Compliance with recommended Varroa destructor treatment regimens improves the survival of honey bee colonies over winter.

The ectoparasitic mite Varroa destructor affects honey bee colony health and survival negatively, thus compelling beekeepers to treat their colonies every year. A broadly used mite control regimen is based on two organic molecules: formic and oxalic acids. To ensure optimal efficiency, several applications of these acids at pre-defined time points are recommended. These recommendations are mainly based on experiments conducted under controlled conditions. Studies evaluating the effectiveness under natural field conditions are lacking. We enrolled 30 beekeepers in a longitudinal study in three cantons in Switzerland and monitored the management and health of their colonies for two years. We assessed compliance with mite control recommendations and measured V. destructor infestation rates, indexes of colony productivity (brood size and honey harvest), and colony mortality in 300 colonies. We observed a 10-fold increased risk of colony death when beekeepers deviated slightly from the recommended treatment regimen compared to compliant beekeepers (odds ratio: 11.9, 95% CI: 2.6-55.2, p = 0.002). The risk of colony death increased 25-fold in apiaries with substantial deviations from the recommendations (odds ratio: 50.4, 95% CI: 9.7-262.5, p < 0.0001). The deviations led to increased levels of V. destructor infestation ahead of wintering, which was likely responsible for colony mortality. After communicating the apparent link between low compliance and poor colony survival at the end of the first year to the beekeepers, we observed better compliance and colony survival in the second year. Our results highlight the positive impact of compliance with the recommended V. destructor treatment regimen on the health of honeybee colonies and the need to better communicate the consequences of deviating from the recommendations to improve compliance. Compliance also occasionally decreased, which hints at concept implementation constraints that could be identified and possibly addressed in detail with the help of social sciences to further promote honey bee health.

Identifying the climatic drivers of honey bee disease in England and Wales.

Honey bee colony health has received considerable attention in recent years, with many studies highlighting multifactorial issues contributing to colony losses. Disease and weather are consistently highlighted as primary drivers of colony loss, yet little is understood about how they interact. Here, we combined disease records from government honey bee health inspections with meteorological data from the CEDA to identify how weather impacts EFB, AFB, CBP, varroosis, chalkbrood and sacbrood. Using R-INLA, we determined how different meteorological variables influenced disease prevalence and disease risk. Temperature caused an increase in the risk of both varroosis and sacbrood, but overall, the weather had a varying effect on the six honey bee diseases. The risk of disease was also spatially varied and was impacted by the meteorological variables. These results are an important step in identifying the impacts of climate change on honey bees and honey bee diseases.

Integrated Pest Management Control of Varroa destructor (Acari: Varroidae), the Most Damaging Pest of (Apis mellifera L. (Hymenoptera: Apidae)) Colonies.

Varroa destructor is among the greatest biological threats to western honey bee (Apis mellifera L.) health worldwide. Beekeepers routinely use chemical treatments to control this parasite, though overuse and mismanagement of these treatments have led to widespread resistance in Varroa populations. Integrated Pest Management (IPM) is an ecologically based, sustainable approach to pest management that relies on a combination of control tactics that minimize environmental impacts. Herein, we provide an in-depth review of the components of IPM in a Varroa control context. These include determining economic thresholds for the mite, identification of and monitoring for Varroa, prevention strategies, and risk conscious treatments. Furthermore, we provide a detailed review of cultural, mechanical, biological, and chemical control strategies, both longstanding and emerging, used against Varroa globally. For each control type, we describe all available treatments, their efficacies against Varroa as described in the primary scientific literature, and the obstacles to their adoption. Unfortunately, reliable IPM protocols do not exist for Varroa due to the complex biology of the mite and strong reliance on chemical control by beekeepers. To encourage beekeeper adoption, a successful IPM approach to Varroa control in managed colonies must be an improvement over conventional control methods and include cost-effective treatments that can be employed readily by beekeepers. It is our intention to provide the most thorough review of Varroa control options available, ultimately framing our discussion within the context of IPM. We hope this article is a call-to-arms against the most damaging pest managed honey bee colonies face worldwide.

Adapted tolerance to virus infections in four geographically distinct Varroa destructor-resistant honeybee populations.

