Selection for outbreeding in Varroa parasitising resistant honey bee (Apis mellifera) colonies

Parasitism is expected to select for counter‐adaptations in the host: driving a coevolutionary arms race. However, human interference between honey bees (Apis mellifera) and Varroa mites removes the effect of natural selection and restricts the evolution of host counter‐adaptations. With full‐sibling mating common among Varroa, this can rapidly select for virulent, highly inbred, Varroa populations. We investigated how the evolution of host resistance could affect the infesting population of Varroa mites. We screened a Varroa‐resistant honey bee population near Toulouse, France, for a Varroa resistance trait: the inhibition of Varroa's reproduction in drone pupae. We then genotyped Varroa which had co‐infested a cell using microsatellites. Across all resistant honey bee colonies, Varroa's reproductive success was significantly higher in co‐infested cells but the distribution of Varroa between singly and multiply infested cells was not different from random. While there was a trend for increased reproductive success when Varroa of differing haplotypes co‐infested a cell, this was not significant. This suggests local mate competition, through the presence of another Varroa foundress in a pupal cell, may be enough to help Varroa overcome host resistance traits; with a critical mass of infesting Varroa overwhelming host resistance. However, the fitness trade‐offs associated with preferentially co‐infesting cells may be too high for Varroa to evolve a mechanism to identify already‐infested cells. The increased reproductive success of Varroa when co‐infesting resistant pupal cells may act as a release valve on the selective pressure for the evolution of counter resistance traits: helping to maintain a stable host–parasite relationship.     

Biology and control of Varroa destructor

The ectoparasitic honey bee mite Varroa destructor was originally confined to the Eastern honey bee Apis cerana. After a shift to the new host Apis mellifera during the first half of the last century, the parasite dispersed world wide and is currently considered the major threat for apiculture. The damage caused by Varroosis is thought to be a crucial driver for the periodical colony losses in Europe and the USA and regular Varroa treatments are essential in these countries. Therefore, Varroa research not only deals with a fascinating host–parasite relationship but also has a responsibility to find sustainable solutions for the beekeeping.This review provides a survey of the current knowledge in the main fields of Varroa research including the biology of the mite, damage to the host, host tolerance, tolerance breeding and Varroa treatment. We first present a general view on the functional morphology and on the biology of the Varroa mite with special emphasis on host–parasite interactions during reproduction of the female mite. The pathology section describes host damage at the individual and colony level including the problem of transmission of secondary infections by the mite. Knowledge of both the biology and the pathology of Varroa mites is essential for understanding possible tolerance mechanisms in the honey bee host. We comment on the few examples of natural tolerance in A. mellifera and evaluate recent approaches to the selection of Varroa tolerant honey bees. Finally, an extensive listing and critical evaluation of chemical and biological methods of Varroa treatments is given.This compilation of present-day knowledge on Varroa honey bee interactions emphasizes that we are still far from a solution for Varroa infestation and that, therefore, further research on mite biology, tolerance breeding, and Varroa treatment is urgently needed.     

Regular dorsal dimples and damaged mites of <Emphasis Type="Italic">Varroa destructor</Emphasis> in some Iranian honey bees (<Emphasis Type="Italic">Apis mellifera</Emphasis>)

The frequency of damaged Varroa destructor Anderson and Trueman (Mesostigmata: Varroidae) found on the bottom board of hives of the honey bee, Apis mellifera L. (Hymenoptera: Apidae) has been used as an indicator of the degree of tolerance or resistance of honey bee colonies against mites. However, it is not clear that this measure is adequate. These injuries should be separated from regular dorsal dimples that have a developmental origin. To investigate damage to Varroa mites and regular dorsal dimples, 32 honey bee (A. mellifera) colonies were selected from four Iranian provinces: Isfahan, Markazi, Qazvin, and Tehran. These colonies were part of the National Honey bee Breeding Program that resulted in province-specific races. In April, Varroa mites were collected from heavily infested colonies and used to infest the 32 experimental colonies. In August, 20 of these colonies were selected (five colonies from each province). Adult bees from these colonies were placed in cages and after introducing mites, damaged mites were collected from each cage every day. The average percentage of injured mites ranged from 0.6 to 3.0% in four provinces. The results did not show any statistical differences between the colonies within provinces for injuries to mites, but there were some differences among province-specific lines. Two kinds of injuries to the mites were observed: injuries to legs and pedipalps, and injuries to other parts of the body. There were also some regular dorsal dimples on dorsal idiosoma of the mites that were placed in categories separate from mites damaged by bees. This type of classification helps identifying damage to mites and comparing them with developmental origin symptoms, and may provide criteria for selecting bees tolerant or resistant to this mite.     

