Do Varroa destructor (Acari: Varroidae) mite flows between Apis mellifera (Hymenoptera: Apidae) colonies bias colony infestation evaluation for resistance selection?

Since the global invasion of the ectoparasitic mite Varroa destructor (Anderson and Trueman), selection of mite-resistant honey bee (Apis mellifera L.) colonies appears challenging and has to date not broadly reduced colony mortality. The low published estimated heritability values for mite infestation levels could explain the limited genetic progresses obtained so far. We hypothesize that intercolonial horizontal mite transmission could differentially affect the single colonies located in a given apiary and therefore invisibly bias colony infestation phenotypes. This bias may be lower in regions with lower colony density, providing suitable conditions to set up evaluation apiaries. To verify these hypotheses, we monitored mite infestation and reinvasion in experimental colonies, as well as infestation in neighboring colonies belonging to beekeepers in three areas with variable colony densities in the canton of Bern, Switzerland during three consecutive beekeeping seasons. Mite immigration fluctuated between apiaries and years and significantly contributed to colony infestation level. Depending on apiary and year, 17-48% of the mites present in the experimental colonies at the time of the summer oxalic acid final treatment potentially derived from mite immigration that had occurred since mid-spring. Mite immigration was not linked to local colony density or the infestation levels of beekeepers' colonies located within 2 km. Our results do not prove that apiaries for colony evaluation should necessarily be established in areas with low colony density. However, they highlight the high impact of beekeeping management practices on mite colony infestation levels.

Investigation of landscape risk factors for the recent spread of varroa mite (Varroa destructor) in European honeybee (Apis mellifera) colonies in New South Wales, Australia.

In June 2022, an exotic pest of the European honeybee (Apis mellifera), the varroa mite (Varroa destructor), was detected in surveillance hives at the Port of Newcastle, New South Wales (NSW). Previously, Australia remained the only continent free of the varroa mite. In September 2023, the National Management Group decided to shift the focus of the response from eradication to management. It is estimated that the establishment of varroa mite in Australia could lead to more than $70 million in losses each year due to greatly reduced pollination services. Currently, there are no reported studies on the epidemiology of varroa mite in NSW because it is such a recent outbreak, and there is little knowledge of the factors associated with the presence of V. destructor in the Australian context. We sourced publicly available varroa mite outbreak reports from June 22 to December 19, 2022, to determine if urbanization, land use, and distance from the incursion site are associated with the detection of varroa mite infestation in European honeybee colonies in NSW. The outcome investigated was epidemic day, relative to the first detected premises (June 22, 2022). The study population was comprised of 107 premises, which were declared varroa-infested. The median epidemic day was day 37 (July 29, 2022), and a bimodal distribution was observed from the epidemic curve, which was reflective of an intermittent source pattern of spread. We found that premises were detected to be infected with varroa mite earlier in urban areas [median epidemic day 25 (July 17, 2022)] compared to rural areas [median epidemic day 37.5 (July 29, 2022)]. Infected premises located in areas without cropping, forests, and irrigation were detected earlier in the outbreak [median epidemic days 23.5 (July 15, 2022), 30 (July 22, 2022), and 15 (July 7, 2022), respectively] compared to areas with cropping, forests, and irrigation [median epidemic days 50 (August 11, 2022), 43 (August 4, 2022), and 47 (August 8, 2022), respectively]. We also found that distance from the incursion site was not significantly correlated with epidemic day. Urbanization and land use are potential factors for the recent spread of varroa mite in European honeybee colonies in NSW. This knowledge is essential to managing the current varroa mite outbreak and preventing future mass varroa mite spread events.

Varroa destructor shapes the unique viral landscapes of the honey bee populations of the Azores archipelago.