The ectoparasitic mite, Varroa destructor, is unarguably the leading cause of honeybee (Apis mellifera) mortality worldwide through its role as a vector for lethal viruses, in particular, strains of the Deformed wing virus (DWV) and Acute bee paralysis virus (ABPV) complexes. Several honeybee populations across Europe have well-documented adaptations of mite-resistant traits but little is known about host adaptations towards the virus infections vectored by the mite. The aim of this study was to assess and compare the possible contribution of adapted virus tolerance and/or resistance to the enhanced survival of four well-documented mite-resistant honeybee populations from Norway, Sweden, The Netherlands and France, in relation to unselected mite-susceptible honeybees. Caged adult bees and laboratory reared larvae, from colonies of these four populations, were inoculated with DWV and ABPV in a series of feeding infection experiments, while control groups received virus-free food. Virus infections were monitored using RT-qPCR assays in individuals sampled over a time course. In both adults and larvae the DWV and ABPV infection dynamics were nearly identical in all groups, but all mite-resistant honeybee populations had significantly higher survival rates compared to the mite-susceptible honeybees. These results suggest that adapted virus tolerance is an important component of survival mechanisms.

Africanized honey bees in Colombia exhibit high prevalence but low level of infestation of Varroa mites and low prevalence of pathogenic viruses.

The global spread of the ectoparasitic mite Varroa destructor has promoted the spread and virulence of highly infectious honey bee viruses. This phenomenon is considered the leading cause for the increased number of colony losses experienced by the mite-susceptible European honey bee populations in the Northern hemisphere. Most of the honey bee populations in Central and South America are Africanized honey bees (AHBs), which are considered more resistant to Varroa compared to European honey bees. However, the relationship between Varroa levels and the spread of honey bee viruses in AHBs remains unknown. In this study, we determined Varroa prevalence and infestation levels as well as the prevalence of seven major honey bee viruses in AHBs from three regions of Colombia. We found that although Varroa exhibited high prevalence (92%), its infestation levels were low (4.5%) considering that these populations never received acaricide treatments. We also detected four viruses in the three regions analyzed, but all colonies were asymptomatic, and virus prevalence was considerably lower than those found in other countries with higher rates of mite-associated colony loss (DWV 19.88%, BQCV 17.39%, SBV 23.4%, ABPV 10.56%). Our findings indicate that AHBs possess a natural resistance to Varroa that does not prevent the spread of this parasite among their population, but restrains mite population growth and suppresses the prevalence and pathogenicity of mite-associated viruses.

Reduced proinsecticide activation by cytochrome P450 confers coumaphos resistance in the major bee parasite Varroa destructor.

Varroa destructor is one of the main problems in modern beekeeping. Highly selective acaricides with low toxicity to bees are used internationally to control this mite. One of the key acaricides is the organophosphorus (OP) proinsecticide coumaphos, that becomes toxic after enzymatic activation inside Varroa We show here that mites from the island Andros (AN-CR) exhibit high levels of coumaphos resistance. Resistance is not mediated by decreased coumaphos uptake, target-site resistance, or increased detoxification. Reduced proinsecticide activation by a cytochrome P450 enzyme was the main resistance mechanism, a powerful and rarely encountered evolutionary solution to insecticide selection pressure. After treatment with sublethal doses of [14C] coumaphos, susceptible mite extracts had substantial amounts of coroxon, the activated metabolite of coumaphos, while resistant mites had only trace amounts. This indicates a suppression of the P450 (CYP)-mediated activation step in the AN-CR mites. Bioassays with coroxon to bypass the activation step showed that resistance was dramatically reduced. There are 26 CYPs present in the V. destructor genome. Transcriptome analysis revealed overexpression in resistant mites of CYP4DP24 and underexpression of CYP3012A6 and CYP4EP4 RNA interference of CYP4EP4 in the susceptible population, to mimic underexpression seen in the resistant mites, prevented coumaphos activation and decreased coumaphos toxicity.

Chemical detection triggers honey bee defense against a destructive parasitic threat.

Invasive species events related to globalization are increasing, resulting in parasitic outbreaks. Understanding of host defense mechanisms is needed to predict and mitigate against the consequences of parasite invasion. Using the honey bee Apis mellifera and the mite Varroa destructor, as a host-parasite model, we provide a comprehensive study of a mechanism of parasite detection that triggers a behavioral defense associated with social immunity. Six Varroa-parasitization-specific (VPS) compounds are identified that (1) trigger Varroa-sensitive hygiene (VSH, bees' key defense against Varroa sp.), (2) enable the selective recognition of a parasitized brood and (3) induce responses that mimic intrinsic VSH activity in bee colonies. We also show that individuals engaged in VSH exhibit a unique ability to discriminate VPS compounds from healthy brood signals. These findings enhance our understanding of a critical mechanism of host defense against parasites, and have the potential to apply the integration of pest management in the beekeeping sector.

