A scientific note on Bombus (Psithyrus) insularis invasions of bumblebee nests and honeybee hives in the western USA

Abstract

Bumblebees (Bombus) in the subgenus Psithyrus are obligate social parasites that usurp the nests of other bumblebees in order to reproduce. Usurpation occurs after nest establishment by the host species when, upon entry to the nest, the invading Psithyrus kills or suppresses the host queen and dominates host workers to rear her offspring, but considerable behavioral variation occurs in host–parasite interactions among members of the genus (Fisher 1988). While many Psithyrus species are known to parasitize a number of species, few of the relationships are well understood. Of the six North American Psithyrus species, most have been recorded from the nests of several host species, and it is reasonable to assume that each host species reacts differently to an invasion. In addition to usurping bumblebee nests, Plath (1927) noted that females of Psithyrus laboriosus (=Bombus citrinus (Smith)) would attempt to invade honeybee hives (Apis mellifera L.). This was noted as a “dead end,” usually resulting in the death of the invading bee. Subsequently, records of several Bombus species attempting to enter honeybee hives were made by Morse and Gary (1961), but of these records, only 1 of the 28 bumblebees found in 25 honeybee colonies was a social parasite (B. citrinus). The authors attributed these nest invasions to resource robbing and not nest usurpation. Here, we report Bombus insularis (Smith) invading the nest of two bumblebee species in the subgenus Pyrobombus, Bombus huntii Greene, and Bombus bifarius Cresson, and attempted invasions of honeybee hives at four apiaries by B. insularis. During the summer of 2012, numerous female B. insularis were observed near bumblebee nest boxes and honeybee hives in Utah, USA. Although observations from previous years were not quantified, it is uncommon to see dead B. insularis around honeybee hives; for example in 2013 at apiary 1, we found no B. insularis at three colonies during weekly inspection. The exception was in 2007 when several dead B. insularis were observed at hive entrances at apiary 1. That year, a B. insularis was observed entering a honeybee hive and was forced out within a minute. When the invader exited the hive, it had several honeybee workers attacking her, by biting her legs and tegulae; however, the parasite managed to escape and fly away. The number of invasions in 2012 by B. insularis provided an opportunity to study aspects of nest usurpation and hive invasion. We collected dead B. insularis from the ground in front of honeybee hives at four apiary sites from 12 June to 12 August (Table I). Apiary 1 was inspected weekly for dead Psithyrus in front of the hives, and apiaries 2–4 were Electronic supplementary material The online version of this article (doi:10.1007/s13592-014-0271-8) contains supplementary material, which is available to authorized users.