The role of epistatic interactions underpinning resistance to parasitic Varroa mites in haploid honey bee (Apis mellifera) drones

The Red Queen hypothesis predicts that host–parasite coevolutionary dynamics can select for host resistance through increased genetic diversity, recombination and evolutionary rates. However, in haplodiploid organisms such as the honeybee (Apis mellifera), models suggest the selective pressure is weaker than in diploids. Haplodiploid sex determination, found in A. mellifera, can allow deleterious recessive alleles to persist in the population through the diploid sex with negative effects predominantly expressed in the haploid sex. To overcome these negative effects in haploid genomes, epistatic interactions have been hypothesized to play an important role. Here, we use the interaction between A. mellifera and the parasitic mite Varroa destructor to test epistasis in the expression of resistance, through the inhibition of parasite reproduction, in haploid drones. We find novel loci on three chromosomes which explain over 45% of the resistance phenotype. Two of these loci interact only additively, suggesting their expression is independent of each other, but both loci interact epistatically with the third locus. With drone offspring inheriting only one copy of the queen's chromosomes, the drones will only possess one of two queen alleles throughout the years‐long lifetime of the honeybee colony. Varroa, in comparison, completes its highly inbred reproductive cycle in a matter of weeks, allowing it to rapidly evolve resistance. Faced with the rapidly evolving Varroa, a diversity of pathways and epistatic interactions for the inhibition of Varroa reproduction could therefore provide a selective advantage to the high levels of recombination seen in A. mellifera. This allows for the remixing of phenotypes despite a fixed queen genotype.     

Selection for barriers between honey bees and a devastating parasite

Rivaling pesticides and a dearth of flowers, the parasitic mite Varroa destructor presents a tremendous threat to western honey bees, Apis mellifera. A longstanding, but minor, pest for the Asian honey bee Apis cerana, these obligate bee parasites feast on developing and adult A. mellifera across several continents. Varroa reproduction is limited to a short window when developing bee pupae are concealed in wax cells. Mated females target developing bees just before pupation and then have about one day to initiate reproduction, eventually laying one male and up to several female offspring. Female mites often fail to reproduce at all, instead waiting in cells until their bee host finishes development and then hitching dangerous rides on a succession of adult bees for up to several weeks, before scouting for a new host pupa. In this issue of Molecular Ecology, Conlon et al. (2019) have explored mite reproductive success via a clever and thought‐provoking association study. In so doing, they have identified a protein whose actions could be integral to the dance between bees and their mite parasites.     

Deformed wing virus associated with <Emphasis Type="Italic">Tropilaelaps mercedesae</Emphasis> infesting European honey bees (<Emphasis Type="Italic">Apis mellifera</Emphasis>)

Mites in the genus Tropilaelaps (Acari: Laelapidae) are ectoparasites of the brood of honey bees (Apis spp.). Different Tropilaelaps subspecies were originally described from Apis dorsata, but a host switch occurred to the Western honey bee, Apis mellifera, for which infestations can rapidly lead to colony death. Tropilaelaps is hence considered more dangerous to A. mellifera than the parasitic mite Varroa destructor. Honey bees are also infected by many different viruses, some of them associated with and vectored by V. destructor. In recent years, deformed wing virus (DWV) has become the most prevalent virus infection in honey bees associated with V. destructor. DWV is distributed world-wide, and found wherever the Varroa mite is found, although low levels of the virus can also be found in Varroa free colonies. The Varroa mite transmits viral particles when feeding on the haemolymph of pupae or adult bees. Both the Tropilaelaps mite and the Varroa mite feed on honey bee brood, but no observations of DWV in Tropilaelaps have so far been reported. In this study, quantitative real-time RT-PCR was used to show the presence of DWV in infested brood and Tropilaelaps mercedesae mites collected in China, and to demonstrate a close quantitative association between mite-infested pupae of A. mellifera and DWV infections. Phylogenetic analysis of the DWV sequences recovered from matching pupae and mites revealed considerable DWV sequence heterogeneity and polymorphism. These polymorphisms appeared to be associated with the individual brood cell, rather than with a particular host.     

