Population genetics of ectoparasitic mites Varroa spp. in Eastern and Western honey bees

Amitraz Resistance in French Varroa Mite Populations-More Complex Than a Single-Nucleotide Polymorphism

Resistance against amitraz in Varroa mite populations has become a subject of interest in recent years due to the increasing reports of the reduced field efficacy of amitraz treatments, especially from some beekeepers in France and the United States. The loss of amitraz as a reliable tool to effectively reduce Varroa mite infestation in the field could severely worsen the position of beekeepers in the fight to keep Varroa infestation rates in their colonies at low levels. In this publication, we present data from French apiaries, collected in the years 2020 and 2021. These data include the field efficacy of an authorized amitraz-based Varroa treatment (Apivar® ,Véto-pharma, France) and the results of laboratory sensitivity assays of Varroa mites exposed to the reference LC90 concentration of amitraz. In addition, a total of 240 Varroa mites from Eastern, Central, and Southern regions in France that were previously classified as either "sensitive" or "resistant" to amitraz in a laboratory sensitivity assay were genotyped. The genetic analyses of mite samples are focused on the β-adrenergic-like octopamine receptor, which is considered as the main target site for amitraz in Varroa mites. Special attention was paid to a single-nucleotide polymorphism (SNP) at position 260 of the ORβ-2R-L gene that was previously associated to amitraz resistance in French Varroa mites, Varroa. Our findings confirm that amitraz resistance occurs in patches or "islands of resistance", with a less severe reduction in treatment efficacy compared to pyrethroid resistance or coumaphos resistance in Varroa mites. The results of our genetic analyses of Varroa mites call into question the hypothesis of the SNP at position 260 of the ORβ-2R-L gene being directly responsible for amitraz resistance development.

Genetic variability of the honey bee mite, Varroa destructor, from a humid continental climatic region of Canada, and temperate and tropical climatic regions of Mexico

Varroa destructor is a damaging mite of Western honey bees (Apis mellifera). Genetic variability of the mite in different regions of the world could be related to the movement of infested bees or other factors, such as climate. In this study, V. destructor samples were collected from tropical and temperate climate regions of Mexico, and a humid continental climate region of Canada. COX-1 AFLPs showed that all the mites were the Korean haplotype. Four microsatellites revealed nine haplogroups from the continental climate region of Canada, compared to three haplogroups from the tropical and temperate climate regions of Mexico. CytII-ATP sequences showed seven haplogroups from the humid continental climate region vs. two haplogroups from the temperate region and one haplogroup from the tropical region. CytB sequences revealed seven haplogroups from Canada vs. three from Mexico. A comparison of the cytB sequences of the samples from Canada and Mexico to those from a worldwide collection showed that one sequence, designated the cytB1 type, predominated, comprising 57% of the 86 sequences; it clustered with similar sequences that comprised 80% of the sequences, designated family A. CytB1 was predominant in Mexico, but not in Canada. The other 20% of sequences were in families B and C, and all those samples originated from East and Southeast Asia. The microsatellite, cytII-ATP, and cytB markers, all showed higher variability in mites collected in Canada than in Mexico, which could be related to the cooler climate or an earlier invasion and/or multiple mite invasions in Canada.

A systematic review of honey bee (Apis mellifera, Linnaeus, 1758) infections and available treatment options

BACKGROUND

Honey bees and honeycomb bees are very valuable for wild flowering plants and economically important crops due to their role as pollinators. However, these insects confront many disease threats (viruses, parasites, bacteria and fungi) and large pesticide concentrations in the environment. Varroa destructor is the most prevalent disease that has had the most negative effects on the fitness and survival of different honey bees (Apis mellifera and A. cerana). Moreover, honey bees are social insects and this ectoparasite can be easily transmitted within and across bee colonies.

OBJECTIVE

This review aims to provide a survey of the diversity and distribution of important bee infections and possible management and treatment options, so that honey bee colony health can be maintained.

METHODS

We used PRISMA guidelines throughout article selection, published between January 1960 and December 2020. PubMed, Google Scholar, Scopus, Cochrane Library, Web of Science and Ovid databases were searched.

