Queens fly from early May (occasionally from mid April in favourable, southern localities) to early October, workers from the beginning of July to late September, and males from mid July to late September. The flight season starts later and finishes earlier in the northern parts of its range. (Løken, 1973)
Parasites and predators: Bombus campestris is a probable social parasite of this species (Hoffer, 1882, 1888, 1889; R.C.L. Perkins, 1945). The mutillid wasp Mutilla europaea is also cited as a parasite (Peeters, Raemakers & Smit,1999) in the Netherlands.
Remarks: Throughout much of its euro-siberian range, populations of this species are very variable in colour, some individuals being almost totally black. This very dark form does not occur in Britain, where the species remains rather constant incolour, dark forms being restricted to only a very small percentage of males and small workers. Yarrow (1977)considered the British population to be sufficiently distinct from those on the continent, to be warranted subspecific status, subspecies anglicus being proposed by him. His publication includes details of the variabilty of this species in both Britain and Europe.
Nomenclature: The orthography of Schrank (1802) employs a long 's' (similar to 'f' or 'f'), a common practice of the period. This convention has since changed and recent authors have consistently used 's'.
When Day (1979) came to fix the application of A. muscorum Linnaeus (see the comments on B. muscorum), he had no reason to believe that Linnaeus had not described this taxon from the syntype specimen that was subsequently described as lectotype (= B. humilis Illiger). This action brought B. humilis Illiger into subjective junior synonymy with B. muscorum (Linnaeus).
To reaffirm the traditional usage of B. muscorum and B. humilis, a case was made to ICZN by Løken et al. (1994). This sought an Opinion from ICZN (ICZN, 1996) that set aside by use of its Plenary Power (ICZN, 1985: Articles 78b, 79) the lectotype designation for A. muscorum by Day from application of the Code (ICZN, 1985) and then designated a neotype (ICZN, 1996: 64) to conserve the traditional usage of B. muscorum and B. humilis (ICZN, 1985: Article 75).
However, Warncke (1986) recognised B. fulvescens (Schrank) as questionably conspecific with B. humilis. I have seen no type specimens, but the description is consistent with this interpretation. B. fulvescens is therefore likely to be the oldest available name for this species.
Although B. fulvescens may be the oldest available name for the present interpretation of this species, the name B. humilis has been in common use for the species since 1950 (e.g. case and references in Løken et al., 1994). In contrast, I know of no publications using the name B. fulvescens (Schrank) since 1950. Warncke (1986:98) followed the listing of this name with 'Art. 23b', which is a reference to purpose of the Principle of Priority (ICZN, 1985). I agree that, in the interests of stability (ICZN, 1999: Article 23), prevailing usage be maintained (see the comments on B. muscorum) (in prep.).