Displacement of Tasmanian Native Megachilid Bees by the Recently Introduced Bumblebee Bombus Terrestris (Linnaeus, 1758) (Hymenoptera:Apidae)

The assertion that the recently introduced large earth bumblebee, Bombus terrestris, does not compete with other bees was investigated by examining the impact of B. terrestris on the foraging of two species of Chalicodoma (Megachilidae) on flowers of Gompholobium huegelii (Fabaceae). Chalicodoma spp. spent less time at each flower during the afternoon in quadrats where B. terrestris foraged than at quadrats from which B. terrestris was excluded, indicating that standing crops of nectar were depleted by B. terrestris during the course of the day. This decline in resources was associated with reductions in the numbers of flowers visited, and the amount of time spent foraging, by Chalicodoma spp. It is proposed that the native bees avoided foraging in the quadrat where B. terrestris occurred as a response to reduced foraging efficiency in that situation. Thus, B. terrestris displaced these two species of Chalicodoma through competition for a limited resource. The high densities at which B. terrestris occurred, together with its ability to forage at lower ambient temperatures than the native bees, exacerbated the impact of this exotic species. The possibility that this will adversely affect pollination in G. huegelii is also discussed.