RESULTS

We have collected 132 articles and retained 106 articles for this study. The data obtained revealed that V. destructor and Nosema spp. were found to be the major pathogens of honey bees worldwide. The impact of these infections can result in the incapacity of forager bees to fly, disorientation, paralysis, and death of many individuals in the colony. We find that both hygienic and chemical pest management strategies must be implemented to prevent, reduce the parasite loads and transmission of pathogens. The use of an effective miticide (fluvalinate-tau, coumaphos and amitraz) now seems to be an essential and common practice required to minimise the impact of Varroa mites and other pathogens on bee colonies. New, alternative biofriendly control methods, are on the rise, and could be critical for maintaining honey bee hive health and improving honey productivity.

CONCLUSIONS

We suggest that critical health control methods be adopted globally and that an international monitoring system be implemented to determine honey bee colony safety, regularly identify parasite prevalence, as well as potential risk factors, so that the impact of pathogens on bee health can be recognised and quantified on a global scale.

Prospects, challenges and perspectives in harnessing natural selection to solve the ‘varroa problem’ of honey bees

Honey bees, Apis mellifera, of European origin are major pollinators of crops and wild flora. Their endemic and exported populations are threatened by a variety of abiotic and biotic factors. Among the latter, the ectoparasitic mite Varroa destructor is the most important single cause behind colony mortality. The selection of mite resistance in honey bee populations has been deemed a more sustainable solution to its control than varroacidal treatments. Because natural selection has led to the survival of some European and African honey bee populations to V. destructor infestations, harnessing its principles has recently been highlighted as a more efficient way to provide honey bee lineages that survive infestations when compared with conventional selection on resistance traits against the parasite. However, the challenges and drawbacks of harnessing natural selection to solve the varroa problem have only been minimally addressed. We argue that failing to consider these issues could lead to counterproductive results, such as increased mite virulence, loss of genetic diversity reducing host resilience, population collapses or poor acceptance by beekeepers. Therefore, it appears timely to evaluate the prospects for the success of such programmes and the qualities of the populations obtained. After reviewing the approaches proposed in the literature and their outcomes, we consider their advantages and drawbacks and propose perspectives to overcome their limitations. In these considerations, we not only reflect on the theoretical aspects of host-parasite relationships but also on the currently largely neglected practical constraints, that is, the requirements for productive beekeeping, conservation or rewilding objectives. To optimize natural selection-based programmes towards these objectives, we suggest designs based on a combination of nature-driven phenotypic differentiation and human-directed selection of traits. Such a dual strategy aims at allowing field-realistic evolutionary approaches towards the survival of V. destructor infestations and the improvement of honey bee health.

Varroa destructor resistance to tau-fluvalinate: relationship between in vitro phenotypic test and VGSC L925V mutation

BACKGROUND

Varroa destructor is a parasitic mite of the honey bee, Apis mellifera. Its presence in colonies can lead to a collapse within a few years. The use of acaricides has become essential to manage the hive infestation. However, the repeated and possibly incorrect use of acaricide treatments, as tau-fluvalinate, has led to the development of resistance. The in vitro phenotypic test allows the proportion of susceptible or resistant individuals to be known following an exposure to an active substance. In Varroa mites, resistance to tau-fluvalinate is associated with the presence of mutations at the position 925 of the voltage-gated sodium channel (VGSC).

RESULTS

Here, we compared the results obtained with an in vitro phenotypic test against tau-fluvalinate and those obtained with an allelic discrimination assay on 13 treated and untreated Varroa populations in France. The correlation between the phenotype and the genetic profile rate is found to be 0.89 Varroa mites having resistant phenotypic profile have a probability of 63% to present the L925V mutation (resistance detection reliability). However, 97% of the Varroa mites having the susceptible phenotype do not present the L925V mutation (susceptible detection reliability).

