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What is causing the increase in honey bee colony mortality?

Written by Simone Haerri

What do you need to know?

The common honey bee (Apis mellifera) is experiencing high mortality rates in recent years. This is a major concern for the beekeeping industry and for agricultural crops relying on honey bees as their main pollinator.  Several key factors have been linked to colony mortality such as exposure to pesticides, viral infections and the ectoparasitic mite Varroa destructor. Whereas we know how each one of those stressors affects the honey bee individually, not as much is known how combining the different stressors affect the bees.

In this paper, researchers assessed how the combined effect of exposure to neonicotinoid pesticides with parasitism by Varroa mites is affecting honeybees during development from larval stage to adulthood. They found that all health-related honey bee parameters were negatively affected by the presence of the Varroa mites, and some aspects showing a compounded negative effect when both stressors were present at the same time. They found that the combined effect of Varroa mites and pesticide exposure in bees exposed during the larval stage has a long term effect by reducing the weight of newly emerged bees and by affecting the expression of immune and metabolic-related genes,  which in turn might affect the chance to fight off infections, repair damaged tissue, and metabolize toxins.

Why is this research important?

The honeybee (Apis mellifera) is one of the most important pollinators for agricultural crops and it also has a long history of management. Recently, honey bees are experiencing a high level of mortality. There are several important factors linked to the drastic decrease in numbers. The presence of the parasite Varroa destructor, viral infections such as the deformed wing virus (DWV) and exposure to pesticides, like neonicotinoids.

Varroa mites are little tiny arachnids that live on the outside of honey bees and they feed on the fat tissue and hemolymph (blood) of the bees and if present in high enough numbers, they can kill entire honey bee colonies. They can also transmit viruses, such as the deformed wing virus. Honeybees can, to a certain degree, protect themselves against the mites, for example by grooming themselves.

Whereas each of those stressors alone has negative effects on honey bee health, in combination, they can cause even more problems. However, research so far has reported contradictory results when it comes to the exposure to multiple stressors. Understanding that complex interplay among various stressors will help taking measures to reduce honey bee mortality.

What did the researchers do?

The honeybee larvae used for the experiment were originally taken from colonies kept at our Honey Bee Research Centre (  Honeybee larvae were then assigned to different concentrations of a neonicotinoid insecticide, clothianidin, with each concentration replicated twice. To test how exposure to pesticides in combination with being infected by Varroa mites is affecting the honeybees, Varroa mites were added to half of the pesticide groups.  This resulted in eight different treatment groups:

  • Group 1: No exposure to pesticides – no Varroa mites
  • Group 2: No exposure to pesticides – Varroa mites present
  • Group 3, 4 & 5: Exposure to low, medium and high concentration of Clothianidin – each with no Varroa mites
  • Group 6, 7 & 8: Exposure to low, medium and high concentration of Clothianidin – each with Varroa mites present

Clothianidin is a pesticide belonging to the Neonicotinoid family. The pesticide concentrations used for the experiment mimicked the realistic exposure a honey bee larvae would experience during its lifetime by ingesting pollen of plants that wore grown from Clothianidin-treated seeds. Once the bees completed their development, the following parameters were measured:

  • Proportion of emerged bees to test for direct mortality
  • Weight of the emerged bee. Weight can be an indicator of bee health
  • DWV levels. DWV (deformed wing virus) is a viral infection experienced by honeybees than can be transmitted by the mites
  • Number of haemocytes per bee was estimated. Haemocytes are cells used to heal a wound and/or attack a pathogen. Their number is often used as an indicator of strength of the immune system
  • Total gene expressions to have a better understanding of the effect of the stressors on bee health, based on the biological pathways associated with the affected genes.

What did the researchers find?

The researchers found that different parameters measured reacted quite differently to the applied treatments. The number of larvae that successfully developed into adult honey bees (proportion of bees emerged) and the number of haemocytes were decreased when the Varroa mites were present, but were not affected directly by the exposure to Clothianidin. The same was found true for the “Deformed wing virus” levels that were higher in the presence of the Varroa mites but unaffected by exposure to pesticides. On the other hand, the weight of the emerged bee was most dramatically reduced when exposed to both stressors, the pesticide and the Varroa mites. This indicates that the combined effect of the two stressors can magnify the effect of each individual stressor and significantly reduce honey bee health.

The study found that clothianidin alone affected genes related to metabolism and Varroa affected genes associated to immune responses. When the stressors were combined, the number of up and down-regulated genes was higher compared to the stressors alone, showing that the interaction between stressors could affect more aspects of bee health.

About the researchers

Dr. Nuria Morfin is a post-doctoral researcher at the School of Environmental Sciences. Dr. Paul H. Goodwin and Dr. Ernesto Guzman-Novoa are faculty members at the School of Environmental Sciences, University of Guelph, ON, Canada.


Honey Bee, Apis mellifera, pollinator, pesticides, neonicotinoids, Varroa mite, parasites, overwinter mortality, honeybee, pollination, beekeeping, deformed wing virus


Morfin, N., Goodwin, P. H., & Guzman-Novoa, E. (2020). Interaction of Varroa destructor and Sublethal Clothianidin Doses during the Larval Stage on Subsequent Adult Honey Bee (Apis mellifera L.) Health, Cellular Immunity, Deformed Wing Virus Levels and Differential Gene Expression. Microorganisms8(6), 858.