The worldwide dispersal of the ectoparasitic mite Varroa destructor from its Asian origins has fundamentally transformed the relationship of the honey bee (Apis mellifera) with several of its viruses, via changes in transmission and/or host immunosuppression. The extent to which honey bee-virus relationships change after Varroa invasion is poorly understood for most viruses, in part because there are few places in the world with several geographically close but completely isolated honey bee populations that either have, or have not, been exposed long-term to Varroa, allowing for separate ecological, epidemiological, and adaptive relationships to develop between honey bees and their viruses, in relation to the mite's presence or absence. The Azores is one such place, as it contains islands with and without the mite. Here, we combined qPCR with meta-amplicon deep sequencing to uncover the relationship between Varroa presence, and the prevalence, load, diversity, and phylogeographic structure of eight honey bee viruses screened across the archipelago. Four viruses were not detected on any island (ABPV-Acute bee paralysis virus, KBV-Kashmir bee virus, IAPV-Israeli acute bee paralysis virus, BeeMLV-Bee macula-like virus); one (SBV-Sacbrood virus) was detected only on mite-infested islands; one (CBPV-Chronic bee paralysis virus) occurred on some islands, and two (BQCV-Black queen cell virus, LSV-Lake Sinai virus,) were present on every single island. This multi-virus screening builds upon a parallel survey of Deformed wing virus (DWV) strains that uncovered a remarkably heterogeneous viral landscape featuring Varroa-infested islands dominated by DWV-A and -B, Varroa-free islands naïve to DWV, and a refuge of the rare DWV-C dominating the easternmost Varroa-free islands. While all four detected viruses investigated here were affected by Varroa for one or two parameters (usually prevalence and/or the Richness component of ASV diversity), the strongest effect was observed for the multi-strain LSV. Varroa unambiguously led to elevated prevalence, load, and diversity (Richness and Shannon Index) of LSV, with these results largely shaped by LSV-2, a major LSV strain. Unprecedented insights into the mite-virus relationship were further gained from implementing a phylogeographic approach. In addition to enabling the identification of a novel LSV strain that dominated the unique viral landscape of the easternmost islands, this approach, in combination with the recovered diversity patterns, strongly suggests that Varroa is driving the evolutionary change of LSV in the Azores. This study greatly advances the current understanding of the effect of Varroa on the epidemiology and adaptive evolution of these less-studied viruses, whose relationship with Varroa has thus far been poorly defined.

Varroa destructor and deformed wing virus interaction increases incidence of winter mortality in honey bee colonies.

Winter mortality of honey bee colonies represents a major source of economic loss for the beekeeping industry. The objectives of this prospective study were to estimate the incidence risk of winter colony mortality in southwestern Quebec, Canada and to evaluate and quantify the impact of the associated risk factors. A total of 242 colonies from 31 apiaries was selected for sampling in August 2017. The presence of Varroa destructor, Vairimorpha (Nosema) spp., Melissococcus plutonius, deformed wing virus (DWV), and viruses of the acute-Kashmir-Israeli complex (AKI complex) was investigated in each colony. Management practices of the various colonies were obtained from a questionnaire. The incidence risk of colony mortality during the winter of 2017-2018 was estimated to be 26.5% [95% confidence interval (CI): 15.4 to 40.3]. In logistic regression modeling of winter mortality in colonies, an interaction was discovered between V. destructor and DWV; the detection of ≥ 1 V. destructor mites per 100 bees was associated with higher odds of mortality (3.46, 95% CI: 1.35 to 8.90) compared to colonies with < 1 mite per 100 bees, but only in DWV-positive colonies. There were more colony losses in apiaries from beekeepers owning 1 to 5 colonies than in apiaries from beekeepers owning over 100 colonies, which suggests that beekeeper experience and/or type of management are important contributors to winter colony mortality. Assuming a causal relationship, the results of this study suggest that up to 9% of all colony mortalities in the population could have been prevented by reducing the level of V. destructor to < 1 mite per 100 bees in all colonies.

