Queens are active from mid May (rarely the end of April) to late September, workers from early July to early September, and males from mid July to September.
Parasites: Bombus rupestris has been cited as a social parasite of this species (Hopper, 1901; May, 1959). The conopid fly Physocephala rufipes has been reported as a parasitoid of the adult B. sylvarum (K.G.V. Smith, 1969).
Remarks: Three males and a worker of a very rare, all-black variety with a reddish tail (var.nigrescens Perez), have been collected at Seaford, East Sussex in August, 1921 by C.H. Mortimer (Mortimer, 1922b; specimens survive in the NHML). This form has also been found in Norway and Denmark (Løken, 1958, 1973), Sweden, France, Bulgaria and Spain. In Norway it has been observed in nests containing typical individuals (Løken, 1973, who also provides a detailed discussion on melanism in this species). This melanic coloration affects all three castes and such specimens could easily be mistaken for B. ruderarius or old examples of the common B. lapidarius.
Nomenclature: The orthography of Linnaeus (1761) 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’.