Queens fly from the end of March to early November, workers from mid June to the beginning of November, and males from mid June to at least mid October.
Parasites and predators: Bombus campestris is a social parasite in the nests of this species (Sladen, 1912; Cumber, 1949; Alford, 1975; Peeters, Raemakers & Smit, 1999).
Remarks: Some very pale coloured individuals closely resemble both B. humilis and B. muscorum, so care is necessary when identifying such examples (particularly when in the field). Four geographical races or forms of this species have been described (originally as subspecies) from the British Isles. These are all largely distinguishable on the quantity of black hairs present on the mesonotum and gastral tergites.
Taxonomic status: Warncke (1986) listed B. cognatus as a synonym of B. muscorum, possibly following Stephens (1846), who wrote of B. cognatus: 'Closely allied to Bo. Muscorum, of which the examples I possess may be immature specimens'. Pagliano (1995) listed B. cognatus as a species separate from both B. muscorum and B. pascuorum, but without any explanation.
Saunders (1896:366-367) wrote 'I have re-examined the type of cognatus, Steph., ... F. Smith placed it in the British Museum collection ...'. Saunders considered this specimen to be conspecific with B. agrorum (Fabricius), continuing: 'It is certainly not the species known on the Continent as cognatus'.
A female in the NHML collection bears the following labels: (1) a red-edged printed 'Type'; (2) 'cognatus.' in handwriting identical to that of F. Smith; (3) '= agrorum / I.H.H.Y.' in handwriting identical to that of I. Yarrow; (4) 'B.M. Type / HYM. / 17B.1163'. I have examined this specimen and am unaware of any reason (other than minor differences in colour pattern) why it should not be considered the type of B. cognatus and conspecific with B. pascuorum.
Nomenclature: The orthography of Scopoli (1763) and Fabricius (1775) 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'.
Løken (1973) listed B. cognatus Stephens, 1846, as a nomen nudum, citing Sherborn (1925). However, the reference by Sherborn is to Stephens (1829), therefore this does not affect the use of the name B. cognatus Stephens, 1846.