La mortalité hivernale des colonies d'abeilles est une cause importante de pertes économiques en apiculture. Cette étude prospective visait à estimer le risque d'incidence de mortalité hivernale des colonies d'abeilles et les facteurs de risque associés dans le sud-ouest du Québec (Canada). Au total, 242 colonies provenant de 31 ruches ont été sélectionnées en août 2017. La présence de Varroa destructor, de Vairimorpha (Nosema) spp., de Melissococcus plutonius, du virus des ailes déformées (DWV) et des virus du complexe AKI ont été évalués. Les pratiques de régie ont été obtenues selon un questionnaire. Le risque de mortalité des colonies à l'hiver 2017­2018 a été estimé à 26,5 % (95 % CI : 15,4 à 40,3). Dans un modèle de régression logistique, la détection de ≥ 1 mite de V. destructor par 100 abeilles était associée à des cotes plus élevées de mortalité (3,46, 95 % CI : 1,35 à 8,90) comparativement aux colonies avec < 1 mite par 100 abeilles, mais seulement pour les colonies positives au DWV. Les ruchers d'apiculteurs possédant entre 1 et 5 colonies présentaient une mortalité plus élevée comparativement à ceux d'apiculteurs possédant plus de 100 colonies, suggérant une influence de l'expérience ou du type de régie. En assumant une relation causale, les résultats de cette étude suggèrent que jusqu'à 9 % de toutes les mortalités hivernales observées dans la population auraient pu être prévenues en réduisant le niveau d'infestation par V. destructor à moins d'une mite per 100 abeilles dans toutes les colonies.(Traduit par les auteurs).

Analysis of Varroa Mite Colony Infestation Level Using New Open Software Based on Deep Learning Techniques.

Varroa mites, scientifically identified as Varroa destructor, pose a significant threat to beekeeping and cause one of the most destructive diseases affecting honey bee populations. These parasites attach to bees, feeding on their fat tissue, weakening their immune systems, reducing their lifespans, and even causing colony collapse. They also feed during the pre-imaginal stages of the honey bee in brood cells. Given the critical role of honey bees in pollination and the global food supply, controlling Varroa mites is imperative. One of the most common methods used to evaluate the level of Varroa mite infestation in a bee colony is to count all the mites that fall onto sticky boards placed at the bottom of a colony. However, this is usually a manual process that takes a considerable amount of time. This work proposes a deep learning approach for locating and counting Varroa mites using images of the sticky boards taken by smartphone cameras. To this end, a new realistic dataset has been built: it includes images containing numerous artifacts and blurred parts, which makes the task challenging. After testing various architectures (mainly based on two-stage detectors with feature pyramid networks), combination of hyperparameters and some image enhancement techniques, we have obtained a system that achieves a mean average precision (mAP) metric of 0.9073 on the validation set.

Characterization of a Molecular Clone of Deformed Wing Virus B.

Honey bees (Apis mellifera) play a crucial role in agriculture through their pollination activities. However, they have faced significant health challenges over the past decades that can limit colony performance and even lead to collapse. A primary culprit is the parasitic mite Varroa destructor, known for transmitting harmful bee viruses. Among these viruses is deformed wing virus (DWV), which impacts bee pupae during their development, resulting in either pupal demise or in the emergence of crippled adult bees. In this study, we focused on DWV master variant B. DWV-B prevalence has risen sharply in recent decades and appears to be outcompeting variant A of DWV. We generated a molecular clone of a typical DWV-B strain to compare it with our established DWV-A clone, examining RNA replication, protein expression, and virulence. Initially, we analyzed the genome using RACE-PCR and RT-PCR techniques. Subsequently, we conducted full-genome RT-PCR and inserted the complete viral cDNA into a bacterial plasmid backbone. Phylogenetic comparisons with available full-length sequences were performed, followed by functional analyses using a live bee pupae model. Upon the transfection of in vitro-transcribed RNA, bee pupae exhibited symptoms of DWV infection, with detectable viral protein expression and stable RNA replication observed in subsequent virus passages. The DWV-B clone displayed a lower virulence compared to the DWV-A clone after the transfection of synthetic RNA, as evidenced by a reduced pupal mortality rate of only 20% compared to 80% in the case of DWV-A and a lack of malformations in 50% of the emerging bees. Comparable results were observed in experiments with low infection doses of the passaged virus clones. In these tests, 90% of bees infected with DWV-B showed no clinical symptoms, while 100% of pupae infected with DWV-A died. However, at high infection doses, both DWV-A and DWV-B caused mortality rates exceeding 90%. Taken together, we have generated an authentic virus clone of DWV-B and characterized it in animal experiments.