CONCLUSION

The L925V mutation explains most of the resistance to tau-fluvalinate in V. destructor in the populations tested. However, other mutations or types of resistance may also be involved to explain the survival of Varroa mites in the phenotypic test. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Genetic diversification of an invasive honey bee ectoparasite across sympatric and allopatric host populations

Invasive parasites are major threats to biodiversity. The honey bee ectoparasite, Varroa destructor, has shifted host and spread almost globally several decades ago. This pest is generally considered to be the main global threat to Western honey bees, Apis mellifera, although the damages it causes are not equivalent in all its new host's populations. Due to the high virulence of this parasite and the viruses it vectors, beekeepers generally rely on acaricide treatments to keep their colonies alive. However, some populations of A. mellifera can survive without anthropogenic mite control, through the expression of diverse resistance and tolerance traits. Such surviving colonies are currently found throughout the globe, with the biggest populations being found in Sub-Saharan Africa and Latin America. Recently, genetic differences between mite populations infesting surviving and treated A. mellifera colonies in Europe were found, suggesting that adaptations of honey bees drive mite evolution. Yet, the prevalence of such co-evolutionary adaptations in other invasive populations of V. destructor remain unknown. Using the previous data from Europe and novel genetic data from V. destructor populations in South America and Africa, we here investigated whether mites display signs of adaptations to different host populations of diverse origins and undergoing differing management. Our results show that, contrary to the differences previously documented in Europe, mites infesting treated and untreated honey bee populations in Africa and South America are genetically similar. However, strong levels of genetic differentiation were found when comparing mites across continents, suggesting ongoing allopatric speciation despite a recent spread from genetically homogenous lineages. This study provides novel insights into the co-evolution of V. destructor and A. mellifera, and confirms that these species are ideal to investigate coevolution in newly established host-parasite systems.

The first steps toward a global pandemic: Reconstructing the demographic history of parasite host switches in its native range

Host switching allows parasites to expand their niches. However, successful switching may require suites of adaptations and also may decrease performance on the old host. As a result, reductions in gene flow accompany many host switches, driving speciation. Because host switches tend to be rapid, it is difficult to study them in real-time, and their demographic parameters remain poorly understood. As a result, fundamental factors that control subsequent parasite evolution, such as the size of the switching population or the extent of immigration from the original host, remain largely unknown. To shed light on the host switching process, we explored how host switches occur in independent host shifts by two ectoparasitic honey bee mites (Varroa destructor and V. jacobsoni). Both switched to the western honey bee (Apis mellifera) after being brought into contact with their ancestral host (Apis cerana), ~70 and ~12 years ago, respectively. Varroa destructor subsequently caused worldwide collapses of honey bee populations. Using whole-genome sequencing on 63 mites collected in their native ranges from both the ancestral and novel hosts, we were able to reconstruct the known temporal dynamics of the switch. We further found multiple previously undiscovered mitochondrial lineages on the novel host, along with the genetic equivalent of tens of individuals that were involved in the initial host switch. Despite being greatly reduced, some gene flow remains between mites adapted to different hosts. Our findings suggest that while reproductive isolation may facilitate the fixation of traits beneficial for exploiting the new host, ongoing genetic exchange may allow genetic amelioration of inbreeding effects.

Haplotype Analysis of Varroa destructor and Deformed Wing Virus Using Long Reads

As a phoretic parasite and virus vector, the mite Varroa destructor and the associated Deformed wing virus (DWV) form a lethal combination to the honey bee, Apis mellifera. Routine acaricide treatment has been reported to reduce the diversity of mites and select for tolerance against these treatments. Further, different DWV strains face selective pressures when transmitted via mites. In this study, the haplotypes of Varroa mites and associated DWV variants were quantified using long reads. A single haplotype dominated the mite mitochondrial gene cytochrome oxidase subunit I, reflecting an ancient bottleneck. However, highly polymorphic genes were present across the mite genome, suggesting the diversity of mites could be actively maintained at a regional level. DWV detected in both mites and honey bees show a dominant variant with only a few low-frequency alternate haplotypes. The relative abundances of DWV haplotypes isolated from honey bees and mites were highly consistent, suggesting that some variants are favored by ongoing selection.

