Bumblebees compete with native pollinators. In Tasmania they feed readily on the flowers of blue gums (Eucalyptus globulus) and black gums (E. ovata), which are the main breeding-season foods of the critically endangered swift parrot. The national recovery plan for the swift parrot mentions bumblebee competition, and the risk of bumblebees spreading to the mainland – where swift parrots feed in winter – as a threatening process. Research on swift parrot habitat around Hobart shows that bumblebees reduce nectar yields on these trees. On one black gum studied, they made up more than 90% of visitors to the flowers.
Another risk of bumblebees is their potential to worsen weed problems by serving as superior pollinators for northern hemisphere plants adapted for bumblebee pollination. Scientists have noted ‘clear evidence for a positive link between the spread of weeds and the presence of introduced bees’. Bumblebees are desired crop pollinators for tomatoes and capsicums because they are better than honeybees at moving pollen between the flowers of these plants, and that holds true for some weeds as well. Australia has many plants with the potential to become serious weeds that are currently minor weeds or not weeds at all because they lack the bumblebee pollinators with which they evolved. Also, major weeds such as blackberry (Rubus fruticosus), foxglove (Digitalis purpurea), gorse (Ulex europaeus), Paterson’s curse (Echium plantagineum) and broom (Cytisus scoparius) could benefit from pollination by bumblebees.
As well as Tasmania, this bumblebee has established in Japan, Chile and Argentina, escaping from greenhouses where it is used for crop pollination. In Japan it outnumbers native bumblebees in some settings, and is blamed for a decline in one native species (B. hypocrita sapporoensis).
Beekeepers are concerned about the prospect of bumblebees reducing yields in honeybee hives by competing for nectar.