Resin foraging dynamics in Varroa destructor-infested hives: a case of medication of kin?

t Social insects have evolved colony behavioral, physiological, and organiza. tional adaptations (social immunity) to reduce the risks of parasitization and/or disease transmission. The collection of resin from various plants and its use in the hive as propolis is a clear example of behavioral defense. For Apis mellifera, an increased propolis content in the hive may correspond to variations in the microbial load of the colony and to a downregulation of an individual bee's immune response. However, many aspects of such antimicrobial mechanism still need to be clarified. Assuming that bacterial and fungal infection mechanisms differ from the action of a parasite, we studied the resin collection dynamics in Varroa destructor-infested honeybee colonies. Comparative experiments involving hives with different mite infestation levels were conducted in order to assess the amount of resin collected and propolis quality within the hive, over a 2-year period (2014 and 2015). Our study demonstrates that when A. mellifera colonies are under stress because of Varroa infestation, an increase in the number of resin foragers is recorded, even if a general intensification of the foraging activity is not observed. A reduction in the total polyphenolic content in propolis produced in infested versus uninfested hives was also noticed. Considering that different propolis types show varying levels of inhibition against a variety of honey bee pathogens in vitro, it would be very important to study the effects against Varroa of two diverse types of propolis: from Varroa-free and from Varroa-infested hives.     

Reproduction of parasitic mites Varroa destructor in original and new honeybee hosts

The ectoparasitic mite, Varroa destructor, shifted host from the eastern honeybee, Apis cerana, to the western honeybee, Apis mellifera. Whereas the original host survives infestations by this parasite, they are lethal to colonies of its new host. Here, we investigated a population of A. cerana naturally infested by the V. destructor Korea haplotype that gave rise to the globally invasive mite lineage. Our aim was to better characterize traits that allow for the survival of the original host to infestations by this particular mite haplotype. A known major trait of resistance is the lack of mite reproduction on worker brood in A. cerana. We show that this trait is neither due to a lack of host attractiveness nor of reproduction initiation by the parasite. However, successful mite reproduction was prevented by abnormal host development. Adult A. cerana workers recognized this state and removed hosts and parasites, which greatly affected the fitness of the parasite. These results confirm and complete previous observations of brood susceptibility to infestation in other honeybee host populations, provide new insights into the coevolution between hosts and parasites in this system, and may contribute to mitigating the large‐scale colony losses of A. mellifera due to V. destructor.     

Evaluation of the Distribution and Impacts of Parasites,Pathogens, and Pesticides on Honey Bee (Apis mellifera) Populations in East Africa

In East Africa, honey bees (Apis mellifera) provide critical pollination services and income for small-holder farmers and rural families. While honey bee populations in North America and Europe are in decline, little is known about the status of honey bee populations in Africa. We initiated a nationwide survey encompassing 24 locations across Kenya in 2010 to evaluate the numbers and sizes of honey bee colonies, assess the presence of parasites (Varroa mites and Nosema microsporidia) and viruses, identify and quantify pesticide contaminants in hives, and assay for levels of hygienic behavior. Varroa mites were present throughout Kenya, except in the remote north. Levels of Varroa were positively correlated with elevation, suggesting that environmental factors may play a role in honey bee host-parasite interactions. Levels of Varroa were negatively correlated with levels of hygienic behavior: however, while Varroa infestation dramatically reduces honey bee colony survival in the US and Europe, in Kenya Varroa presence alone does not appear to impact colony size. Nosema apis was found at three sites along the coast and one interior site. Only a small number of pesticides at low concentrations were found. Of the seven common US/European honey bee viruses, only three were identified but, like Varroa, were absent from northern Kenya. The number of viruses present was positively correlated with Varroa levels, but was not correlated with colony size or hygienic behavior. Our results suggest that Varroa, the three viruses, and Nosema have been relatively recently introduced into Kenya, but these factors do not yet appear to be impacting Kenyan bee populations. Thus chemical control for Varroa and Nosema are not necessary for Kenyan bees at this time. This study provides baseline data for future analyses of the possible mechanisms underlying resistance to and the long-term impacts of these factors on African bee populations.     