Comparison of Brain Gene Expression Profiles Associated with Auto-Grooming Behavior between Apis cerana and Apis mellifera Infested by Varroa destructor.

The grooming behavior of honeybees serves as a crucial auto-protective mechanism against Varroa mite infestations. Compared to Apis mellifera, Apis cerana demonstrates more effective grooming behavior in removing Varroa mites from the bodies of infested bees. However, the underlying mechanisms regulating grooming behavior remain elusive. In this study, we evaluated the efficacy of the auto-grooming behavior between A. cerana and A. mellifera and employed RNA-sequencing technology to identify differentially expressed genes (DEGs) in bee brains with varying degrees of grooming behavior intensity. We observed that A. cerana exhibited a higher frequency of mite removal between day 5 and day 15 compared to A. mellifera, with day-9 bees showing the highest frequency of mite removal in A. cerana. RNA-sequencing results revealed the differential expression of the HTR2A and SLC17A8 genes in A. cerana and the CCKAR and TpnC47D genes in A. mellifera. Subsequent homology analysis identified the HTR2A gene and SLC17A8 gene of A. cerana as homologous to the HTR2A gene and SLC17A7 gene of A. mellifera. These DEGs are annotated in the neuroactive ligand-receptor interaction pathway, the glutamatergic synaptic pathway, and the calcium signaling pathway. Moreover, CCKAR, TpnC47D, HTR2A, and SLC17A7 may be closely related to the auto-grooming behavior of A. mellifera, conferring resistance against Varroa infestation. Our results further explain the relationship between honeybee grooming behavior and brain function at the molecular level and provide a reference basis for further studies of the mechanism of honeybee grooming behavior.

Molecular phylogeny reveals Varroa mites are not a separate family but a subfamily of Laelapidae.

Varroa mites, notorious for parasitizing honeybees, are generally classified as Varroidae. Their extremely modified morphologies and behaviors have led to debates regarding their phylogenetic position and classification as an independent family. In this study, two different datasets were employed to reconstruct the phylogenies of Varroa mites and related Laelapidae species: (1) 9257 bp from the whole 13 mitochondrial protein-coding genes of 24 taxa, (2) 3158 bp from 113 taxa using Sanger sequencing of four nuclear loci. Both mitochondrial and nuclear analyses consistently place Varroa mites within the Laelapidae. Here we propose to place Varroa mites in the subfamily Varroinae stat. nov., which represents a highly morphologically adapted group within the Laelapidae. Ancestral state reconstructions reveal that bee-associated lifestyles evolved independently at least three times within Laelapidae, with most phoretic traits originating from free-living ancestors. Our revised classification and evolutionary analyses will provide new insight into understanding the Varroa mites.

First detection and prevalence of Apis mellifera filamentous virus in Apis mellifera and Varroa destructor in the Republic of Korea.

Apis mellifera filamentous virus (AmFV) is a double-stranded DNA virus that infects Apis mellifera bees. To our knowledge, this is the first comprehensive study aiming to detect and analyse the genetic diversity and prevalence of AmFV in Korean honeybee colonies. Phylogenetic analysis based on baculovirus repeat open reading frame-N gene (Bro-N) sequences revealed that AmFV isolates from the Republic of Korea (ROK) fell into two distinct lineages, with genetic origins in Switzerland and China, with nucleotide similarities of 98.3% and 98.2%, respectively. Our prevalence analysis demonstrated a noteworthy infection rate of AmFV in 545 honeybee colonies, reaching 33.09% in 2022 and increasing to 44.90% by 2023. Intriguingly, we also detected AmFV in Varroa destructor mites, highlighting their potential role as vectors and carriers of AmFV. The presence of AmFV was correlated with an increased infection rate of sacbrood virus, deformed wing virus, Lake Sinai virus 2, black queen cell virus, and Nosema ceranae in honeybee colonies. These findings provide valuable insight into the prevalence and potential transmission mechanisms of AmFV in honeybee colonies in the ROK. The results of this study may be instrumental in the effective management of viral infections in honeybee apiaries.

