Epidemiological and molecular analysis of the main pathogens of Apis mellifera and their importance in triggering colony losses
Sandra Barroso Arévalo defended the PhD Thesis at the Faculty of Veterinary Medicine of the Complutense University of Madrid
November 29th, 2019
The number of honey bees has been in constant and progressive decline since the second half of the twentieth century. These insects play a key role as pollinators and in the maintenance of global biodiversity. Colony losses include two kind of phenomena. The first one is “Colony collapse disorder” (CCD), which consists of a rapid decrease in bee population, but honey/breeding/pollen stores remain adequate and bees do not show disease symptoms. The second one is winter mortality after wintering. In addition to the impact on the beekeeping sector, colony losses also affect food production, environmental sustainability and ecosystems.
There are many factors involved in CCD and winter losses, such as environmental pollutants, the global expansion of pathogens, phytosanitary agents, invasive species, inadequate management, and climate, among others. Although the relative importance of each of these factors remains unknown, there is an agreement on the importance of pathogens, environmental conditions and bee´s immune system. Among pathogens, the Varroa destructor mite and the deformed wing virus (DWV) seem to play a key role in triggering colony losses. When both agents affect the colony, the immune system response is decisive. Other important factors are the environment around the colony and honey bee´s nutrition. Therefore, the present doctoral thesis, entitled “Epidemiological and molecular analysis of the main pathogens of the Apis mellifera and their importance in the triggering colony collapse”, has focused on the study of the main infectious and parasitic agents affecting honey bee colonies, taking into account the honey bee´s immune system and environmental conditions. The results of the findings of this doctoral thesis have been reflected in four scientific articles published in scientific journals.
The first objective focused on the study of DWV sequence. Here, we have provided the whole sequence for two DWV variants in Spain (DWV-A and DWV-B). In addition, our results revealed that nucleotide differences may have an impact on virus virulence, since DWV-A levels were higher in colonies with worse health status.
The second objective was to study honey bee´s immune system in relation to DWV-varroa and colony mortality. Four immune genes were analysed and compared to health parameters. Potential immune markers were developed, which may provide useful information about colony health.
The third objective consisted of the study of honey bee´s nutrition and environment, and their relationship with colony health. First, pollen diversity role in colony health was examined (sub objective 3.1), with no significant results. Second, environmental conditions around the apiaries were evaluated (sub objective 3.2). Worse environmental conditions were related to worse health status.
The forth objective focused on the relationship between DWV and varroa with colony strengh over time. Our findings showed that high DWV and varroa levels were associated with poor colony vigour and lower survival capacity. Therefore, previous results from this thesis were validated and the need of evaluating both pathogens was emphasised.
Thus, this thesis represents a major progress in the study of some of the most important factors for colony health from a multidisciplinary perspective, combining molecular, environmental and epidemiological approaches. Therefore, the findings from this thesis have provided new tools for colony monitoring and control, improving the knowledge of key factors like honey bee´s immune system and DWV genetic.
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