An investigation on the multiple stress factors affecting honey bee health

Honey bees (Apis mellifera L.) play a vital role in ecosystems and are essential for conservation of both plant biodiversity and agricultural production. In the last decades a serious decline of bee colonies has been observed in many countries in the northern hemisphere, often followed by colony losses. This worrying phenomenon is due to the interaction among a number of stress factors, including parasites and pathogens (i.e. Varroa destructor and deformed wing virus), agrochemicals, the availability and quality of food resources and environmental conditions. To understand how different stress agents (both abiotic and biotic) might positively or negatively interact is fundamental to plan possible actions to maintain and restore bee health. For this purpose, during the PhD, several experiments were carried out under laboratory condition following a "from detail to general" approach, initially focusing on the interaction between two factors and then gradually incorporating other stressors and assessing how they interfere with the system. Firstly, we investigated the possible interaction between pollen, an essential nutritional resource for bees, and a xenobiotic substance. Specifically, we considered the toxic alkaloid nicotine that can be found both in nectar and pollen of some plant species. This experiment was replicate both early and late in the season to see if seasonality and hence viral infection, can influence the results. Interestingly, the simultaneous administration of pollen and nicotine resulted in a negative effect on bee survival only late in the season, in the presence of high viral loads. We also investigated the above-mentioned interaction after inhibiting the proper functioning of the detoxification system allowing bees to deal with harmful secondary metabolites and xenobiotics. If the detoxification system was compromised, a detrimental effect of nicotine was noted also early in the season, supporting the importance of detoxification. Interestingly, pollen seemed to promote detoxification. However, late in the season, the presence of the virus made the system less predictable. After confirming the positive role of pollen both in virus free and virus infected bees we investigated which pollen component is associated to its beneficial effect. For this purpose, three different types of pollen were administered to the bees. Since the polar fraction of pollen seems to play a key role in this respect, we decided to test one of major flavonoids in this pollen: quercetin. However, this compound, at the dose tested here, did not significantly increase the survival of caged bees. Then, we assessed how the alkaloid nicotine interacts with other stress factors that honey bees can be exposed to: a lower than normal hive temperature (32 °C), pollen deprivation and V. destructor, the most dangerous ectoparasite of honey bees. To this purpose, a four-factors factorial experiment was carried out. Further than confirming that both Varroa infestation and a low temperature play a negative role under most conditions, the experiment allowed to identify three significant interactions between factors that open up new avenues of investigation. Finally, a systems biology approach was used to gain insights into various interactions among the factors that may affect honey bee health. Thus, a conceptual model was created and subsequently validated with dedicated laboratory experiments. This model highlighted a critical positive feed-back loop between virus and immunity; as a consequence, the presence of an immune-suppressive virus creates bistability. Hence, the survival of bees in presence of another stressor, such as a pesticide, does not depend only on that stressor’s level but also on the bee's initial condition.