By Erica Mather 06/08/2015


The spread of bee parasites has been found to be mediated by flowers, according to new research from the UK.  Infected bees deposit parasites onto flowers as they forage.  The parasites are then dispersed by other bees that visit the same flowers, and even spread between bee species.  These results reveal flowers to be ‘hotspots’ of pathogen dispersal, aiding the spread of harmful diseases to pollinators.

Landing on flowers to gather pollen and nectar is a risky business for bees.  A relationship that could not have seemed more symbiotic is now shown to be otherwise, and parasites are to blame.

The substantial population decline of several pollinator species is sparking great interest in the stress factors that are affecting them, including parasites.  The emergence of the Varroa mite has been associated with honeybee colony loss in many countries.  Increasing evidence emphasises the significance of parasite transmission between different pollinators more than is currently accepted.

Researchers in the UK, Peter Graystock and colleagues conducted an experiment to explore the possibility that shared flower use has a profound role in the transmission of pollinator parasites, a minimally investigated area.  Previous research has not been able to confidently determine whether the parasites detected in bee pollen originated from the flower or from the foraging bee itself.  Other studies question the mode of parasite transmission and therefore cannot make conclusions about shared flower use.

The authors state in the paper published in Proceedings of the Royal Society B that while some plant-pollinator systems are specific, most often flowers are visited by a number of different pollinator species.  Therefore, it could be speculated that the contact between parasites and non-host species during shared flower use may be of great importance for the interaction between parasites and pollinators.

Graystock and colleagues investigated the potential for non-host species to vector parasites – studying both honeybees and bumblebees for their capacity to act as transient hosts, dispersing parasites widely to other flowers thereby transferring parasites between each other.

It was discovered that parasites from honeybees were dispersed onto flowers, which serve as their ‘platform’ from where they ‘hitchhike’ on bumblebees to further flowers and back to colonies.  This was also the case for bumblebee parasites and honeybee vectors.  This tactic ensures that parasites continue to spread amongst future unsuspecting foragers, targeting their desired host species both directly and indirectly.

The researchers are confident in their experimental setup and conclude the “parasite contamination detected had originated from the parasite provider bees and not from outside the experiment or from latent infections in the vector bees.”

How do parasites hitchhike?  In addition to on the bodies of bees, parasites were discovered in internal tissue samples, suggesting that they were ingested by the bees whilst foraging on contaminated flowers, or during the subsequent grooming of other bees.  A particular bumblebee parasite, Crithidia bombi is vectored by honeybees and is even able to survive after being ingested.

Even though the experiment excluded additional factors found naturally, there was a significant level of parasite dispersal within just 3 hours.  These results imply that parasite dispersal in the natural environment is widespread.

“Our findings suggest the need to widen parasite screening regimes for imported/exported bees and flower products to include parasites that may be vectored by the bees or flowers, and which may pose a potentially devastating threat to naive pollinator communities,” the authors write.

Experimental set-up. The movement of bees (black, solid arrows) and potential movement of parasites (red, dashed arrows) during experiments in which either honeybees provided parasites and bumblebees were the vectors (a,b), or vice versa (c,d). Initially, the bees providing parasites were allowed to forage on a set of flowers (a,c). The parasite provider bees were then excluded, and the vector bees allowed to forage on both sets of flowers (b,d). Flowers consisted of a mix of the flat-formed V. tricolor flowers and bell-shaped C. cochleariifolia flowers.
Experimental set-up: The movement of bees (black, solid arrows) and potential movement of parasites (red, dashed arrows) during experiments in which either honeybees provided parasites and bumblebees were the vectors (a,b), or vice versa (c,d). Initially, the bees providing parasites were allowed to forage on a set of flowers (a,c). The parasite provider bees were then excluded, and the vector bees allowed to forage on both sets of flowers (b,d). Flowers consisted of a mix of the flat-formed V. tricolor flowers and bell-shaped C. cochleariifolia flowers.

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