The decline of bumble bees in the United States has been well-documented and wide-spread, with reduced abundance of bees across the country and continent. Bumble bees are immensely important to the survival of many plants and animals, but also to the survival of humans because of the interconnection we all share. Bumble bees provide us with many ecological services, the most important being pollination; without pollination our planet would be a very different place because pollination has such a direct effect on plant populations. As plant populations begin to weaken or fail, we would experience a cascade of negative effects throughout all levels, and possibly the collapse of select ecosystems. All organisms experience negative species-interactions, but bumble bee decline is hypothesized to be in large part due to the introduction of non-native bee species into their natural habitat.

One non-native competitor, the honey bee, is widely known for their commercial agricultural use, but bumble bees are also kept in commercial greenhouses for their unique ability to use buzz pollination. Buzz pollination, or the rapid beating of their wings while feeding causing more pollen to collect on their bodies, is crucial to the pollination of tomatoes and sweet peppers due to the unique shape of their inflorescence. Honey bees are widely prevalent in many urban and rural habitats and stray commercial bumble bees often escape the unsealed greenhouses, finding themselves amongst wild bee populations and competing with each other for floral resources. One study found competition levels to be as high as a 90% overlap in the plants visited by both honey bees and native bumble bees in the US, and in Japan there was a 70% overlap found in floral resources used between native bumble bee species and the introduced Bombus terrestris⁵. Not only do they create competition for native species, the pathogens and parasites they carry readily pass onto the native bees they encounter, who have little defense against the infections. Pathogen spillover can occur when healthy populations meet closely-related infected individuals, allowing disease to spread. This often is seen when feral animals escape from domestic populations where disease is rampant². Pathogen spillover is widely televised when the disease acutely and directly impacts humans (e.g. AIDS, SARS, and H5N1 influenza), but much less attention is paid when the impact affects non-human species⁴. Such transmissions can cause cases of diseases that are temporarily widespread and severely prevalent within a species or group of animals (e.g. the 2005 Avian Flu Epidemic), otherwise known as epizootic events.

There are several varieties of pathogens and parasites that can be transmitted between bumble bee species, of particular note and concern, is Nosema bombi (microsporidia) and similar infectious parasites. N. bombi reduces reproductive performance and lowers the survival rate of workers¹, and these intracellular fungal pathogens are easily contracted by wild bees from their commercial counterparts when meeting at floral resources. It has been found that colonies of commercial bees often have significantly higher pathogen concentrations than wild bee colonies far from greenhouses². The figure below shows a comparison of pathogen prevalence between wild and commercial bumble bees, revealing a significantly higher rate of infection among commercial bees than in their wild cousins.

The proportions of bumble bees infected by parasites such as N. bombi in SW Ontario in 2004 and 2005. The x-axis shows testing locations, where n equals the number of bees counted².

 

Bombus terrestris has been a common choice amongst commercial bumble bee keepers in its native Europe³ since the 1980s and two field studies have found that N. bombi pathogens are present in more than 50% of B. terrestris colonies¹. It is theorized that N. bombi travelled to North America in the 1990s after an attempt to expand the market prompted the transfer of native bee species B. impatiens and B. occidentalis to and then from Europe for colony rearing in facilities also used to raise B. terrestris¹. These reimported commercialized bees brought the pathogens with them, spreading them to wild native bees. The figure below displays graphs of the presence of N. bombi in the colonies of five North American Bombus species and B. terrestris, showing the prevalence of infection after the introduction of B. terrestris and European-bred North American species in the 1990s. Thus far stable species of bumble bees in North America show significantly lower N. bombi infection rates than species that are declining¹. In fact, B. occidentalis was abandoned from commercial use in greenhouses shortly after introduction to the market due to the infestation of N. bombi in colony rearing stock³. Shortly after, wild populations of the once plentiful species began to steeply decline¹.

The number of specimens found infected with Nosema sp. vs total number of bees screened. Pink bars represent infected bees and blue bars represent uninfected bees (y axis left). The dashed lines show the proportion of individuals infected vs uninfected (y axis right)¹.

1. Cameron SA, Lim HC, Lozier JD, Duennes MA, Thorp R. Test of the invasive pathogen hypothesis of bumble bee decline in North America. Proceedings of the National Academy of Sciences. 2016;113(16):4386–4391. doi:10.1073/pnas.1525266113
2. Colla SR, Otterstatter MC, Gegear RJ, Thomson JD. Plight of the Bumble Bee: Pathogen Spillover from Commercial to Wild Populations. Biological Conservation. 2006;129(4):461–467. doi:10.1016/j.biocon.2005.11.013
3. Koch JB, Strange JP. The Status of Bombus occidentalis and B. moderatus in Alaska with Special Focus on Nosema bombi Incidence. Northwest Science. 2012 . 86(3): 212–220. doi: 10.3955/046.086.0306
4. Otterstatter MC, Thomson JD. Does Pathogen Spillover from Commercially Reared Bumble Bees Threaten Wild Pollinators? PLoS ONE. 2008;3(7). doi:10.1371/journal.pone.0002771
5. Potts SG, Biesmeijer JC, Kremen C, Neumann P, Schweiger O, Kunin WE. Global pollinator declines: trends, impacts and drivers. Trends in Ecology & Evolution. 2010;25(6):345–353. doi:10.1016/j.tree.2010.01.007