Genetics and human management in Varroa destructor parasitism Open Access

Dynes, Travis (Summer 2018)

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European honey bee colonies have experienced widespread losses in the past decades, a particular concern due to the importance that honey bees play in agricultural services critical in both economic terms and to human health. In the first half of the 20th century the obligate ectoparasitic mite Varroa destructor made a sustained host switch from the Asian honey bee to the European honey bee. Since that time V. destructor has spread around the world and become the largest pathogenic threat currently facing the beekeeping industry.

When Varroa arrived in the United States in the 1980’s beekeepers were able to effectively treat Varroa infections with miticides. However, the mites quickly developed resistance to these miticides. This was unusual considering Varroa is characterized by a lack of genetic diversity. Our research was able to show that there was more genetic diversity at fine scales than would have been predicted. This implies that there was also more transmission of mites between colonies than would have been predicted. Human management of honey bee colonies places colonies in densities that more than three orders of magnitude greater than would be found in feral colonies. This increased density has implications for parasite transmission, colony health and survival. We found that increased density leads to more potential for disease transmission, decreased colony health and productivity, and increased winter mortality.

According to the virulence-transmission theory, Varroa populations evolving under these different management intensities (from feral to heavily managed) may face different selection pressures for population growth and virulence. Our research was consistent with this hypothesis for population growth. However, our virulence results suggest that there are genotype by genotype interactions that are occurring.

Table of Contents

Chapter 1

1.1 Introduction                                                                                                                                           1

Chapter 2: Fine scale genetic structure of Varroa destructor, an ectoparasitic mite of the honey bee (Apis mellifera)

2.1 Introduction                                                                                                                               6

2.2 Materials and Methods                                                                                                             8

2.3 Results                                                                                                                                    11

           Table 2.1 Population (apiary) pairwise FST values                                                           13

           Table 2.2 AMOVA analysis                                                                                             14

           Figure 2.1 Fine scale allelic diversity compared to global allelic diversity                      15

           Figure 2.2 Allelic diversity in ten apiaries using a rarefaction approach                         16

           Figure 2.3 AMOVA change in source of population variation over time                        16

2.4 Discussion                                                                                                                              17

2.5 Supplementary Materials                                                                                                        22

           Table 2.S1 Observed and expected heterozygosity levels within apiaries                        25

           Table 2.S2 Observed and expected heterozygosity levels within colonies                       26

           Table 2.S3 SMOGD alternative estimates of population differentiation                          27

           Figure 2.S1 Boxplot of Garza-Williamson Index values                                                 28

Chapter 3: GRAPHITE: A graphical environment for scalable in situ video tracking of moving insects

3.1 Introduction                                                                                                                             29

3.2 Experimental Setup                                                                                                                32

           Figure 3.1 Camera housings                                                                                             33

3.3 Modules and Editor                                                                                                                 34

           Figure 3.2 Analysis pipeline                                                                                             35

           Figure 3.3 Active region                                                                                                   36

           Figure 3.4 Tag detection from a single video frame                                                         38

           Figure 3.5 Tag tracks                                                                                                        41

           Figure 3.6 Graphical user interface                                                                                   42

3.4 Evaluations                                                                                                                              43

3.5 Conclusions and Future Directions                                                                                         44

3.6 Supplementary Material                                                                                                          45

           Figure 3.S1 Tagged bees                                                                                                  45

           Figure 3.S2 Editor                                                                                                             45

Chapter 4: Reduced density and visually complex apiaries reduce parasite load and promote overwintering survival in honey bees

4.1 Introduction                                                                                                                             46

4.2 Materials and Methods                                                                                                           49

           Figure 4.1 Scale representation of apiary arrangement from above                                 50

4.3 Results                                                                                                                                    55

           Figure 4.2 Honey production over time                                                                            56

           Figure 4.3 Mite count by sticky board                                                                              57

           Figure 4.4 Mite levels in brood                                                                                        58

           Figure 4.5 Survival curves and winter survival                                                                59

           Figure 4.6 Representation of drift within each of the apiaries                                         60

4.4 Discussion                                                                                                                              61

Chapter 5: Assessing virulence of Varroa destructor mites from different honey bee management regimes

5.1 Introduction                                                                                                                             67

5.2 Materials and Methods                                                                                                           70

           Table 5.1 Mite inoculation sources within each apiary                                                    71

5.3 Results                                                                                                                                    76

           Figure 5.1 Measures of mite abundance by treatment                                                     76

           Figure 5.2 Number of frames of brood by treatment                                                       78

           Figure 5.3 Survival curves by mite treatment                                                                  78

5.4 Discussion                                                                                                                              79

Chapter 6

6.1 Dissertation Conclusion                                                                                                          85

Bibliography                                                                                                                            88

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