Evaluating the seasonal efficacy of commonly used chemical treatments on Varroa destructor (Mesostigmata: Varroidae) population resurgence in honey bee colonies.

The purpose of this research was to determine how common chemical treatments influence Varroa destructor (Anderson and Trueman) population resurgence rates (defined as time posttreatment for mite populations to reach 3 mites/100 adult bees) in managed honey bee (Apis mellifera L.) colonies seasonally. We conducted 2 experiments that followed the same basic protocol to address this purpose. We established 6 treatment groups in Experiment 1 in the fall of 2014: untreated control, Apivar, Apistan, CheckMite+, ApiLifeVar, and Mite Away II applied to 10 colonies per treatment. In Experiment 2, we applied 8 chemical treatments to each of 4 seasonal (spring, summer, fall, and winter) cohorts of honey bee colonies to determine how mite populations are influenced by the treatments. The treatments/formulations tested were Apivar, Apistan, Apiguard, MAQS, CheckMite+, oxalic acid (dribble), oxalic acid (shop towels), and amitraz (shop towels soaked in Bovitraz). In Experiment 1, Apivar and Mite Away II were able to delay V. destructor resurgence for 2 and 6 months, respectively. In Experiment 2, Apiguard, MAQS, oxalic acid (dribble), and Bovitraz treatments were effective at delaying V. destructor resurgence for at least 2 months during winter and spring. Only the Bovitraz and MAQS treatments were effective at controlling V. destructor in the summer and fall. Of the 2 amitraz-based treatments, the off-label Bovitraz treatment was the only treatment to reduce V. destructor populations in every season. The data gathered through this study allow for the refinement of treatment recommendations for V. destructor, especially regarding the seasonal efficacy of each miticide and the temporal efficacy posttreatment.

Combined treatment with amitraz and thymol to manage Varroa destructor mites (Acari: Varroidae) in Apis mellifera honey bee colonies (Hymenoptera: Apidae).

The parasitic mite Varroa destructor (Anderson and Trueman) is one of the greatest stressors of Apis mellifera (L.) honey bee colonies. When Varroa infestations reach damaging levels during fall, rapid control is necessary to minimize damage to colonies. We performed a field trial in the US Southeast to determine if a combination of registered treatments (Apivar, amitraz-based; and Apiguard, thymol-based) could provide rapid and effective control of Varroa. We compared colonies that received this combination treatment against colonies that received amitraz-based positive control treatments: (i) Apivar alone; or (ii) amitraz emulsifiable concentrate ("amitraz EC"). While not registered, amitraz EC is used by beekeepers in the United States in part because it is thought to control Varroa more rapidly and effectively than registered products. Based on measurements of Varroa infestation rates of colonies after 21 days of treatment, we found that the combination treatment controlled Varroa nearly as rapidly as the amitraz EC treatment: this or other combinations could be useful for Varroa management. At the end of the 42-day trial, colonies in the amitraz EC group had higher bee populations than those in the Apivar group, which suggests that rapid control helps reduce Varroa damage. Colonies in the combination group had lower bee populations than those in the amitraz EC group, which indicates that the combination treatment needs to be optimized to avoid damage to colonies.

Effects of natural treatments on the varroa mite infestation levels and overall health of honey bee (Apis mellifera) colonies.

