Bombus (Psithyrus) variabilis (Cresson)

Tripodi, Amber D. & Szalanski, Allen L., 2015, The bumble bees (Hymenoptera: Apidae: Bombus) of Arkansas, fifty years later, Journal of Melittology 2015 (50), pp. 1-17 : 13-14

Bombus (Psithyrus) variabilis (Cresson)

‘Variable Cuckoo Bumble Bee’

Prior to our examination, only a single record of this species in Arkansas existed in the literature. Chandler & McCoy (1965) listed a single record from Washington County, but without including any additional collection information. Three specimens of B. variabilis collected in Washington Co. during our target historical period were among the specimens in the UAAM collection (September, 1900; August 15, 1906; October 1, 1961), yet no specimens for our contemporary period were present ( Fig. 1). However, three additional male specimens that were collected outside of our historical and contemporary periods are present in the UAAM. Two specimens were collected in the northwest portion of the state (Franklin Co., October 5, 1976 and Washington Co., September 29, 1993); the other was collected in eastern Arkansas (Desha Co., August 7, 1966). Bombus variabilis was not recovered in our surveys or in Warriner’s (2011) extensive Arkansas grassland surveys. With so few records, there is no suggestion of a change in the occurrence of B. variabilis between the historic (2.6%) and contemporary periods (0%, Fig. 3), and its status in the state remains unclear.

Records for this species are both temporally and spatially sporadic throughout eastern North America ( Williams et al., 2014). The species is a member of the social parasite subgenus Psithyrus Lepeletier , whose host is B. pensylvanicus . Its unusual life history may help explain its rarity. Lacking a foraging worker caste, species of Psithyrus are nest-bound and less likely to be encountered in typical field surveys. Also, as obligate nest parasites, their abundance is bound to be lower than that of their host. Still, there are indications that B. variabilis is declining and deserves further study. Its host, B. pensylvanicus , is also suspected to be on the decline ( Cameron et al., 2011; Colla et al., 2012), and an obligate parasite is likely to follow the same population trends as its host. Across its range, B. variabilis has dramatically declined both in abundance relative to other species of Bombus and in geographic persistence, leading to a recommendation that it be classified as “critically endangered” ( Colla et al., 2012). As in the case of B. fraternus , we urge that studies aiming to determine the conservation status of this rarer species take into consideration the inherent difficulties in accurately sampling species with low detectability before drawing conclusions on its stability.

GLOSSA LENGTH: Unknown, not reported.

ADULT ACTIVE PERIOD: Unknown, records in Arkansas from August–September.

PREFERRED PLANTS: Unknown, not observed.