Genetic analysis and screening of pyrethroid resistance mutations in Varroa destructor populations from Turkey

Varroa destructor is the most common ectoparasite of the Western honey bee (Apis mellifera L.) worldwide and poses a serious threat to bee health. Synthetic acaricides, particularly pyrethroids, are frequently used to control Varroa mites. However, long-term and repeated use of synthetic pyrethroids has led to the development of resistance. In this study, we report on the presence of resistance mutations in the voltage-gated sodium channel in V. destructor populations from Turkish beekeeping areas. Two resistance mutations, L925V and L925I, that were previously associated with pyrethroid resistance, were found in more than 75% of the populations. A general correlation between the presence of mutations and the history of acaricide usage was observed for the sampled hives. In addition, we show there is only a low genetic distance among the sampled V. destructor populations, based on the analysis of three mitochondrial genes: cytochrome b (cytb), ATP synthase subunit 6 (atp6), and cytochrome c oxidase subunit III (cox3). Revealing the presence and geographical distribution of pyrethroid resistance mutations in V. destructor populations from Turkish apiaries will contribute to create more effective mite management programmes.

Population genetics and host specificity of Varroa destructor mites infesting eastern and western honeybees

In a globalized world, parasites are often brought in contact with new potential hosts. When parasites successfully shift host, severe diseases can emerge at a large cost to society. However, the evolutionary processes leading to successful shifts are rarely understood, hindering risk assessment, prevention, or mitigation of their effects. Here, we screened populations of Varroa destructor, an ectoparasitic mite of the honeybee genus Apis, to investigate their genetic structure and reproductive potential on new and original hosts. From the patterns identified, we deduce the factors that influenced the macro- and microevolutionary processes that led to the structure observed. Among the mite variants identified, we found two genetically similar populations that differed in their reproductive abilities and thus in their host specificity. These lineages could interbreed, which represents a threat due to the possible increased virulence of the parasite on its original host. However, interbreeding was unidirectional from the host-shifted to the nonshifted native mites and could thus lead to speciation of the former. The results improve our understanding of the processes affecting the population structure and evolution of this economically important mite genus and suggest that introgression between shifted and nonshifted lineages may endanger the original host.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10340-020-01322-7.

Reproduction of ectoparasitic mites in a coevolved system: Varroa spp.-Eastern honey bees, Apis cerana

Parasite host shifts can impose a high selective pressure on novel hosts. Even though the coevolved systems can reveal fundamental aspects of host-parasite interactions, research often focuses on the new host-parasite relationships. This holds true for two ectoparasitic mite species, Varroa destructor and Varroa jacobsonii, which have shifted hosts from Eastern honey bees, Apis cerana, to Western honey bees, Apis mellifera, generating colony losses of these pollinators globally. Here, we study infestation rates and reproduction of V. destructor and V. jacobsonii haplotypes in 185 A. cerana colonies of six populations in China and Thailand to investigate how coevolution shaped these features. Reproductive success was mostly similar and low, indicating constraints imposed by hosts and/or mite physiology. Infestation rates varied between mite haplotypes, suggesting distinct local co-evolutionary scenarios. The differences in infestation rates and reproductive output between haplotypes did not correlate with the virulence of the respective host-shifted lineages suggesting distinct selection scenarios in novel and original host. The occasional worker brood infestation was significantly lower than that of drone brood, except for the V. destructor haplotype (Korea) from which the invasive lineage derived. Whether mites infesting and reproducing in atypical intraspecific hosts (i.e., workers and queens) actually predisposes for and may govern the impact of host shifts on novel hosts should be determined by identifying the underlying mechanisms. In general, the apparent gaps in our knowledge of this coevolved system need to be further addressed to foster the adequate protection of wild and managed honey bees from these mites globally.

Varroa destructor: how does it harm Apis mellifera honey bees and what can be done about it?