The survival of the honey bee (Apis mellifera), which has a crucial role in pollination and ecosystem maintenance, is threatened by many pathogens, including parasites, bacteria, fungi and viruses. The ectoparasite Varroa destructor is considered the major cause of the worldwide decline in honey bee colony health. Although several synthetic acaricides are available to control Varroa infestations, resistant mites and side effects on bees have been documented. The development of natural alternatives for mite control is therefore encouraged. The study aims at exploring the effects of cinnamon and oregano essential oils (EOs) and of a mixed fruit cocktail juice on mite infestation levels and bee colony health. A multi-method study including hive inspection, mite count, molecular detection of fungal, bacterial and viral pathogens, analysis of defensin-1, hymenoptaecin and vitellogenin immune gene expression, colony density and honey production data, was conducted in a 20-hive experimental apiary. The colonies were divided into five groups: four treatment groups and one control group. The treatment groups were fed on a sugar syrup supplemented with cinnamon EO, oregano EO, a 1:1 mixture of both EOs, or a juice cocktail. An unsupplemented syrup was, instead, used to feed the control group. While V. destructor affected all the colonies throughout the study, no differences in mite infestation levels, population density and honey yield were observed between treatment and control groups. An overexpression of vitellogenin was instead found in all EO-treated groups, even though a significant difference was only found in the group treated with the 1:1 EO mixture. Viral (DWV, CBPV and BQCV), fungal (Nosema ceranae) and bacterial (Melissococcus plutonius) pathogens from both symptomatic and asymptomatic colonies were detected.

Heightened sensitivity in high-grooming honey bees (Hymenoptera: Apidae).

Honey bees use grooming to defend against the devastating parasite Varroa destructor Anderson and Trueman. We observed the grooming responses of individual bees from colonies previously chosen for high- and low-grooming behavior using a combination of mite mortality and mite damage. Our aim was to gain insight into specific aspects of grooming behavior to compare if high-grooming bees could discriminate between a standardized stimulus (chalk dust) and a stimulus of live Varroa mites and if bees from high-grooming colonies had greater sensitivity across different body regions than bees from low-grooming colonies. We hypothesized that individuals from high-grooming colonies would be more sensitive to both stimuli than bees from low-grooming colonies across different body regions and that bees would have a greater response to Varroa than a standardized irritant (chalk dust). Individuals from high-grooming colonies responded with longer bouts of intense grooming when either stimulus was applied to the head or thorax, compared to sham-stimulated controls, while bees from low-grooming colonies showed no differences between stimulated and sham-stimulated bees. Further, high-grooming bees from colonies with high mite damage exhibited greater grooming to Varroa than high-grooming colonies with only moderate mite damage rates. This study provides new insights into Varroa-specific aspects of grooming, showing that although a standardized stimulus (chalk dust) may be used to assess general grooming ability in individual bee grooming assays, it does not capture the same range of responses as a stimulus of Varroa. Thus, continuing to use Varroa mites in grooming assays should help select colonies with more precise sensitivity to Varroa.

Neonicotinoid exposure increases Varroa destructor (Mesostigmata: Varroidae) mite parasitism severity in honey bee colonies and is not mitigated by increased colony genetic diversity.

Agrochemical exposure is a major contributor to ecological declines worldwide, including the loss of crucial pollinator species. In addition to direct toxicity, field-relevant doses of pesticides can increase species' vulnerabilities to other stressors, including parasites. Experimental field demonstrations of potential interactive effects of pesticides and additional stressors are rare, as are tests of mechanisms via which pollinators tolerate pesticides. Here, we controlled honey bee colony exposure to field-relevant concentrations of 2 neonicotinoid insecticides (clothianidin and thiamethoxam) in pollen and simultaneously manipulated intracolony genetic heterogeneity. We showed that exposure increased rates of Varroa destructor (Anderson and Trueman) parasitism and that while increased genetic heterogeneity overall improved survivability, it did not reduce the negative effect size of neonicotinoid exposure. This study is, to our knowledge, the first experimental field demonstration of how neonicotinoid exposure can increase V. destructor populations in honey bees and also demonstrates that colony genetic diversity cannot mitigate the effects of neonicotinoid pesticides.

The efficacy of 1-allyloxy-4-propoxybenzene (3c{3,6}) against Varroa destructor mites in honey bee colonies from Maryland, USA.

