Evaluation of the nutritional status of worker honey bees (Apis mellifera ligustica S., 1806) across temporal patterns through morphological analysis

Global honey bee colony losses represents a significant and complex issue in modern apiculture. Regardless of climate, most losses occur in winter, which is a particularly challenging period for Apis mellifera. In fact, in order to deal with the harsh winter conditions of temperate climates of the northern hemisphere, honey bees evolved a seasonal caste system (summer Vs. winter bees) with fundamental physiological processes underlying survival dictated by nutrition. Whereas the overwintering of honey bees in northern regions has been well studied, much less is known regarding the seasonal dynamics of these social insects in southern temperate climates. Because honey bee colony losses are not restricted to cold climates, and nutrition is a key factor in overwintering success, there is a pressing need to deepen our understanding of honey bee seasonality and nutrition in southern latitudes. Furthermore, given the complex aetiology of honey bee colony losses, identification of novel markers of honey bee health and nutrition would prove beneficial in the context of ongoing research and monitoring efforts. Lastly, since a higher survival rate of local subspecies adapted to specific climatic conditions has been shown, knowledge of southern honey bee populations is of increasing interest, especially in the face of accelerated climate change the world is facing in present days. The various publications in this dissertation add to the current knowledge of these subject areas. Firstly, an up-to-date literature review containing a centralised overview of honey bee worker dynamics, division of labour, and seasonality in temperate climates of the northern hemisphere is presented. A detailed description of the relevant physiology of summer and winter bees illustrates how A. mellifera has adapted molecular pathways into an effective social system for the division of labour as well as a bimodal, biannual worker caste system. The mechanism behind honey bee seasonality governed by a multitude of internal and external factors with varying sensitivity is described and research gaps are highlighted. Secondly, the results of the preliminary evaluation and proof of concept for the use of robust measurements of individual size (head width, thoracal width and length, abdominal width and length, and total body length) as possible markers for honey bee health and nutrition are reported. Specifically, abdominal and total body length were identified through correlation analysis as suitable candidate markers in forager bees, while head width showed promise as a marker across temporal patterns. Thirdly, the long-term monitoring of selected individual size measurements of the locally adapted A. m. ligustica allowed for the first description of the honey bee nutritional year-cycle in a southern temperate climate. Analysis of data according to sampling date, climate (temperature, precipitation, and daylength), and flower diversity, revealed two critical timepoints for honey bee nutrition: summer and winter dearth. Seasonality was in accordance with our previous findings, with a peak in nutrient storage indicating the short presence of winter bees. The noted importance of precipitation for honey bee nutrition in the study area demonstrates how climate change could pose a threat to European honey bee populations in the future. Finally, size measurements were shown to vary significantly between nurse and forager bees further illustrating the potential value of selected morphological markers. The findings reported in this dissertation provide insight that could benefit ongoing research efforts and inform management or conservation strategies with the ultimate goal of improving overwintering success and preventing unnecessary colony losses. Present results can furthermore be used as a cornerstone for future research on honey bee nutrition and seasonality in southern temperate climates.

Honey bee colony losses represents a significant issue in modern apiculture. Regardless of climate, most losses occur in winter which is a particularly challenging period for A. mellifera. In fact, in order to deal with harsh winter conditions honey bees evolved a seasonal caste system with fundamental physiological processes dictated by nutrition. Whereas the overwintering of honey bees in northern regions has been well studied, much less is known regarding the seasonal dynamics of these insects in southern temperate climates. The goal of this dissertation is hence to approach this knowledge gap, especially from a nutritional point of view. Firstly, an up-to-date literature review on honey bee seasonality is presented providing necessary background information. A detailed description of summer and winter bee physiology shows how A. mellifera has adapted molecular pathways into a social system of division of labour as well as a biannual worker caste system governed by various internal and external factors. Next, the use of robust individual size measurements as possible novel markers for honey bee health and nutrition was investigated. Results of this preliminary evaluation revealed abdominal length, total body length, and head width as suitable candidate markers. These findings were then applied in the long-term monitoring of Italian bees allowing for the first description of the honey bee nutritional year-cycle in a southern temperate climate. Analysis of data according to date, climate, and flower diversity revealed two critical timepoints for honey bees: summer and winter dearth. Additionally, the noted importance of precipitation in the study area demonstrates how climate change could pose a threat to European honey bee populations in the future. These findings provide insight that could benefit ongoing research efforts and inform management or conservation strategies to improve overwintering success and prevent unnecessary colony losses