Since its migration from the Asian honey bee (Apis cerana) to the European honey bee (Apis mellifera), the ectoparasitic mite Varroa destructor has emerged as a major issue for beekeeping worldwide. Due to a short history of coevolution, the host–parasite relationship between A. mellifera and V. destructor is unbalanced, with honey bees suffering infestation effects at the individual, colony and population levels. Several control solutions have been developed to tackle the colony and production losses due to Varroa, but the burden caused by the mite in combination with other biotic and abiotic factors continues to increase, weakening the beekeeping industry. In this synthetic review, we highlight the main advances made between 2015 and 2020 on V. destructor biology and its impact on the health of the honey bee, A. mellifera. We also describe the main control solutions that are currently available to fight the mite and place a special focus on new methodological developments, which point to integrated pest management strategies for the control of Varroa in honey bee colonies.

Inventory of Varroa destructor susceptibility to amitraz and tau-fluvalinate in France

Varroa destructor is one of the greatest threats for the European honeybee, Apis mellifera. Acaricides are required to control mite infestation. Three conventional chemical acaricide substances are used in France: tau-fluvalinate, flumethrin and amitraz. Tau-fluvalinate was used for over 10 years before experiencing a loss of effectiveness. In 1995, bioassay trials showed the first mite resistance to tau-fluvalinate. In some countries, amitraz was widely used, also leading to resistance of V. destructor to amitraz. In France, some efficiency field tests showed a loss of treatment effectiveness with amitraz. We adapted the bioassay from Maggi and collaborators to determine mite susceptibility to tau-fluvalinate and amitraz in France in 2018 and 2019. The lethal concentration (LC) which kills 90% of susceptible mite strains (LC₉₀) is 0.4 and 12 µg/mL for amitraz and tau-fluvalinate, respectively. These concentrations were chosen as the determining factors to evaluate mite susceptibility. Some mites, collected from different apiaries, present resistance to amitraz and tau-fluvalinate (71% of the mite samples show resistance to amitraz and 57% to tau-fluvalinate). As there are few active substances available in France, and if mite resistance to acaricides continues to increase, the effectiveness of the treatments will decrease and therefore more treatments per year will be necessary. To prevent this situation, a new strategy needs to be put in place to include mite resistance management. We suggest that a bioassay would be a good tool with which to advise the policymakers.

Varroa mite evolution: a neglected aspect of worldwide bee collapses?

While ectoparasitic Varroa mites cause minimal damage to their co-evolved ancestral host, the eastern honey bee (Apis cerana), they devastate their novel host, the western honey bee (Apis mellifera). Over several decades, the host switch caused worldwide population collapses, threatening global food security. Varroa management strategies have focused on breeding bees for tolerance. But, can Varroa overcome these counter-adaptations in a classic coevolutionary arms race? Despite increasing evidence for Varroa genetic diversity and evolvability, this eventuality has largely been neglected. We therefore suggest a more holistic paradigm for studying this host-parasite interaction, one in which 'Varroa-tolerant' bee traits should be viewed as a shared phenotype resulting from Varroa and honey bee interaction.

Varroa destructor: A Complex Parasite, Crippling Honey Bees Worldwide

The parasitic mite, Varroa destructor, has shaken the beekeeping and pollination industries since its spread from its native host, the Asian honey bee (Apis cerana), to the naïve European honey bee (Apis mellifera) used commercially for pollination and honey production around the globe. Varroa is the greatest threat to honey bee health. Worrying observations include increasing acaricide resistance in the varroa population and sinking economic treatment thresholds, suggesting that the mites or their vectored viruses are becoming more virulent. Highly infested weak colonies facilitate mite dispersal and disease transmission to stronger and healthier colonies. Here, we review recent developments in the biology, pathology, and management of varroa, and integrate older knowledge that is less well known.

