Parasite and virus dynamics in the honeybee Apis mellifera unicolor on a tropical island recently invaded by Varroa destructor.

In La Réunion, the established honeybee subspecies Apis mellifera unicolor, an endemic subspecies of African lineage, is facing considerable challenges. Since the introduction of the Varroa destructor mite in 2017 high colony losses have been recorded. We investigated the dynamics of V. destructor and two viruses, the Deformed Wing Virus (DWV), known to be transmitted by the mite, and the Chronic Bee Paralysis Virus (CBPV), in A. m. unicolor. Colonies from two apiaries located at 300 and 900 m a.s.l were monitored twice for one year without any acaricide treatment. The brood area, V. destructor infestation rates, DWV and CBPV prevalence and load were recorded monthly. A. m. unicolor maintained brood rearing throughout the year. Varroa destructor infestation resulted in high colony mortality (up to 85 %) and high phoretic mite rates (up to 52 mites per hundred bees). The establishment of DWV in colonies occurred after that of V. destructor and the mite infestation rate had a significant effect on the virus prevalence and load. CBPV appeared only transiently throughout the surveys. The data showed that, in tropical colonies with permanent brood rearing, V. destructor and DWV can reach high levels, but are still subject to seasonal variations that appear to be influenced by environmental conditions. This suggests that beekeeping practices could be adapted by favouring sites and periods for transhumance or acaricide treatment.

Caught in the act: the invasion of a viral vector changes viral prevalence and titre in native honeybees and bumblebees.

Novel transmission routes change pathogen landscapes and may facilitate disease emergence. The varroa mite is a virus vector that switched to western honeybees at the beginning of the last century, leading to hive mortality, particularly in combination with RNA viruses. A recent invasion of varroa on the French island of Ushant introduced vector-mediated transmission to one of the last varroa-naive native honeybee populations and caused rapid changes in the honeybee viral community. These changes were characterized by a drastic increase in deformed wing virus type B prevalence and titre in honeybees, as well as knock-on effects in bumblebees, particularly in the year following the invasion. Slow bee paralysis virus also appeared in honeybees and bumblebees, with a 1 year delay, while black queen cell virus declined in honeybees. This study highlights the rapid and far-reaching effects of vector-borne transmission that can extend beyond the directly affected host species, and that the direction of the effect depends on the pathogen's virulence.

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.

Brood indicators are an early warning signal of honey bee colony loss-a simulation-based study.

Honey bees (Apis mellifera) are exposed to multiple stressors such as pesticides, lack of forage, and diseases. It is therefore a long-standing aim to develop robust and meaningful indicators of bee vitality to assist beekeepers While established indicators often focus on expected colony winter mortality based on adult bee abundance and honey reserves at the beginning of the winter, it would be useful to have indicators that allow detection of stress effects earlier in the year to allow for adaptive management. We used the established honey bee simulation model BEEHAVE to explore the potential of different indicators such as population size, number of capped brood cells, flight activity, abundance of Varroa mites, honey stores and a brood-bee ratio. We implemented two types of stressors in our simulations: 1) parasite pressure, i.e. sub-optimal Varroa treatment by the beekeeper (hereafter referred as Biotic stress) and 2) temporal forage gaps in spring and autumn (hereafter referred as Environmental stress). Neither stressor type could be detected by bee abundance or honey reserves at the end of the first year. However, all response variables used in this study did reveal early warning signals during the course of the year. The most reliable and useful measures seem to be related to brood and the abundance of Varroa mites at the end of the year. However, while in the model we have full access to time series of variables from stressed and unstressed colonies, knowledge of these variables in the field is challenging. We discuss how our findings can nevertheless be used to develop practical early warning indicators. As a next step in the interactive development of such indicators we suggest empirical studies on the importance of the number of capped brood cells at certain times of the year on bee population vitality.

Identification and pathogen detection of a Neocypholaelaps species (Acari: Mesostigmata: Ameroseiidae) from beehives in the Republic of Korea.

