I recently read Dave Goulson’s excellent book “A Sting in the Tale” and learnt a lot about bumblebees. Although I was aware of the global trade in honeybees, I hadn’t realised that there was an equivalent trade in bumblebees. Some crops such as tomatoes and peppers require buzz-pollination, the rapid vibration of the flower. Bumblebees do this very well and are now used extensively by commercial growers of tomatoes and other crops. To supply the demand for these useful insects there are at least thirty factories producing bumblebees for shipping all over the world. The numbers are staggering with European factories producing up to a million nests per year. This is big business with huge financial rewards but keeping so many insects together in one place risks the rapid spread of disease unless stringent hygiene precautions are observed. To complicate matters, commercially-reared bumblebees are fed pollen from honeybees so that they are potentially exposed to all the diseases that affect honeybees.
But what about wild bumblebees? What happens when a wild bumblebee forages at a flower that has already been visited by a honeybee? Are the bumblebees exposed to honeybee diseases and what might the consequences be?
Last week’s Nature magazine carried an article addressing this issue. The team who carried out the work were from Royal Holloway London, Queen’s University Belfast and Exeter University. They showed that some honeybee diseases are indeed a problem for wild bumblebees and could be causing a decline in these wild pollinators. They studied two diseases: the fungal parasite Nosema which weakens honeybee colonies, and Deformed Wing Virus (DWV) which causes abnormalities in the wings and abdomen of infected honeybees as well as severely curtailing their lifespan.
The starting point for the work was to test whether these honeybee diseases could actually infect bumblebees. The researchers inoculated bumblebees (B.terrestris) with DWV or Nosema, and found that bumblebees were indeed susceptible to infection by both diseases. In the case of DWV, infection led to reduced survival of B.terrestris workers. For Nosema, although it could infect bumblebees, this did not reduce their lifespan.
Having established that the two honeybee diseases could infect bumblebees, the researchers examined the incidence of the two diseases. They performed a large scale study on the prevalence of DWV and Nosema in honeybees and bumblebees across 26 sites in Great Britain and the Isle of Man. DWV was found in 36% of honeybees tested and in 11% of bumblebees tested. For many of the infected bumblebees, the virus was active showing that the bumblebees were not simply acting as carriers. Nosema was less prevalent being found in 9% of honeybees and 7% of bumblebees. When the geographical distribution was analysed, there was some evidence for clustering, indicating disease hotspots. Hotspots for DWV were found in the south west and east of Great Britain and for Nosema in the south east. By analysing the distribution of the two diseases they were also able to show that the prevalence of DWV in honeybees influenced the prevalence of DWV in bumblebees, implying local transmission between the two insects. Local transmission was confirmed by analysing the form (nucleotide sequence) of the virus present in the two types of bee collected from the same site.
Honeybees have a high prevalence of DWV, a consequence of infestation of colonies by Varroa mites. The most obvious conclusion from this new work is that honeybee DWV is spreading to wild bumblebees. This probably occurs when the two types of bee forage in the same environment. Because DWV infection of bumblebees reduces their lifespan, the spread of this pathogen could be contributing to the decline in bumblebee numbers.
Both honeybees and bumblebees are important pollinators and need to be maintained. Their loss would have immense financial implications. This research shows that disease control in honeybee populations, for example through the efforts of beekeepers, has important implications for the health of other pollinators as well.