A salivary chitinase of Varroa destructor influences host immunity and mite's survival.

Varroa destructor is an ectoparasite of honey bees and an active disease vector, which represents one of the most severe threats for the beekeeping industry. This parasitic mite feeds on the host's body fluids through a wound in the cuticle, which allows food uptake by the mother mite and its progeny, offering a potential route of entrance for infecting microorganisms. Mite feeding is associated with saliva injection, whose role is still largely unknown. Here we try to fill this gap by identifying putative host regulation factors present in the saliva of V. destructor and performing a functional analysis for one of them, a chitinase (Vd-CHIsal) phylogenetically related to chitinases present in parasitic and predatory arthropods, which shows a specific and very high level of expression in the mite's salivary glands. Vd-CHIsal is essential for effective mite feeding and survival, since it is apparently involved both in maintaining the feeding wound open and in preventing host infection by opportunistic pathogens. Our results show the important role in the modulation of mite-honey bee interactions exerted by a host regulation factor shared by different evolutionary lineages of parasitic arthropods. We predict that the functional characterization of Varroa sialome will provide new background knowledge on parasitism evolution in arthropods and the opportunity to develop new bioinspired strategies for mite control based on the disruption of their complex interactions with a living food source.

Advances and perspectives in selecting resistance traits against the parasitic mite Varroa destructor in honey bees.

BACKGROUND: In spite of the implementation of control strategies in honey bee (Apis mellifera) keeping, the invasive parasitic mite Varroa destructor remains one of the main causes of colony losses in numerous countries. Therefore, this parasite represents a serious threat to beekeeping and agro-ecosystems that benefit from the pollination services provided by honey bees. To maintain their stocks, beekeepers have to treat their colonies with acaricides every year. Selecting lineages that are resistant to infestations is deemed to be a more sustainable approach. REVIEW: Over the last three decades, numerous selection programs have been initiated to improve the host-parasite relationship and to support honey bee survival in the presence of the parasite without the need for acaricide treatments. Although resistance traits have been included in the selection strategy of honey bees, it has not been possible to globally solve the V. destructor problem. In this study, we review the literature on the reasons that have potentially limited the success of such selection programs. We compile the available information to assess the relevance of selected traits and the potential environmental effects that distort trait expression and colony survival. Limitations to the implementation of these traits in the field are also discussed. CONCLUSIONS: Improving our knowledge of the mechanisms underlying resistance to V. destructor to increase trait relevance, optimizing selection programs to reduce environmental effects, and communicating selection outcomes are all crucial to efforts aiming at establishing a balanced relationship between the invasive parasite and its new host.

Diagnosis of Varroosis Based on Bee Brood Samples Testing with Use of Semiconductor Gas Sensors.

Varroosis is a dangerous and difficult to diagnose disease decimating bee colonies. The studies conducted sought answers on whether the electronic nose could become an effective tool for the efficient detection of this disease by examining sealed brood samples. The prototype of a multi-sensor recorder of gaseous sensor signals with a matrix of six semiconductor gas sensors TGS 823, TGS 826, TGS 832, TGS 2600, TGS 2602, and TGS 2603 from FIGARO was tested in this area. There were 42 objects belonging to 3 classes tested: 1st class-empty chamber (13 objects), 2nd class-fragments of combs containing brood sick with varroosis (19 objects), and 3rd class-fragments of combs containing healthy sealed brood (10 objects). The examination of a single object lasted 20 min, consisting of the exposure phase (10 min) and the sensor regeneration phase (10 min). The k-th nearest neighbors algorithm (kNN)-with default settings in RSES tool-was successfully used as the basic classifier. The basis of the analysis was the sensor reading value in 270 s with baseline correction. The multi-sensor MCA-8 gas sensor signal recorder has proved to be an effective tool in distinguishing between brood suffering from varroosis and healthy brood. The five-time cross-validation 2 test (5 × CV2 test) showed a global accuracy of 0.832 and a balanced accuracy of 0.834. Positive rate of the sick brood class was 0.92. In order to check the overall effectiveness of baseline correction in the examined context, we have carried out additional series of experiments-in multiple Monte Carlo Cross Validation model-using a set of classifiers with different metrics. We have tested a few variants of the kNN method, the Naïve Bayes classifier, and the weighted voting classifier. We have verified with statistical tests the thesis that the baseline correction significantly improves the level of classification. We also confirmed that it is enough to use the TGS2603 sensor in the examined context.