In this study, we identified a new strain of the genus Neocypholaelaps from the beehives of Apis mellifera colonies in the Republic of Korea (ROK). The Neocypholaelap sp. KOR23 mites were collected from the hives of honeybee apiaries in Wonju, Gangwon-do, in May 2023. Morphological and molecular analyses based on 18S and 28S rRNA gene regions conclusively identified that these mites belong to the genus Neocypholaelaps, closely resembling Neocypholaelaps sp. APGD-2010 that was first isolated from the United States. The presence of 9 of 25 honeybee pathogens in these mite samples suggests that Neocypholaelaps sp. KOR23 mite may act as an intermediate vector and carrier of honeybee diseases. The identification of various honeybee pathogens within this mite highlights their significance in disease transmission among honeybee colonies. This comprehensive study provides valuable insights into the taxonomy and implications of these mites for bee health management and pathogen dissemination.

Analysis of X-ray irradiation effects on the mortality values and hemolymph immune cell composition of Apis mellifera and its parasite, Varroa destructor.

Varroa destructor is one of the most destructive enemies of the honey bee, Apis mellifera all around the world. Several control methods are known to control V. destructor, but the efficacy of several alternative control methods remains unexplored. Irradiation can be one of these unknown solutions but before practical application, the effectiveness, and the physiological effects of ionizing radiation on the host and the parasite are waiting to be tested. Therefore, the objective of our study was to investigate the effects of different doses (15, 50, 100, and 150 Gy) of high-energy X-ray irradiation through mortality rates and hemocyte composition changes in A. mellifera workers and record the mortality rates of the parasite. The mortality rate was recorded during short-term (12, 24, and 48 h) and long-term periods (3, 6, 12, 18, and 24d). The sensitivity of the host and the parasite in case of the higher doses of radiation tested (50, 100, and 150 Gy) been demonstrated by total mortality of the host and 90 % of its parasite has been observed on the 18th day after the irradiation. V. destructor showed higher sensitivity (1.52-times higher than the adult honey bee workers) at the lowest dose (15 Gy). A. mellifera hemocytes were influenced significantly by radiation dosage and the elapsed time after treatment. The higher radiation doses increased plasmatocyte numbers in parallel with the decrease in prohemocyte numbers. On the contrary, the numbers of granulocytes and oencoytes increased in the treated samples, but the putative effects of the different dosages on the recorded number of these hemocyte types could not be statistically proven. In summary, based on the outcome of our study X-ray irradiation can be deemed an effective tool for controlling phoretic V. destructor. However, further research is needed to understand the physiological response of the affected organisms.

Interaction of acetamiprid, Varroa destructor, and Nosema ceranae in honey bees.

Health of honey bees is threatened by a variety of stressors, including pesticides and parasites. Here, we investigated effects of acetamiprid, Varroa destructor, and Nosema ceranae, which act either alone or in combination. Our results suggested that interaction between the three factors was additive, with survival risk increasing as the number of stressors increased. Although exposure to 150 µg/L acetamiprid alone did not negatively impact honey bee survival, it caused severe damage to midgut tissue. Among the three stressors, V. destructor posed the greatest threat to honey bee survival, and N. ceranae exacerbated intestinal damage and increased thickness of the midgut wall. Transcriptomic analysis indicated that different combinations of stressors elicited specific gene expression responses in honey bees, and genes involved in energy metabolism, immunity, and detoxification were altered in response to multiple stressor combinations. Additionally, genes associated with Toll and Imd signalling, tyrosine metabolism, and phototransduction pathway were significantly suppressed in response to different combinations of multiple stressors. This study enhances our understanding of the adaptation mechanisms to multiple stressors and aids in development of suitable protective measures for honey bees. ENVIRONMENTAL IMPLICATION: We believe our study is environmentally relevant for the following reasons: This study investigates combined effects of pesticide, Varroa destructor, and Nosema ceranae. These stressors are known to pose a threat to long-term survival of honey bees (Apis mellifera) and stability of the ecosystems. The research provides valuable insights into the adaptive mechanisms of honey bees in response to multiple stressors and developing effective conservation strategies. Further research can identify traits that promote honey bee survival in the face of future challenges from multiple stressors to maintain the overall stability of environment.

