Queens fly from late March or early April to mid September, workers from mid June to early September (exceptionally at the end of the month), and males from mid June to mid September, rarely early October. Some queens must commence nesting in March, as G.M. Spooner (pers. comm.) has noted a worker on 18th April on the Isles of Scilly, and I have encountered workers flying into a nest on April 20th in Hampshire. There are indications that this species possesses the ability to complete a second nesting cycle in some seasons (Sladen, 1912; Prys-Jones, 1982).
Parasites: Bombus (=Psithyrus) barbutellus is a social parasite in the nests of this species (Sladen, 1912; Muller, 1936; Cumber, 1949; Alford, 1975; Peeters, Raemakers & Smit, 1999).
Taxonomic status: The Spanish B. asturiensis has been considered a separate species from B. hortorum by Rasmont (1983, 1988), although they have been treated as conspecific by Pittioni (1939c), Tkalcu (1975), Ornosa (1986a, 1986b, 1991), Castro (1988, 1993) and, more recently, by Rasmont et al. (1995). The two taxa are closely similar.
The Spanish B. reinigiellus has also been considered both as conspecific with B. hortorum (Castro, 1987) and as a separate species (e.g. Rasmont, 1983; Castro, 1988; Ornosa, 1991). The two taxa are allopatric (Rasmont, 1983), with B. reinigiellus being narrowly restricted to the Sierra Nevada of Spain, possibly as a disjunct peripheral population. B. reinigiellus is closely similar to B. hortorum, although subtle differences in characters of colour and morphology have been described (e.g. Rasmont, 1983; Castro, 1988; Ornosa, 1991). From the material I have examined, the morphological differences appear to be analogous to the variation between mainland and island populations of B. terrestris (see the comments on B. terrestris).
Depending upon the species concept embraced, such subtle differences as those between B. reinigiellus and B. hortorum might be expected even within a single population and I shall treat all three taxa as conspecific for the present. More evidence is awaited.
Introductions: B. hortorum has been introduced into New Zealand (e.g. Gurr, 1957; Macfarlane & Gurr, 1995) (see the comments on B. ruderatus, B. subterraneus and B. terrestris). It occurs in Iceland, where it has also probably been introduced (Prys-Jones et al., 1981) (see the comments on B. lucorum).