PCR-based detection of a tracheal mite of the honey bee Acarapis woodi

The effects of the tracheal mite Acarapis woodi on the health of honey bees have been neglected since the prevalence of Varroa mites to Apis mellifera colonies. However, tracheal mite infestation of honey bee colonies still occurs worldwide and could impose negative impact on apiculture. The detection of A. woodi requires the dissection of honey bees followed by microscopic observation of the tracheal sacs. We thus developed PCR methods to detect A. woodi. These methods facilitate rapid and sensitive detection of A. woodi in many honey bee samples for epidemiologic surveys.     

Genetic bases of tolerance to Varroa destructor in honey bees (Apis mellifera L.)

Currently, the Varroa destructor mite is the most serious parasite of honey bees (Apis mellifera) and has become a nearly cosmopolitan species. The mite not only causes damage by feeding on the haemolymph of honey bees, but it also transmits viruses, which have been implicated in colony collapse disorder. The major research goal has been to breed mite-tolerant honey bee lines in order to reduce the amount of pesticide used, because pesticides can promote the evolution of resistance in mites. In this review, we describe different behavioural traits and genes that may be part of the defence against the Varroa mite. Specifically, we review grooming behaviour, Varroa-sensitive hygiene and the suppression of mite reproduction. A large number of candidate genes have been identified by Quantitative Trait Loci studies, and through gene expression studies their function and effect have been elucidated. Results from the studies discussed can be used in apiary practice.     

Honey bee waggle dance communication increases diversity of pollen diets in intensively managed agricultural landscapes

The benefits of honey bee dance communication for colony performance in different resource environments are still not well understood. Here, we test the hypothesis that directional dance communication enables honey bee colonies to maintain a diverse pollen diet, especially in landscapes with low resource diversity. To test this hypothesis, we placed 24 Apis mellifera L. colonies with either intact or experimentally disrupted dance communication in eight agricultural landscapes that differed in the diversity of flowering plants and in the dominance of mass‐flowering crops. Pollen from incoming foragers was collected and identified via DNA metabarcoding. Disrupting dance communication affected the way the diversity of honey bee pollen diets was impacted by the dominance of mass‐flowering crops in available flower resources (p = .04). With increasing dominance of mass‐flowering crops in resource environments, foragers of colonies with intact communication foraged on an increasing proportion of available plant genera (p = .01). This was not the case for colonies with disrupted dance communication (p = .5). We conclude that the honey bee dance communication benefits pollen foraging on diverse plant resources and thereby contributes to high quality nutrition in environments with low‐resource diversity.     

Influence of resistant honey bee hosts on the life history of the parasite Acarapis woodi

Non-infested, young adult honey bees (Apis mellifera L.) of two stocks were exposed to tracheal mites (Acarapis woodi (Rennie)) in infested colonies to determine how divergent levels of susceptibility in host bees differentially affect components of the mite life history. Test bees were retrieved after exposure and dissected to determine whether resistance is founded on the reduced success of gravid female (foundress) mites to enter the host tracheae, on the suppressed reproduction by foundress mites once established in host tracheae or on both. Cohorts of 30–60 bees from each of ten resistant colonies and eight susceptible colonies were tested in eight trials (three to five colonies per stock per trial) having exposure durations of 4, 9 or 21 days. The principal results were that lower percentages of resistant bees than of susceptible bees routinely became infested by foundress mites, individual infested susceptible bees often had more foundress mites than individual infested resistant bees did and mite fecundity was similar in both host types. The infestation percentage results corresponded well with similar results from a prior field test of these stocks and, thus, suggest that the bioassay is useful for assessing honey bee resistance to A. woodi.     