Varroa destructor Oud (Acari: Varroidae) is a harmful ectoparasite of Apis mellifera L. honey bees causing widespread colony losses in Europe and North America. To control populations of these mites, beekeepers have an arsenal of different treatments, including both chemical and nonchemical options. However, nonchemical treatments can be labor intensive, and Varroa has gained resistance to some conventional pesticides, and the use of other chemical treatments is restricted temporally (e.g., cannot be applied during periods of honey production). Thus, beekeepers require additional treatment options for controlling mite populations. The compound 1-allyloxy-4-propoxybenzene (3c{3,6}) is a diether previously shown to be a strong feeding deterrent against Lepidopteran larvae and a repellent against mosquitoes and showed promise as a novel acaricide from laboratory and early field trials. Here we test the effect of the compound, applied at 8 g/brood box on wooden release devices, on honey bees and Varroa in field honey bee colonies located in Maryland, USA, and using a thymol-based commercial product as a positive control. 3c{3,6} had minimal effect on honey bee colonies, but more tests are needed to determine whether it affected egg production by queens. Against Varroa3c{3,6} had an estimated efficacy of 78.5%, while the positive control thymol product showed an efficacy of 91.3%. 3c{3,6} is still in the development stage, and the dose or application method needs to be revisited.

Effects of summer treatments against Varroa destructor on viral load and colony performance of Apis mellifera colonies in Eastern Canada.

Despite the use of various integrated pest management strategies to control the honey bee mite, Varroa destructor, varroosis remains the most important threat to honey bee colony health in many countries. In Canada, ineffective varroa control is linked to high winter colony losses and new treatment options, such as a summer treatment, are greatly needed. In this study, a total of 135 colonies located in 6 apiaries were submitted to one of these 3 varroa treatment strategies: (i) an Apivar® fall treatment followed by an oxalic acid (OA) treatment by dripping method; (ii) same as in (i) with a summer treatment consisting of formic acid (Formic Pro™); and (iii) same as in (i) with a summer treatment consisting of slow-release OA/glycerin pads (total of 27 g of OA/colony). Treatment efficacy and their effects on colony performance, mortality, varroa population, and the abundance of 6 viruses (acute bee paralysis virus [ABPV], black queen cell virus [BQCV], deformed wing virus variant A [DWV-A], deformed wing virus variant B [DWV-B], Israeli acute paralysis virus [IAPV], and Kashmir bee virus [KBV]) were assessed. We show that a strategy with a Formic Pro summer treatment tended to reduce the varroa infestation rate to below the economic fall threshold of 15 daily varroa drop, which reduced colony mortality significantly but did not reduce the prevalence or viral load of the 6 tested viruses at the colony level. A strategy with glycerin/OA pads reduced hive weight gain and the varroa infestation rate, but not below the fall threshold. A high prevalence of DWV-B was measured in all groups, which could be related to colony mortality.

Faster-growing parasites threaten host populations via patch-level population dynamics and higher virulence; a case study in Varroa mites (Mesostigmata: Varroidae) and honey bees (Hymenoptera: Apidae).

Honey bee parasites remain a critical challenge to management and conservation. Because managed honey bees are maintained in colonies kept in apiaries across landscapes, the study of honey bee parasites allows the investigation of spatial principles in parasite ecology and evolution. We used a controlled field experiment to study the relationship between population growth rate and virulence (colony survival) of the parasite Varroa destructor (Anderson and Trueman). We used a nested design of 10 patches (apiaries) of 14 colonies to examine the spatial scale at which Varroa population growth matters for colony survival. We tracked Varroa population size and colony survival across a full year and found that Varroa populations that grow faster in their host colonies during the spring and summer led to larger Varroa populations across the whole apiary (patch) and higher rates of neighboring colony loss. Crucially, this increased colony loss risk manifested at the patch scale, with mortality risk being related to spatial adjacency to colonies with fast-growing Varroa strains rather than with Varroa growth rate in the colony itself. Thus, within-colony population growth predicts whole-apiary virulence, demonstrating the need to consider multiple scales when investigating parasite growth-virulence relationships.