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.

Lactobacillus salivarius A3iob Reduces the Incidence of Varroa destructor and Nosema Spp. in Commercial Apiaries Located in the Northwest of Argentina.

Lactobacillus salivarius A3iob was administered to productive colonies belonging to commercial apiaries of small beekeepers (around 30-50 hives each one), from four departments of the province of Jujuy (Argentina): Yala, Tilquiza, El Carmen, and Los Alisos. The incidence of Varroa destructor and Nosema spp., before and after winter, was monitored during 2 years of study (2014-2015). Depending on the geographical location of each apiary and the application time, a monthly dose of the bacteria (105 CFU/mL) reduced the levels of varroasis between 50 and 80%. Interestingly, L. salivarius A3iob cells remitted the percentage of the mites to undetectable values in an apiary treated with flumethrin (at Yala, Yungas region).On the other hand, the spore levels of Nosema spp. in the lactobacilli-treated colonies also depended on the apiary and the year of application, but a significant decrease was mainly observed in the post-winter period. However, at Rivera (El Carmen's department), no significant changes were detected in both parameters.These results obtained after 2 years of work suggest that delivering L. salivarius A3iob cells to the bee colonies can become a new eco-friendly tool to cooperate with the control of these bees' pests.

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.

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.

Impact of nutritional stress on the honeybee colony health.

Honeybees Apis mellifera are important pollinators of wild plants and commercial crops. For more than a decade, high percentages of honeybee colony losses have been reported worldwide. Nutritional stress due to habitat depletion, infection by different pests and pathogens and pesticide exposure has been proposed as the major causes. In this study we analyzed how nutritional stress affects colony strength and health. Two groups of colonies were set in a Eucalyptus grandis plantation at the beginning of the flowering period (autumn), replicating a natural scenario with a nutritionally poor food source. While both groups of colonies had access to the pollen available in this plantation, one was supplemented with a polyfloral pollen patty during the entire flowering period. In the short-term, colonies under nutritional stress (which consumed mainly E. grandis pollen) showed higher infection level with Nosema spp. and lower brood and adult bee population, compared to supplemented colonies. On the other hand, these supplemented colonies showed higher infection level with RNA viruses although infection levels were low compared to countries were viral infections have negative impacts. Nutritional stress also had long-term colony effects, because bee population did not recover in spring, as in supplemented colonies did. In conclusion, nutritional stress and Nosema spp. infection had a severe impact on colony strength with consequences in both short and long-term.

Health status of honey bee colonies (Apis mellifera) and disease-related risk factors for colony losses in Austria.

Austrian beekeepers frequently suffered severe colony losses during the last decade similar to trends all over Europe. This first surveillance study aimed to describe the health status of Austrian bee colonies and to analyze the reasons for losses for both the summer and winter season in Austria. In this study 189 apiaries all over Austria were selected using a stratified random sampling approach and inspected three times between July 2015 and spring 2016 by trained bee inspectors. The inspectors made interviews with the beekeepers about their beekeeping practice and the history of the involved colonies. They inspected a total of 1596 colonies for symptoms of nine bee pests and diseases (four of them notifiable diseases) and took bee samples for varroa mite infestation analysis. The most frequently detected diseases were three brood diseases: Varroosis, Chalkbrood and Sacbrood. The notifiable bee pests Aethina tumida and Tropilaelaps spp. were not detected. During the study period 10.8% of the 1596 observed colonies died. Winter proved to be the most critical season, in which 75% of the reported colony losses happened. Risks for suffering summer losses increased significantly, when colonies were weak in July, had queen problems or a high varroa mite infestation level on bees in July. Risks for suffering winter losses increased significantly, when the colonies had a high varroa mite infestation level on bees in September, were weak in September, had a queen older than one year or the beekeeper had few years of beekeeping experience. However, the effect of a high varroa mite infestation level in September had by far the greatest potential to raise the winter losses compared to the other significant factors.

Knock-on community impacts of a novel vector: spillover of emerging DWV-B from Varroa-infested honeybees to wild bumblebees.

