Tougher controls on importing bumblebees are needed to protect native bees, scientists have urged after a study revealed widespread disease in colonies being brought into the UK.
Around 40,000 to 50,000 commercially reared bumblebee colonies are imported into the UK each year, to pollinate greenhouse crops such as tomatoes, to boost pollination of other plants such as strawberries and even for use in gardens.
But DNA testing of 48 colonies of buff-tailed bumblebees purchased from European producers found that more than three-quarters (77%) of the colonies were carrying parasites.
All but one of 25 samples of pollen supplied with the colonies as food for the bees were also infected with parasites, the study published in the Journal of Applied Ecology showed.
Further research by the team from the universities of Leeds, Stirling and Sussex found that the parasites carried by the commercially reared bumblebees were viable and could infect both bumblebees and honeybees. The researchers warned that commercially imported bumblebees could interact with wild bees and honeybees, spreading disease by visiting the same flowers as the other species.
Non-native subspecies of buff-tailed bumblebees are imported for use in greenhouses, but as the buildings have windows and vents to control temperature, they can forage outside while wild bees or honeybees can get in to feed on the crops.
Native subspecies of the bumblebee are used in open polytunnels and are even being marketed for use in people's gardens, both situations where they can potentially spread disease to wild species.
Disease is one of the major issues facing bees, which are important pollinators of crops and wild plants. Two of the UK's 25 bumblebee species have become extinct and a further eight have seen significant declines since 1940. Research in North America, South America and Japan suggests that parasites introduced from commercial colonies can be a major cause of population declines in wild bumblebees, with one species in Argentina facing extinction in a decade.
The findings of the UK study suggest that the current rules on ensuring imported bees are disease-free were not strong enough, the scientists said. They called for tighter regulations which would include checking bees on arrival in the UK, better screening by producers, and the development of disease-free pollen food.
Friends of the Earth's Paul de Zylva said: "Limp regulation has already let in ash die-back and doomed millions of British trees. To avoid the same fate for our wild bumblebees, ministers must ensure bee importers comply with stricter rules."