Know your enemy: investigations on Varroa destructor – A step forward in understanding its biology and promoting in vitro rearing

This PhD thesis offers a comprehensive investigation of the rearing challenges associated with Varroa destructor, the major ectoparasite affecting honey bee colonies worldwide. The introduction chapter details the mite’s biology, morphology, and life cycle, emphasizing its significant detrimental impact on bee health. The reasons for rearing Varroa mites are explained in the same chapter, and the issues regarding the rearing of arthropods ectoparasite are deeply analysed. The key concept of the introduction is the definition of the life history of an organism, which is composed of factors affecting survival and factors affecting reproduction. These factors are the ones that need to be studied in order to replicate the life history of the mite in a laboratory environment. Chapter 2 presents a thorough literature review and state-of-the-art analysis of past research efforts, spanning studies from the 1970s to the present. These studies examined various methods of maintaining and rearing Varroa mites in vitro. The document highlights the complexity of replicating the hive environment, where factors such as temperature, humidity, feeding cues, and host characteristics are crucial for mite survival and reproduction. From this review it was possible to identify which questions, regarding Varroa biology, were still open and could be answered by the present research. From chapter 3 to 10 the experimental work is reported. The research efforts were focused on the following aspects. Optimal host and parasite condition: determining which developmental stage of the host best supports mite longevity (chapter 3) and investigating the influence of mite age and reproductive history on mite’ survival (chapter 4) under artificial conditions. Artificial Feeding System: evaluating whether completely artificial diets can meet the nutritional needs of Varroa mites (chapter 5) and if the physical configuration of rearing chambers (chapter 6) affects mites’ survival; moreover the efficacy of different plastic membranes (chapter 7) in the artificial feeding of Varroa was determined. Mite Physiology: investigating the role of the phoretic phase on mite reproduction (chapter 8) and the reproduction of the mite in the absence of the living host (chapter 9). Host Resistance: exploring preliminary evidence that resistant bee strains might reduce Varroa fertility (chapter 10). The general conclusions are presented in chapter 11, followed by the references cited in this thesis. The appendix includes other scientific works produced during the PhD program.