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Transmission of pathogens between pollinators

A study carried out in eight European countries confirms that pathogens can be transmitted from one species of pollinating insect to another. The health of wild pollinators can therefore be affected when a contaminated colony of honeybees is established nearby. Unlike in previous studies, the results did not show any significant effect of the type of crop grown in bee foraging areas.

The study was conducted as part of the European Poshbee project on bee health, which brought together partners from 14 countries from 2018 to 2023. The pollinating insects in the study were spread out across 128 sites in eight European countries and were established in the vicinity of two types of crops: rapeseed and apple. These countries – Estonia, Germany, Ireland, Italy, the United Kingdom, Spain, Sweden and Switzerland – were chosen to represent geographical areas with different climates. The aim was to study the factors influencing the contamination of pollinating insects by pathogens. 

Eleven pathogens studied

The study focused on three species of pollinators: honeybees, buff-tailed bumblebees, and red mason bees (solitary wild bees). Their contamination by infectious and parasitic agents was assessed before and after the colonies and nests had been established near the crops for 11 to 59 days. The pathogens studied included six viruses, two bacteria, and three parasitic fungi. Ten of these pathogens are common in honeybees. The eleventh, a fungus, tends to infect bumblebees. “The insects had already been carrying certain pathogens before they were released into the field” explains Eric Dubois, a project manager in the Bee Diseases Unit at the ANSES Sophia Antipolis Laboratory. “Even when they came from farms, it was impossible to prevent them from being contaminated. On the other hand, we checked that none of them showed any symptoms of disease”. The insects were released in the vicinity of rapeseed and apple crops at the time of flowering.

More pathogens in honeybees than in the other two pollinators

Some differences between pollinators were identified: honeybees were carrying more species of pathogens and in greater quantities than bumblebees and mason bees. Three viruses were more frequently found: deformed wing virus, black queen cell virus, and sacciform brood virus. There were several indications of pathogens being exchanged between pollinating species. The most significant was that the quantity of the main viruses found in bumblebees and mason bees was proportional to that found in honeybees on the same site. “These insects share the same foraging areas. They deposit pathogens on flowers and can therefore transmit pathogens to other species” explains the scientist. “The lower viral load in bumblebees and mason bees may be due to the fact that the viruses studied are mainly known to target honeybees. They may be less suited to infecting other species, or else wild pollinators may be more resistant to them”.

A risk for vulnerable wild pollinators

Despite their high degree of resistance, wild pollinators do not appear to be totally insensitive to honeybee pathogens: for example, previous studies have described symptoms of deformed wing virus in bumblebees. “We must therefore be vigilant when introducing a colony of honeybees into a protected area to avoid transmitting pathogens to endangered species of pollinators”.  The study carried out as part of the Poshbee project showed that the most common pathogens, which are not necessarily the most virulent, were the best indicator of the risk of exposure to pathogens in pollinators.

Lastly, unlike in earlier studies, the results did not show any effect of the conditions specific to the site of establishment: no significant link was found between nearby crops, the use of plant protection products, the climate, or the duration of exposure in the field, on the one hand, and the diversity or quantity of pathogens found in the insects, on the other. This may have been due to the relatively short period of exposure in the field. Some differences were observed between countries, without there necessarily being a link with their biogeographical zone, but the data will need to be analysed in greater detail to determine the cause.