Survival of the mite Varroa jacobsoni Oud. (Mesostigmata: Varroidae) in broodless colonies of the honey bee Apis mellifera L. (Hymenoptera: Apidae)

Varroa mite free colonies of the honey bee Apis mellifera L. were artificially infested, with either parasitized bees or infested worker brood. Queens were kept in cages to provide broodless conditions during the experiment. Parasites that fell to the bottom of the hive were monitored at 3–4 days intervals for three months. An acaricide treatment was used to recover mites still alive after this time period. Survivorship at each interval was calculated and life table functions of the phoretic mite cohorts were obtained. Trends in survival of Varroa cohorts showed maximum lifespans ranging from 80 to 100 days. Life expectancy of these phoretic cohorts at the beginning of the experiment ranges between 19 to 41, with a mean of 31 days.     

The influence of nest site selection on the population dynamics of Africanized honey bees in an urban landscape

Urban landscapes provide habitat for many species, including domesticated and feral honey bees, Apis mellifera L. (Hymenoptera: Apidae). With recent losses of managed honey bee colonies, there is increasing interest in feral honey bee colonies and their potential contribution to pollination services in agricultural, natural, and urban settings. However, in some regions the feral honey bee population consists primarily of Africanized honey bees. Africanized honey bees (AHB) are hybrids between European honey bees and the African honey bee, Apis mellifera scutellataLepeletier, and have generated economic, ecological, and human health concerns because of their aggressive behavior. In this study, we used two long‐term datasets (7–10 years) detailing the spatial and temporal distribution of AHB colonies in Tucson, AZ, USA, where feral colonies occupy a variety of cavities including water meter boxes. A stage‐structured matrix model was used to elucidate the implications of nest site selection and the effects of colony terminations on the structure and dynamics of the AHB population. Our results suggest that Tucson's AHB population is driven by a relatively small number of ‘source’ colonies that escape termination (ca. 0.165 colonies per km² or 125 colonies in total), although immigrating swarms and absconding colonies from the surrounding area may have also contributed to the stability of the Tucson AHB population. Furthermore, the structure of the population has likely been impacted by the number and spatial distribution of water meter boxes across the city. The study provides an example of how urban wildlife populations are driven by interactions among landscape structure, human management, and behavioral traits conferred by an invasive genotype.     

Susceptibility of small honey bee colonies to invasion by the small hive beetle,Aethina tumida (Coleoptera,Nitidulidae)

Weak and small honey bee colonies are supposed to be more susceptible to infestations by the small hive beetle [Aethina tumida, small hive beetle (SHB)]. To test this, we established 24 nucleus colonies [12 with and 12 without previous SHB removal (= screening)]. Four weeks later, we compared beetle numbers and the occurrence of SHB reproduction to the corresponding full‐sized colonies. Full‐sized colonies with no screening were infested with significantly more SHBs than all other groups (mean ± standard deviation = 46.9 ± 26.7). Regardless of this, none of the full‐sized colonies showed damage or evidence of SHB reproduction. In contrast, five nucleus colonies collapsed and SHB larvae were found in an additional seven colonies. Our study demonstrates that SHB infestation levels which are harmless to full‐sized colonies may have a negative impact on small nucleus colonies.     

Host Specificity in the Honeybee Parasitic Mite,Varroa spp. in Apis mellifera and Apis cerana

The ectoparasitic mite Varroa destructor is a major global threat to the Western honeybee Apis mellifera. This mite was originally a parasite of A. cerana in Asia but managed to spill over into colonies of A. mellifera which had been introduced to this continent for honey production. To date, only two almost clonal types of V. destructor from Korea and Japan have been detected in A. mellifera colonies. However, since both A. mellifera and A. cerana colonies are kept in close proximity throughout Asia, not only new spill overs but also spill backs of highly virulent types may be possible, with unpredictable consequences for both honeybee species. We studied the dispersal and hybridisation potential of Varroa from sympatric colonies of the two hosts in Northern Vietnam and the Philippines using mitochondrial and microsatellite DNA markers. We found a very distinct mtDNA haplotype equally invading both A. mellifera and A. cerana in the Philippines. In contrast, we observed a complete reproductive isolation of various Vietnamese Varroa populations in A. mellifera and A. cerana colonies even if kept in the same apiaries. In light of this variance in host specificity, the adaptation of the mite to its hosts seems to have generated much more genetic diversity than previously recognised and the Varroa species complex may include substantial cryptic speciation.     