Honey bee (Apis mellifera) nurse bee visitation of worker and drone larvae increases Varroa destructor mite cell invasion.

The life cycle of Varroa destructor, the ectoparasitic mite of honey bees (Apis mellifera), includes a dispersal phase, in which mites attach to adult bees for transport and feeding, and a reproductive phase, in which mites invade worker and drone brood cells just prior to pupation to reproduce while their bee hosts complete development. In this study, we wanted to determine whether increased nurse bee visitations of adjacent drone and worker brood cells would increase the likelihood of Varroa mites invading those cells. We also explored whether temporarily restricting the nurses' access to sections of worker brood for 2 or 4 h would subsequently cause higher nurse visitations, and thus, higher Varroa cell invasions. Temporarily precluding larvae from being fed by nurses subsequently led to higher Varroa infestation of those sections in some colonies, but this pattern was not consistent across colonies. Therefore, removing highly infested sections of capped worker brood could be further explored as a potential mechanical/cultural method for mite control. Our results provide more information on how nurse visitations affect the patterns of larval cell invasion by Varroa. Given that the mite's successful reproduction depends on the nurses' ability to visit and feed developing brood, more studies are needed to understand the patterns of Varroa mite invasion of drone and worker cells to better combat this pervasive honey bee parasite.

Effective pest management approaches can mitigate honey bee (Apis mellifera) colony winter loss across a range of weather conditions in small-scale, stationary apiaries.

Managed honey bee (Apis mellifera L.) colonies in North America and Europe have experienced high losses in recent years, which have been linked to weather conditions, lack of quality forage, and high parasite loads, particularly the obligate brood parasite, Varroa destructor. These factors may interact at various scales to have compounding effects on honey bee health, but few studies have been able to simultaneously investigate the effects of weather conditions, landscape factors, and management of parasites. We analyzed a dataset of 3,210 survey responses from beekeepers in Pennsylvania from 2017 to 2022 and combined these with remotely sensed weather variables and novel datasets about seasonal forage availability into a Random Forest model to investigate drivers of winter loss. We found that beekeepers who used treatment against Varroa had higher colony survival than those who did not treat. Moreover, beekeepers who used multiple types of Varroa treatment had higher colony survival rates than those who used 1 type of treatment. Our models found weather conditions are strongly associated with survival, but multiple-treatment type colonies had higher survival across a broader range of climate conditions. These findings suggest that the integrated pest management approach of combining treatment types can potentially buffer managed honey bee colonies from adverse weather conditions.

Relative impacts of Varroa destructor (Mesostigmata:Varroidae) infestation and pesticide exposure on honey bee colony health and survival in a high-intensity corn and soybean producing region in northern Iowa.

The negative effects of Varroa and pesticides on colony health and survival are among the most important concerns to beekeepers. To compare the relative contribution of Varroa, pesticides, and interactions between them on honey bee colony performance and survival, a 2-year longitudinal study was performed in corn and soybean growing areas of Iowa. Varroa infestation and pesticide content in stored pollen were measured from 3 apiaries across a gradient of corn and soybean production areas and compared to measurements of colony health and survival. Colonies were not treated for Varroa the first year, but were treated the second year, leading to reduced Varroa infestation that was associated with larger honey bee populations, increased honey production, and higher colony survival. Pesticide detections were highest in areas with high-intensity corn and soybean production treated with conventional methods. Pesticide detections were positively associated with honey bee population size in May 2015 in the intermediate conventional (IC) and intermediate organic (IO) apiaries. Varroa populations across all apiaries in October 2015 were negatively correlated with miticide and chlorpyrifos detections. Miticide detections across all apiaries and neonicotinoid detections in the IC apiary in May 2015 were higher in colonies that survived. In July 2015, colony survival was positively associated with total pesticide detections in all apiaries and chlorpyrifos exposure in the IC and high conventional (HC) apiaries. This research suggests that Varroa are a major cause of reduced colony performance and increased colony losses, and honey bees are resilient upon low to moderate pesticide detections.