Honey bee stressor networks are complex and dependent on crop and region.

Honey bees play a major role in crop pollination but have experienced declining health throughout most of the globe. Despite decades of research on key honey bee stressors (e.g., parasitic Varroa destructor mites and viruses), researchers cannot fully explain or predict colony mortality, potentially because it is caused by exposure to multiple interacting stressors in the field. Understanding which honey bee stressors co-occur and have the potential to interact is therefore of profound importance. Here, we used the emerging field of systems theory to characterize the stressor networks found in honey bee colonies after they were placed in fields containing economically valuable crops across Canada. Honey bee stressor networks were often highly complex, with hundreds of potential interactions between stressors. Their placement in crops for the pollination season generally exposed colonies to more complex stressor networks, with an average of 23 stressors and 307 interactions. We discovered that the most influential stressors in a network-those that substantively impacted network architecture-are not currently addressed by beekeepers. Finally, the stressor networks showed substantial divergence among crop systems from different regions, which is consistent with the knowledge that some crops (e.g., highbush blueberry) are traditionally riskier to honey bees than others. Our approach sheds light on the stressor networks that honey bees encounter in the field and underscores the importance of considering interactions among stressors. Clearly, addressing and managing these issues will require solutions that are tailored to specific crops and regions and their associated stressor networks.

Population-wide modelling reveals prospects of marker-assisted selection for parasitic mite resistance in honey bees.

In 2019, a joint eight-variant model was published in which eight single nucleotide polymorphisms (SNPs) in seven Apis mellifera genes were associated with Varroa destructor drone brood resistance (DBR, i.e. mite non-reproduction in drone brood). As this model was derived from only one Darwinian Black Bee Box colony, it could not directly be applied on a population-overarching scale in the northern part of Belgium (Flanders), where beekeepers prefer the carnica subspecies. To determine whether these eight SNPs remained associated with the DBR trait on a Flemish colony-broad scope, we performed population-wide modelling through sampling of various A. mellifera carnica colonies, DBR scoring of Varroa-infested drone brood and variant genotyping. Novel eight-variant modelling was performed and the classification performance of the eight SNPs was evaluated. Besides, we built a reduced three-variant model retaining only three genetic variants and found that this model classified 76% of the phenotyped drones correctly. To examine the spread of beneficial alleles and predict the DBR probability distribution in Flanders, we determined the allelic frequencies of the three variants in 292 A. mellifera carnica queens. As such, this research reveals prospects of marker-assisted selection for Varroa drone brood resistance in honeybees.

Some essential oils as potential control agents for varroa mite (Varroa destructor) in infected honey bees (Apis mellifera).

Background: Ecto-parasite, varroa mite, (Varroa destructor), is the primary pest affecting the apiculture sector globally in various regions. Aim: This study examined the toxicity of nine essential oils to Apis mellifera L. and the acaricidal impact of those oils against V. destructor. Methods: The acaricidal effects of nine essential oils, extracted from plant materials were used. In the screening experiment, 10 mg of the active ingredients of the plant material extracts were prepared in an alcohol solution with concentrations of 5%, 10%, and 15%. For each type of plant extract, five female V. destructor were transferred to a Petri dish with five worker bees incubated at 70% humidity and 33°-34° for 2 days, for each treatment four replicates were used compared to the control. Forty-eight hours following treatment, the number of dead and live mites was counted to determine the mortality rate. In the second assay experiment, the best five essential oils of the previous experiment were selected to re-assess their effectiveness on varroa mites and honeybee workers by using a concentration of 15%. Five females of V. destructor were transferred to a Petri dish with 10 adult bees and treated with the solution of the selected oils. Five replicates and control treatments were taken for each sample simultaneously. Dead and live bees were counted for each replicate at 48 hours after treatment. Results: There were no significant differences between the concentrations used of each oil on the rate of death of mites, and its effectiveness ranged between 70.0% and 53.3% compared to the control groups. In addition, the best oil used was bitter melon, with a death rate of 80% at a concentration of 15%, while peppermint oil showed the lowest death rate of 45% at a concentration of (5%). However, all these treatments were statistically highly significant compared with the natural death rate in control (2%). In the second test, the results of the statistical analysis indicated that there were highly significant differences (P0.05 <0.0001) in the average numbers of dead varroa mites compared to the control when using a 15% concentration of five selected oils. On the other hand, there was no statistically significant difference in the honey bee workers' mortality rate between the treatment and control groups (P0.05 <0.3390), and it was relatively low for all treatments except the basil oil, where the bee mortality rate was 16% compared to the control (10%). Conclusion: It is clear from this experiment that bitter melon oil can be used to control varroa mites and it can be considered safe for honey bees as well as for the environment.

