphasis of the more widely shared characters and the more distant affinities for the generic concept, encouraged by the opinion of Michener (1990, 2000) that bumble bees are 'morphologically monotonous' or 'homogeneous' in comparison with variation among species within closely related groups such as Euglossini (orchid bees) and Meliponini (stingless bees). One advantage of a single genus for all bumble bees is that it recognises a group for which evidence of monophyly is particularly strong, so that nomenclature is most likely to remain stable in the future. It also emphasises the many differences from other groups of bees. Use of a single genus Bombus for all bumble bees (Williams, 1991 [pdf]) has now been accepted by most recent authors (e.g. Rasmont & Adamski, 1995; Rasmont et al., 1995; Schwarz et al., 1996; Michener, 2000).

The subgenera within the former genus Psithyrus have often been considered less distinct from one another than have the other subgenera of Bombus (Pittioni, 1939a; Ito, 1985; Williams, 1985b; Michener, 1990) and therefore may be treated as synonyms of Psithyrus (Milliron, 1961; Williams, 1991, 1995). In an alternative treatment, Rasmont et al. (1995) included the former subgenera of the former genus Psithyrus as separate subgenera within the genus Bombus.

NOMENCLATURE: Because the type species of Laboriopsithyrus was misidentified (discussed by Milliron, 1960:99), the choice of type species to be designated should be made to serve nomenclatural stability (ICZN, 1999: Article 70). Williams (1998) suggested that, in the interests of stability (ICZN, 1999: Article 23), the bumble bee species actually involved should be designated (Bombus laboriosus in the sense of Frison, = Bombus citrinus (Smith)), which was wrongly named in the type fixation (ICZN, 1999: Article 70.3.2).

Part of the bumblebee phylogenetic tree including available Psithyrus species from an analysis of DNA sequence data for five genes (Cameron et al. 2007 [pdf]). Values above branches are Bayesian posterior probabilities, values below branches are parsimony bootstrap values. Alternative resolution from parsimony analysis is shown with dotted lines.


bellardii barbutellus bohemicus branickii campestris chinensis citrinus coreanus cornutus	expolitus ferganicus flavidus insularis monozonus morawitzianus norvegicus novus	quadricolor rupestris skorikovi suckleyi sylvestris tibetanus turneri variabilis vestalis

citrinus-group of species

Bombus (Ps.) intrudens (Smith)
intrudens (Smith, 1861:154 [Apathus]) examined
variabilis (Cresson, 1872:284 [Apathus])
guatemalensis (Cockerell, 1912:21 [Psithyrus]) examined
sololensis (Franklin, 1915:173 [Psithyrus]) examined
mysticus (Frison, 1925a:138 [Psithyrus])
5 names

TAXONOMIC STATUS: Specimens in the NHM collection from Mexico and Guatemala labelled 'intrudens' and 'sololensis' appear to me to be closely similar to B. variabilis. Frison (1925a) believed that B. sololensis is a colour form of B. guatemalensis, but still distinguished B. mysticus as a separate species only on the basis of colour pattern. I am unaware of any reason (other than minor differences in colour pattern) why B. variabilis, B. intrudens, B. sololensis, or B. guatemalensis and B. mysticus, should not be considered conspecific.

NOMENCLATURE: A female in the NHM collection has three labels 'Apathus / intrudens / Smith.', '58.135 MEX. / (Oajaca.)', 'Holo- / type' and I am unaware of any problems with this designation. If this is correct and the type is conspecific with B. variabilis, then B. intrudens is the oldest available name for this species. D. Yanega (in litt.) agrees with this interpretation.

Although B. intrudens is the oldest available name for the present interpretation of this species, the name B. variabilis has been in common use for the species since 1950 (e.g. Chandler, 1950; LaBerge & Webb, 1962; Mitchell, 1962; Medler & Carney, 1963; Hobbs, 1966; Plowright & Stephen, 1973; Hurd, 1979; Husband et al., 1980; Michener, 1990; Poole, 1996). I know of no publications using the name B. intrudens for the whole of this species since 1899. It could be argued that, in the interests of stability (ICZN, 1999: Article 23), prevailing usage be maintained (in prep.).