Contrasting impacts of a novel specialist vector on multihost viral pathogen epidemiology in wild and managed bees

Typically, pathogens infect multiple host species. Such multihost pathogens can show considerable variation in their degree of infection and transmission specificity, which has important implications for potential disease emergence. Transmission of multihost pathogens can be driven by key host species and changes in such transmission networks can lead to disease emergence. We study two viruses that show contrasting patterns of prevalence and specificity in managed honeybees and wild bumblebees, black queen cell virus (BQCV) and slow bee paralysis virus (SBPV), in the context of the novel transmission route provided by the virus‐vectoring Varroa destructor. Our key result is that viral communities and RNA virus genetic variation are structured by location, not host species or V. destructor presence. Interspecific transmission is pervasive with the same viral variants circulating between pollinator hosts in each location; yet, we found virus‐specific host differences in prevalence and viral load. Importantly, V. destructor presence increases the prevalence in honeybees and, indirectly, in wild bumblebees, but in contrast to its impact on deformed wing virus (DWV), BQCV and SBPV viral loads are not increased by Varroa presence, and do not show genetic evidence of recent emergence. Effective control of Varroa in managed honeybee colonies is necessary to mitigate further disease emergence, and alleviate disease pressure on our vital wild bee populations. More generally, our results highlight the over‐riding importance of geographical location to the epidemiological outcome despite the complexity of multihost‐parasite interactions.     

Admixture increases diversity in managed honey bees: Reply to De la Rúa et al. (2013)

De la Rúa et al. (2013) express some concerns about the conclusions of our recent study showing that management increases genetic diversity of honey bees (Apis mellifera) by promoting admixture (Harpur et al. 2012). We provide a brief review of the literature on the population genetics of A. mellifera and show that we utilized appropriate sampling methods to estimate genetic diversity in the focal populations. Our finding of higher genetic diversity in two managed A. mellifera populations on two different continents is expected to be the norm given the large number of studies documenting admixture in honey bees. Our study focused on elucidating how management affects genetic diversity in honey bees, not on how to best manage bee colonies. We do not endorse the intentional admixture of honey bee populations, and we agree with De la Rúa et al. (2013) that native honey bee subspecies should be conserved.     

Non-reproducing Varroa jacobsoni Oud. in honey bee worker cells—status of mites or effect of brood cells?

The parasitic mite Varroa jacobsoni Oud. reproduces in sealed honey bee brood cells. Within worker cells a considerable fraction of the mites do not produce offspring. It is investigated whether variation in the ratio of cells without reproduction is caused by properties of the worker brood, or by the state of the mites entering cells. Pieces of brood comb were taken from colonies of 12 different bee lines and were placed simultaneously into highly infested colonies. Non-reproduction was independent of the origin of the brood pieces, indicating a minor role of a variation due to different brood origin. Between colonies used for infestation, however, it differed considerably. A comparison of the proportion of cells without reproduction when infested by one Varroa mite or when infested by two or three Varroa mites showed, that non-reproduction was mainly related to the state of the mites entering cells, and only to a minor degree to an influence of the brood cells. A high ratio of worker cells without reproduction was consistently reported in bee lines which survive the disease without treatment, and a high level of non-reproduction is thus regarded to be a key factor in breeding bees for high Varroa tolerance. The current results indicate, that differences in this trait are only to a minor degree related to differences between bee lines in the ability of the bee brood to induce oviposition. These differences seem rather to depend on other, unknown colony factors influencing the reproductive state of Varroa when they enter cells for reproduction.