Africanized honey bee colonies in Costa Rica: first evidence of its management, brood nest structure and factors associated with varroa mite infestation.

Management, brood nest structure and factors associated with varroa mite infestation were studied in 60 apiaries of Africanized honey bees in the northwest region of the Central Valley of Costa Rica. Apiaries were monitored two times. The first monitoring was taken forward during the rainy season between May and November 2019. The second monitoring during the dry season between February and March 2020. Information about the beekeepers, apiaries and management was collected through a survey. Amount of open and capped brood, honey and pollen were measured in the field. The infestation rate of varroa (IRV) was quantified using standard laboratory methods. A determination of multi-residue pesticides in bee bread was made through GC-MS/MS and LC-MS/MS techniques. According to the results, most of the beekeepers produce honey (96.7%), participate in training activities (82.2%), and change the bee queens annually (70%). The first monitoring was characterized by a lower amount of capped brood and honey reserves compared to the second one. IRV was significantly higher in the first monitoring (6.0 ± 0.4) in comparison with the second one (3.0 ± 0.3) (U Mann-Whitney p < 0.001). The maximum value for the first monitoring exceeds 40%, while this value was close to 25% in the second monitoring. Mite infestation exposed significant differences in relation to the variables associated to the beekeeper's management, i.e., change of bee queen (p = 0.002) or when beekeepers monitor varroa mites (p = 0.004). Additionally, the IRV had inverse correlations (p < 0.01) with the number of comb sides with capped brood (Spearman's rho coefficient = - 0.190), and honey reserves (Spearman's rho coefficient = - 0.168). Furthermore, 23 of 60 bee bread samples presented one to five pesticide residues, being the most frequent antifungal agrochemicals.

Varroa destructor parasitism and Deformed wing virus infection in honey bees are linked to peroxisome-induced pathways.

The ectoparasitic mite Varroa destructor transmits and triggers viral infections that have deleterious effects on honey bee colonies worldwide. We performed a manipulative experiment in which worker bees collected at emergence were exposed to Varroa for 72 h, and their proteomes were compared with those of untreated control bees. Label-free quantitative proteomics identified 77 differentially expressed A. mellifera proteins (DEPs). In addition, viral proteins were identified by orthogonal analysis, and most importantly, Deformed wing virus (DWV) was found at high levels/intensity in Varroa-exposed bees. Pathway enrichment analysis suggested that the main pathways affected included peroxisomal metabolism, cyto-/exoskeleton reorganization, and cuticular proteins. Detailed examination of individual DEPs revealed that additional changes in DEPs were associated with peroxisomal function. In addition, the proteome data support the importance of TGF-ß signaling in Varroa-DWV interaction and the involvement of the mTORC1 and Hippo pathways. These results suggest that the effect of DWV on bees associated with Varroa feeding results in aberrant autophagy. In particular, autophagy is selectively modulated by peroxisomes, to which the observed proteome changes strongly corresponded. This study complements previous research with different study designs and suggests the importance of the peroxisome, which plays a key role in viral infections.

Developing a method to rear Varroa destructor in vitro.