Honey bee survival mechanisms against the parasite Varroa destructor: a systematic review of phenotypic and genomic research efforts

The ectoparasitic mite Varroa destructor is the most significant pathological threat to the western honey bee, Apis mellifera, leading to the death of most colonies if left untreated. An alternative approach to chemical treatments is to selectively enhance heritable honey bee traits of resistance or tolerance to the mite through breeding programs, or select for naturally surviving untreated colonies. We conducted a literature review of all studies documenting traits of A. mellifera populations either selectively bred or naturally selected for resistance and tolerance to mite parasitism. This allowed us to conduct an analysis of the diversity, distribution and importance of the traits in different honey bee populations that can survive V. destructor globally. In a second analysis, we investigated the genetic bases of these different phenotypes by comparing 'omics studies (genomics, transcriptomics, and proteomics) of A. mellifera resistance and tolerance to the parasite. Altogether, this review provides a detailed overview of the current state of the research projects and breeding efforts against the most devastating parasite of A. mellifera. By highlighting the most promising traits of Varroa-surviving bees and our current knowledge on their genetic bases, this work will help direct future research efforts and selection programs to control this pest. Additionally, by comparing the diverse populations of honey bees that exhibit those traits, this review highlights the consequences of anthropogenic and natural selection in the interactions between hosts and parasites.

Genetic variation and heteroplasmy of Varroa destructor inferred from ND4 mtDNA sequences

Varroa destructor, a parasitic mite of the western honey bee, Apis mellifera L., is a serious threat to colonies and beekeeping worldwide. Population genetics studies of the mite have provided information on two mitochondrial haplotypes infecting honey bee colonies, named K and J (after Korea and Japan, respectively, where they were originally identified). On the American continent, the K haplotype is much more prevalent, with the J haplotype only detected in some areas of Brazil. The aims of the present study were to assess the genetic diversity of V. destructor populations in the major beekeeping region of Argentina and to evaluate the presence of heteroplasmy at the nucleotide level. Phoretic mites were collected from managed A. mellifera colonies in ten localities, and four mitochondrial DNA (mtDNA) regions (COXI, ND4, ND4L, and ND5) were analyzed. Based on cytochrome oxidase subunit I (COXI) sequencing, exclusively the K haplotype of V. destructor was detected. Furthermore, two sub-haplotypes (KArg-N1 and KArg-N2) were identified from a variation in ND4 sequences and the frequency of these sub-haplotypes was found to significantly correlate with geographical latitude. The occurrence of site heteroplasmy was also evident for this gene. Therefore, ND4 appears to be a sensitive marker for detecting genetic variability in mite populations. Site heteroplasmy emerges as a phenomenon that could be relatively frequent in V. destructor.

Reproduction of Distinct Varroa destructor Genotypes on Honey Bee Worker Brood

Honey bees play important roles in pollination for many crops and wild plants, but have been facing great threats posed by various pathogens and parasites. Among them, Varroa destructor, an obligate ectoparasite of honey bees, is considered the most damaging. Within the last century, V. destructor shifted from the original host, the Asian honey bee Apis cerana to the new host, the European honey bee A. mellifera. However, the reproduction of Varroa mites, especially of different haplotypes in the two hosts, is still largely unknown. In this study, we first investigated the existing Varroa haplotypes in local colonies in southern China, and then compared the reproduction of different haplotypes on the worker brood of both the original and new hosts by artificial inoculation. We confirmed that there are two haplotypes of V. destructor in southern China, one is the Korea haplotype and the other is the China haplotype, and the two types parasitized different honey bee species. Although Varroa females from A. mellifera (Korea haplotype) are able to reproduce on the worker brood of both honey bee species, they showed better reproductive performance in the new host A. mellifera with significantly higher fecundity (number of offspring per mother mite) and reproductive rate (number of adult daughters per mother mite), suggesting that this parasite gains higher fitness after host shift. The data further showed that a short stay of Varroa females inside the A. cerana worker cells decreased their fecundity and especially the reproductive rate in a time-dependent manner, suggesting that the A. cerana worker cells may inhibit Varroa reproduction. In contrast, Varroa mites derived from A. cerana colonies (China haplotype) were entirely sterile in A. mellifera worker cells during two sequential inoculations, while the control mites from A. mellifera colonies (Korea haplotype) reproduced normally. In addition, all the infertile mites were found to defecate on the abdomen of bee pupae. We have revealed that two haplotypes of V. destructor exhibit differential reproduction on the worker brood of the original and new host honey bees, providing novel insights into the diversity and complexity of the reproduction of V. destructor.