Fungicides and bees : a story of the unexpected : towards a better understanding of inexplicable bee colony disorders in the Walloon Region (Belgium)

For several decades now, an increasing phenomenon of enhanced honey bee colony mortality and morbidity is troubling beekeepers and naturalists. Such negative trends affect also wild pollinators and cast doubt about the sustainability of pollination both of wild plants and crops. These trends are also experienced in Belgium, country showing a level of annual colony losses above 18% in the best years. In addition, Walloon beekeepers describe for many years unspecific symptoms with their colonies like weaken colonies, queen losses, abnormal brood development or full death of the colony without any symptoms of disease. The aim of my work was to bring light in this situation, which revealed unexpected results. A prevalence case study carried out in the region of Wallonia by myself in collaboration with colleagues of the CRA-Wallonia, pointed at fungicides as the stressing factor linked to honey bee colony disorders. These pesticides considered harmless to bees, we started digging to understand the underlying reasons of our field observations. First we considered if they were just markers of intensive agriculture, not really having any toxicological effects on bees. Indeed, we found that colonies surrounded by conventionally-farmed crops were more likely to be contaminated by pesticides, in special fungicides. In addition, the colonies in these crop areas were more prone to show disorders. We also found that this contamination does not necessarily come from treated crops, but from wild plants or catch/cover crops in the surroundings or succeeding treated crops within a surface of at least 3 km radius. Then, I tested the direct toxicological effects fungicides on different colony individuals. I started by evaluating the effect of the most frequently found fungicide, boscalid, on be larvae, which did not show to induce increased mortality of immature bee stages. Then I studied the impact of chronic exposure of adult bees. Finally I suggest further research lines to better understand the role of fungicides in honey bee colony disorders, specially when fungicides are in combination with other stressors, and on the indirect effects of fungicides on bee nutrition and microbiology. Finally, I suggest action lines to improve the existing field situation.