Varroa destructor is a significant mite pest of western honey bees (Apis mellifera). Developing a method to rear and maintain populations of V. destructor in vitro would provide year-round access to the mites, allowing scientists to study their biology, behavior, and control more rapidly. In this study, we determined the impact of various rearing parameters on V. destructor survival and reproduction in vitro. This was done by collecting V. destructor from colonies, placing them in gelatin capsules containing honey bee larvae, and manipulating the following conditions experimentally: rearing temperature, colony source of honey bee larva, behavioral/developmental stages of V. destructor and honey bee larva, and mite:bee larva ratio. Varroa destructor survival was significantly impacted by temperature, colony source of larvae and mite behavioral stage. In addition, V. destructor reproduction was significantly impacted by mite: larva ratio, larval developmental stage, colony source of larva, and temperature. The following conditions optimized mite survival and reproduction in vitro: using a 4:1 mite:larva ratio, beginning the study with late stage uncapped larvae, using mites collected from adult bees, maintaining the rearing temperature at 34.5° C, and screening larval colony source. Ultimately, this research can be used to improve V. destructor in vitro rearing programs.

Geographical distribution of pyrethroid resistance mutations in Varroa destructor across Türkiye and a European overview.

Varroa destructor Anderson & Trueman (Acari: Varroidae) is of paramount significance in modern beekeeping, with infestations presenting a primary challenge that directly influences colony health, productivity, and overall apicultural sustainability. In order to control this mite, many beekeepers rely on a limited number of approved synthetic acaricides, including the pyrethroids tau-fluvalinate, flumethrin and organophosphate coumaphos. However, the excessive use of these substances has led to the widespread development of resistance in various beekeeping areas globally. In the present study, the occurrence of resistance mutations in the voltage-gated sodium channel (VGSC) and acetylcholinesterase (AChE), the target-site of pyrethroids and coumaphos, respectively, was examined in Varroa populations collected throughout the southeastern and eastern Anatolia regions of Türkiye. All Varroa samples belonged to the Korean haplotype, and a very low genetic distance was observed based on cytochrome c oxidase subunit I (COI) gene sequences. No amino acid substitutions were determined at the key residues of AChE. On the other hand, three amino acid substitutions, (L925V/I/M), previously associated with pyrethroid resistance, were identified in nearly 80% of the Turkish populations. Importantly, L925M, the dominant mutation in the USA, was detected in Turkish Varroa populations for the first time. To gain a more comprehensive perspective, we conducted a systematic analysis of the distribution of pyrethroid resistance mutations across Europe, based on the previously reported data. Varroa populations from Mediterranean countries such as Türkiye, Spain, and Greece exhibited the highest frequency of resistance mutation. Revealing the occurrence and geographical distribution of pyrethroid resistance mutations in V. destructor populations across the country will enhance the development of more efficient strategies for mite management.

Properties of essential oils absorbed on the surface of cardboard pieces after using atmospheric-pressure plasma treatments to develop long-lasting Varroa miticides in honeybees (Apis mellifera).

The ectoparasitic mite, Varroa destructor is the most serious widespread pest of managed honeybees (Apis mellifera). Several acaricide products, which include essential oils, have been proposed for mite control. In this study, we aimed to apply atmospheric-pressure plasma to modify a cardboard piece surface in order to prolong the delivery of essential oils for controlling Varroa in honeybee colonies. Absorption capacity, release rates and evaporation rates of essential oils were determined. Cardboard piece showed a higher absorption capacity of cinnamon compared to citronella and clove. Surface modification of cardboard pieces using argon plasma at different gas flow rates and treatment durations, significantly affected the absorption of clove oil. Additionally, the release rate of cinnamon, citronella and clove was significantly enhanced after argon plasma treatments. Evaporation of cinnamon was dramatically increased by plasma treatment at 6-h of incubation. The highest evaporation rate was obtained by plasma-treated cardboard piece at a gas flow rate of 0.5 Lpm for 60 s (0.2175 ± 0.0148 µl/g•h). Efficiency of plasma-treated cardboard piece, impregnated with essential oils, was also investigated for Varroa control in honeybee colonies. In the first experiment, formic acid 65% (v/v) showed the highest efficiency of 90.60% and 81.59% with the percent of mite infestation was 0.23 ± 0.13% and 0.47 ± 0.19% at 21 and 35 days, respectively after treatment. The efficacy of cardamon oil (5% (v/v)) delivered using plasma-treated cardboard pieces was 57.71% (0.70 ± 0.16% of mite infestation) at day 21 of experiment. However, the delivery of cardamon oil at the concentration of 1% and 5% (v/v) by untreated cardboard piece had 16.93% and 24.05% of efficacy to control mites. In the 2nd experiment, the application of plasma-treated cardboard pieces impregnated with 5% (v/v) clove oil induced a 38.10% reduction in the population of Varroa mites followed by 5% (v/v) of cardamon with 30% efficiency. Although, the infestation rate of Varroa in colonies was not significant different between treatments, essential oils delivered using plasma-treated cardboard pieces tended to decrease Varroa population in the treated colonies. Hence, atmospheric-pressure plasma for the modification of other materials, should be further investigated to provide alternative control treatment applications against honeybee mites.

