Distribution, Genetic Diversity, Host Defense, and Control of Varroa Destructor (Acari: Varroidae), a Pest of Honeybees (Apis Mellifera) (Hymenoptera: Apidae) in Ethiopia
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Date
2024-06
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Addis Ababa University
Abstract
Honeybees, Apis mellifera (Hymenoptera: Apidae), are the most important social insects that provide vital economic and ecosystem benefits to humans. However, the recent dramatic colony losses and overall bee population declines have jeopardized the apicultural industry while causing high pressures on agricultural crop production and food security. Various biotic and abiotic stressors as well as their synergetic effects thought to be responsible for global colony decline and loss of honeybee populations. The ecto-parasitic mite, Varroa destructor (Anderson and Trueman), is one of the most recognized threat to honeybee health, causing dramatic colony losses worldwide. The parasite is known as the most harmful, and globally distributed pest of European honeybees (A. mellifera) that successfully infests both managed and wild colonies and play a prominent role in the decline of global honeybee populations. This in turn impacts both ecological and economic benefits of the honeybees, and becoming a threat for global food security. The occurrence of V. destructor mites in Ethiopia was reported for the first time in 2010; however, information on the distribution, genetic diversity, and host-parasite interactions of the mite is limited in the country. Therefore, this dissertation was designed to investigate the overall distribution, genetic diversity, host defense mechanisms, and non-chemical control of V. destructor in Ethiopian honeybee colonies. For this purpose, field assessments, laboratory diagnostics employing various techniques, and advanced molecular tools were used in this thesis research.
A diagnostic survey was undertaken between October 2020 and July 2022 to determine the prevalence and infestation levels of mites within selected areas across three regions: Oromia, Amhara, and the Southern Nations, Nationalities, and Peoples Region (SNNPR). In total, 360 samples were thoroughly collected from 39 apiaries and analyzed using established protocols of varroa mite research. To elucidate the genetic diversity and population structure of varroa mites, mitochondrial DNA sequencing targeting the Cytb and COX1 gene regions, along with single nucleotide polymorphism (SNP) marker analysis were conducted using the standard procedures of molecular techniques. Genetic diversity assessments, population structure analysis, clustering, and analysis of molecular variance (AMOVA) were subsequently performed to determine variability within varroa genotypes collected from different localities. Furthermore, investigation was undertaken to explore the host behavioral defense mechanisms used by local honeybees
(A.m. bandasii) against varroa mites. This involved evaluating key indicators of resistance traits, including hygienic behaviors (HB) and grooming behaviors (GB) of local honeybees. In addition, the potential efficacy of propolis extract in controlling varroa parasitism was tested under laboratory bioassay conditions.
The findings of this study showed that varroa mites were widely distributed in all the three regions of Ethiopia, with prevalence rates of 95.8% in Oromia, 85.2% in Amhara and 71.9% in the SNNP regional states. Mite infestation levels were lower in local hives (2.6 ± 5.9) than in frame hives (5.0 ± 0.9). In addition, the infestation level was significantly higher in brood bees (5.6 ± 0.8) compared to adult bees (1.93 ± 0.17) (p < 0.001). Genotyping of varroa samples using mtDNA gene sequencing, combined with 169 Cytb and 96 COX1 sequences from Gene Bank, revealed that only one haplotype, the Korean strain (K1 haplotype), was present in Ethiopia, suggesting a very low mite genetic diversity. SNP markers analysis also revealed that V. destructor was the only species detected in the local honeybee colonies. The phylogenetic cluster analysis of SNP markers grouped the mite populations into three hypothetical clusters, all of which shared a common ancestor of the Korean strain. The average allele count, genetic diversity, and polymorphic information content (PIC) values were 1.53, 0.14, and 0.29, respectively. Analysis of molecular variance (AMOVA) showed a very low genetic variation (2%) among varroa populations in Ethiopia, suggesting that the varroa introduced to the country was possibly originated from a single ancestor.
The present study showed that local honeybee, A. m. bandasii colonies expressed stronger hygienic behavior (97.6±3.4%) after 48 h. There was a strong negative correlation between the HB and varroa infestation rates (r = -0.81, P = 0.001), suggesting that HB has potential to reduce the mite population in colonies. Likewise, assessment of GB also indicated higher mean daily fallen mites per colony (16.3±10.2), of which about 80% of the total fallen mites were damaged, and eventually dead. These combined behavioral traits have evolved in African honeybees and have greatly contributed to their survival in the face of the catastrophic effects of the V. destructor mite. In this study, the ethanolic extract of propolis was very efficient and could kill 100% of the varroa mites within 4 hr post treatment. This highlights the potential of natural propolis extract in controlling varroa mite infestations.
In general, the results of this thesis provided valuable insights into the current varroa mite parasitism in local honeybee populations in Ethiopia and suggest for regular monitoring and management to mitigate the mites‘ possible impact on the health and productivity of honeybees. Furthermore, there is a pressing need for awareness creation among local beekeepers, government bodies, and different stakeholders towards designing long-term and sustainable control strategies.
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Keywords
Apis Mellifera, Brood, Infestation, Prevalence, Varroa Destructor, Markers, Genetic Variation, Population, Grooming Behavior, Hygienic Behavior, Varroacidal, Propolis, Extract, Ethanol, Mortality