Novel transmission routes can directly impact the evolutionary ecology of infectious diseases, with potentially dramatic effect on host populations and knock-on effects on the wider host community. The invasion of Varroa destructor, an ectoparasitic viral vector in Western honeybees, provides a unique opportunity to examine how a novel vector affects disease epidemiology in a host community. This specialist honeybee mite vectors deformed wing virus (DWV), an important re-emerging honeybee pathogen that also infects wild bumblebees. Comparing island honeybee and wild bumblebee populations with and without V. destructor, we show that V. destructor drives DWV prevalence and titre in honeybees and sympatric bumblebees. Viral genotypes are shared across hosts, with the potentially more virulent DWV-B overtaking DWV-A in prevalence in a current epidemic. This demonstrates disease emergence across a host community driven by the acquisition of a specialist novel transmission route in one host, with dramatic community level knock-on effects.

Mite bombs or robber lures? The roles of drifting and robbing in Varroa destructor transmission from collapsing honey bee colonies to their neighbors.

When honey bee colonies collapse from high infestations of Varroa mites, neighboring colonies often experience surges in their mite populations. Collapsing colonies, often called "mite bombs", seem to pass their mites to neighboring colonies. This can happen by mite-infested workers from the collapsing colonies drifting into the neighboring colonies, or by mite-free workers from the neighboring colonies robbing out the collapsing colonies, or both. To study inter-colony mite transmission, we positioned six nearly mite-free colonies of black-colored bees around a cluster of three mite-laden colonies of yellow-colored bees. We then monitored the movement of bees between the black-bee and yellow-bee colonies before, during, and after mite-induced collapse of the yellow-bee colonies. Throughout the experiment, we monitored each colony's mite level. We found that large numbers of mites spread to the black-bee colonies (in both nearby and distant hives) when the yellow-bee colonies collapsed from high mite infestations and became targets of robbing by the black-bee colonies. We conclude that "robber lures" is a better term than "mite bombs" for describing colonies that are succumbing to high mite loads and are exuding mites to neighboring colonies.

Neonicotinoids and ectoparasitic mites synergistically impact honeybees.

The Western honeybee, Apis mellifera, is the most important managed pollinator globally and has recently experienced unsustainably high colony losses. Synergistic interactions among stressors are believed to be primarily responsible. However, despite clear evidence of strong effect on honeybee longevity of widely-employed neonicotinoid insecticides and of the ubiquitous ectoparasitic mite Varroa destructor, no data exist to show synergistic effects between these two stressors. Even though neonicotinoids had no significant impact by themselves, we here show for the first time a synergistic time-lag interaction between mites and neonicotinoids that resulted in significantly reduced survival of long-lived winter honeybees. Even though these mites are potent vectors of viruses, the virus-insecticide interaction had no significant impact. The data suggest a previously overlooked mechanism possibly explaining recent unsustainably high losses of managed A. mellifera honeybee colonies in many regions of the world. Future mitigation efforts should concentrate on developing sustainable agro-ecosystem management schemes that incorporate reduced use of neonicotinoids and sustainable solutions for V. destructor mites.

Reduced density and visually complex apiaries reduce parasite load and promote honey production and overwintering survival in honey bees.

Managed honey bee (Apis mellifera) colonies are kept at much greater densities than naturally occurring feral or wild colonies, which may have detrimental effects on colony health and survival, disease spread, and drifting behavior (bee movement between natal and non-natal colonies). We assessed the effects of a straightforward apiary management intervention (altering the density and visual appearance of colonies) on colony health. Specifically, we established three "high density / high drift" ("HD") and three "low density / low drift" ("LD") apiary configurations, each consisting of eight bee colonies. Hives in the HD apiary configuration were of the same color and placed 1m apart in a single linear array, while hives in the LD apiary configuration were placed 10m apart at different heights, facing outwards in a circle, and made visually distinctive with colors and symbols to reduce accidental drift between colonies. We investigated disease transmission and dynamics between the apiary configurations by clearing all colonies of the parasitic mite Varroa destructor, and subsequently inoculating two randomly-chosen colonies per apiary with controlled mite doses. We monitored the colonies for two years and found that the LD apiary configuration had significantly greater honey production and reduced overwinter mortality. Inoculation and apiary management intervention interacted to affect brood mite levels, with the highest levels in the inoculated colonies in the HD configuration. Finally, foragers were more than three times more likely to drift in the HD apiary configurations. Our results suggest that a relatively straightforward management change-placing colonies in low-density visually complex circles rather than high-density visually similar linear arrays-can provide meaningful benefits to the health and productivity of managed honey bee colonies.