Effects of feeding with a protein liquid supplement on productivity, mortality and health of Apis mellifera hives in southwestern Spain.

Colony collapse disorder (CCD) has affected bees worldwide in recent decades, with southwestern Spain being no exception. This disorder is one of the main causes of Apis mellifera mortality and is believed to be caused by environmental, social and sanitary conditions. Dietary supplementation can help to improve some parameters of the general status and sanitary condition of bees, such as infestation by certain recurrent pathogens, including Varroa destructor and Nosema ceranae, by enhancing immune and social response. Thus, the aim of this study was to test a liquid hydrolysed protein supplement on the health and general status of the hive in several apiaries with access to the same natural food and under similar climatic conditions. We selected two groups of ten hives (supplemented by either placebo or protein) from five apiaries where the number of adult bees, amount of brood (open and operculated), honey and pollen reserves, infestation by V. destructor, N. ceranae, deformed wing virus (DWV) and chronic bee paralysis virus (CBPV) were measured. Additionally, we assess the expression of four immune system-related genes and a gene encoding vitellogenin. At the end of this work, treated hives showed a significant increase in open brood and a decrease in V. destructor infestation. Also, these hives showed a significant decrease in the mortality rate after the cold season. Therefore, supplementation with this product improved the health of the hive and could be a promising tool against bee colony loss.

Seasonal trends of the ABPV, KBV, and IAPV complex in Italian managed honey bee (Apis mellifera L.) colonies.

Acute bee paralysis virus (ABPV), Kashmir bee virus (KBV), and Israeli acute paralysis virus (IAPV) usually persist as covert infections in honey bee colonies. They can cause rapid bee mortality in cases of severe infection, often associated with high Varroa destructor infestation, by which they are transmitted. In various countries, these viruses have been associated with colony collapse. Despite their potential danger, these viruses are often disregarded, and little information is available on their occurrence in many countries, including Italy. In 2021, 370 apiaries representing all of the Italian regions were investigated in four different months (June, September, November, and March) for the presence of ABPV, KBV, and IAPV. IAPV was not found in any of the apiaries investigated, whereas 16.45% and 0.67% of the samples tested positive for ABPV and KBV, respectively. Most ABPV cases occurred in late summer-autumn in both northern and southern regions. We observed a scattered pattern of KBV-positive colonies that did not allow any seasonal or regional trends to be discerned. Differences observed among regions and months were potentially related to the dynamics of varroa infestation, viral genetic variations, and different climatic conditions resulting in variations in bee behaviour. This study improves our understanding of the circulation of bee viruses and will contribute to better disease prevention and preservation of bee health.

Susceptible and infectious states for both vector and host in a dynamic pathogen-vector-host system.

Deformed wing virus (DWV) is a resurgent insect pathogen of honeybees that is efficiently transmitted by vectors and through host social contact. Continual transmission of DWV between hosts and vectors is required to maintain the pathogen within the population, and this vector-host-pathogen system offers unique disease transmission dynamics for pathogen maintenance between vectors and a social host. In a series of experiments, we measured vector-vector, host-host and host-vector transmission routes and show how these maintain DWV in honeybee populations. We found co-infestations on shared hosts allowed for movement of DWV from mite to mite. Additionally, two social behaviours of the honeybee, trophallaxis and cannibalization of pupae, provide routes for horizontal transmission from bee to bee. Circulation of the virus solely among hosts through communicable modes provides a reservoir of DWV for naïve Varroa to acquire and subsequently vector the pathogen. Our findings illustrate the importance of community transmission between hosts and vector transmission. We use these results to highlight the key avenues used by DWV during maintenance and infection and point to similarities with a handful of other infectious diseases of zoonotic and medical importance.

The Varroa paradox: infestation levels and hygienic behavior in feral scutellata-hybrid and managed Apis mellifera ligustica honey bees.

The Varroa destructor mite is a parasitic threat to managed and feral honey bee colonies around the world. Beekeepers use miticides to eliminate Varroa in commercial hives, but these chemicals can diminish bee health and increase miticide resistance. In contrast, feral honey bees have developed multiple ways to counteract mites without chemical treatment. We compared mite levels, grooming habits, and mite-biting behavior between feral Africanized honey bees (genomically verified Apis mellifera scutellata hybrids) and managed Italian honey bees (A. mellifera ligustica). Surprisingly, there was no difference in mite infestation levels between scutellata-hybrids and managed bees over one year despite the regular use of miticides in managed colonies. We also found no differences in the social immunity responses of the two groups, as measured by their hygienic habits (through worker brood pin-kill assays), self-grooming, and mite-biting behavior. However, we provide the first report that both scutellata-hybrids and managed honey bees bite off mite chemosensory forelegs, which the mites use to locate brood cells for reproduction, to a significantly greater degree than other legs (a twofold greater reduction in foreleg length relative to the most anterior legs). Such biting may impair mite reproduction.

Prevalence and distribution of Varroa destructor and Nosema spp. in symptomatic honey bee colonies across the USA from 2015 to 2022.

USDA-ARS Bee Research Laboratory received symptomatic honey bee (Apis mellifera L.) samples across the United States for disease diagnosis. Here, we present a retrospective study and cartography of ectoparasite Varroa destructor and intracellular microsporidia parasite Nosema spp. These two major parasites were identified in the diseased honey bee samples between 2015 and 2022. Varroa infestation level (VIL) was examined by a wash technique (Mites/100 bees) and calculated as a percentage, while Nosema infection was quantified by microscopical spore count (Million Spores/Bee). Data were analyzed by month, year, state, and by nine geographical climate regions described in the U.S. Of adult bee samples (n = 4039) that were analyzed for Varroa mite infestation, the overall VIL in the U.S. ranged between 0.4 and 30.85%, with an overall national VIL and Varroa prevalence of 8.21% and 85.14%, respectively. Overall monthly data showed VIL constantly exceeded the critical level of 4% except from June to September and reached a maximum of 15% in January and December. Nationwide, VIL significantly (p < 0.001) increased from 2015 to 2018 (1.1-4.7%), plateaued from 2018 to 2021 (4.7-4.5%), followed by a significant decrease in 2022 (3.6%). Significant VIL differences (p < 0.001) were recorded among climate regions, with the highest mite infestation levels in the Upper Midwest region (13.9%) and the lowest in the West region (5.1%). Of adult bee samples (n = 2,994) that were analyzed for Nosema infection, Nosema spore count ranged between (1-16.8) million spores per bee among states, with a national average of 6.8 and a prevalence of 99.7%. The lowest and highest Nosema loads were respectively recorded in the South region (3.1) and Upper Midwest (10.5), a significant difference (p < 0.001). No statistical differences were recorded among the six other climate regions. Overall, VIL and Nosema infection correlated significantly (p < 0.001) with a regression coefficient of (R2 = 0.6). Our data, which originated from ailing bee colonies, showed significantly higher rates of maladies compared to data from healthy colonies obtained by the USDA-APHIS National Honey Bee Survey, demonstrating the role of bee diseases caused by Varroa mite and Nosema in honey bee population declines.