Varroa destructor (Mesostigmata: Varroidae) Parasitism and Climate Differentially Influence the Prevalence, Levels, and Overt Infections of Deformed Wing Virus in Honey Bees (Hymenoptera: Apidae)

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.

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.

Prevalence of Adult Honey Bee (Apis mellifera L.) Pests and Pathogens in the Five Beekeeping Regions of Mexico

Mexico is a major honey producer, but not much information exists about the health status of honey bees (Apis mellifera L.) in the country. This study was conducted to determine the sanitary status of adult honey bees in Mexico's five beekeeping regions. Samples from 369 apiaries were diagnosed to identify pathogens such as Varroa destructor, which was quantified, Acarapis woodi, Nosema spp., and five viruses. Colonies were also inspected for the presence of the small hive beetle (SHB), Aethina tumida. Varroa destructor was found in 83.5% of the apiaries, with the Pacific Coast region having the highest prevalence (>95%) and rates (4.5% ± 0.6). Acarapis woodi was detected in only one apiary from the Pacific Coast, whereas Nosema spp. were prevalent in 48.5% of the apiaries, with the highest and lowest frequencies in the Yucatan Peninsula and North regions (64.6% and 10.2%, respectively). For viruses, deformed wing virus (DWV) was detected in 26.1% of the apiaries, with the highest frequency in the Pacific Coast region (44.7%). Israeli acute paralysis virus (IAPV) was diagnosed in 3.2% of the samples and sacbrood bee virus (SBV) in 23.3% of them, with the highest frequency in the High Plateau region (36.4%). Chronic bee paralysis and Kashmir bee viruses were not detected. SHB prevalence was 25.2% nationwide, with the highest frequency in the Yucatan Peninsula (39.2%). This study shows that the most common parasites of adult honey bees in Mexico are V. destructor and Nosema spp., and that the most prevalent virus is DWV, whereas SHB is highly prevalent in the Yucatan Peninsula. This information could be useful to design disease control strategies for honey bee colonies in different regions of Mexico.

Genotype, but Not Climate, Affects the Resistance of Honey Bees (Apis mellifera) to Viral Infections and to the Mite Varroa destructor

This study was conducted to analyze the effect of genotype and climate on the resistance of honey bee (Apis mellifera) colonies to parasitic and viral diseases. The prevalence and intensity of parasitism by Varroa destructor, or infection by Nosema spp., and four honey bee viruses were determined in 365 colonies of predominantly European or African ancestry (descendants of A. m. scutellata) in subtropical and temperate regions of Mexico. Varroa destructor was the most prevalent parasite (95%), whilst N. ceranae was the least prevalent parasite (15%). Deformed wing virus (DWV) and black queen cell virus (BQCV) were the only viruses detected, at frequencies of 38% and 66%, respectively. Varroa destructor was significantly more prevalent in colonies of European ancestry (p < 0.05), and the intensity of parasitism by V. destructor or infection by DWV and BQCV was also significantly higher in colonies of European descent than in African descent colonies (p < 0.01), although no genotype−parasite associations were found for N. ceranae. Additionally, significant and positive correlations were found between V. destructor and DWV levels, and the abundance of these pathogens was negatively correlated with the African ancestry of colonies (p < 0.01). However, there were no significant effects of environment on parasitism or infection intensity for the colonies of both genotypes. Therefore, it is concluded that the genotype of honey bee colonies, but not climate, influences their resistance to DWV, BQCV, and V. destructor.

Molecular Detection and Differentiation of Arthropod, Fungal, Protozoan, Bacterial and Viral Pathogens of Honeybees

The honeybee Apis mellifera is highly appreciated worldwide because of its products, but also as it is a pollinator of crops and wild plants. The beehive is vulnerable to infections due to arthropods, fungi, protozoa, bacteria and/or viruses that manage to by-pass the individual and social immune mechanisms of bees. Due to the close proximity of bees in the beehive and their foraging habits, infections easily spread within and between beehives. Moreover, international trade of bees has caused the global spread of infections, several of which result in significant losses for apiculture. Only in a few cases can infections be diagnosed with the naked eye, by direct observation of the pathogen in the case of some arthropods, or by pathogen-associated distinctive traits. Development of molecular methods based on the amplification and analysis of one or more genes or genomic segments has brought significant progress to the study of bee pathogens, allowing for: (i) the precise and sensitive identification of the infectious agent; (ii) the analysis of co-infections; (iii) the description of novel species; (iv) associations between geno- and pheno-types and (v) population structure studies. Sequencing of bee pathogen genomes has allowed for the identification of new molecular targets and the development of specific genotypification strategies.

Study of Morphological Features in Pre-Imaginal Honey Bee Impaired by Varroa destructor by Means of Computer Tomography

Simple Summary

The Varroa mite (Varroa destructor) is the most important natural pest of the honey bee, Apis mellifera, worldwide. The extent to which impairments in honey bees occur concomitantly upon infestation by this parasite greatly varies. Inter alia, the Varroa mite causes developmental disorders mediated by deformed wing virus in this host. Although there is a plethora of information regarding the consequences of this parasitism in the fully developed stage, data concerning the pre-imaginal honey bee stage inside the comb are rather scarce. In this study, morphological differences in the main body parts of the honey bee during the development stages of both intact and parasitized larvae were measured inside the comb by means of computed tomography. The images obtained reveal a visualization of the harmful effects of the Varroa mite on the pre-imaginal host. Our results demonstrate that the deformation of certain body parts was due to the presence of the parasite. Deformity, as the most conspicuous sign of infestation, is coupled with a decrease in the total-body size and abdomen size together with a disproportionate ratio of different body parts. In summary, information on the impairment of honey bee development triggered by the Varroa mite gives the opportunity to assess the damage caused by this serious pest, which occurs latently in honey bees.

Abstract

The honey bee (Apis mellifera L. 1778) is an essential element in maintaining the diversity of the biosphere and food production. One of its most important parasites is Varroa destructor, Anderson and Trueman, 2000, which plays a role in the vectoring of deformed wing virus (DWV) in honey bee colonies. Our aim was to measure the potential morphometric changes in the pre-imaginal stage of A. mellifera caused by varroosis by means of computed tomography, hence supplying evidence for the presumable role that V. destructor plays as a virus vector. Based on our results, the developmental disorders in honey bees that ensued during the pre-imaginal stages were evident. The total-body length and abdomen length of parasitized specimens were shorter than those of their intact companions. In addition, the calculated quotients of the total-body/abdomen, head/thorax, and head/abdomen in parasitized samples were significantly altered upon infestation. In our view, these phenotypical disorders can also be traced to viral infection mediated by parasitism, which was confirmed by reverse transcriptase polymerase chain reaction (RT-PCR) analysis. Capitalizing on a non-destructive method, our study reveals the deformation of the honey bee due to mite parasitism and the intermediary role this pest plays in viral infection, inside the brood cell.

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.

The Effect of Covert and Overt Infections on Disease Dynamics in Honey-Bee Colonies

Viral diseases of honey bees are important economically and ecologically and have been widely modelled. The models reflect the fact that, in contrast to the typical case for vertebrates, invertebrates cannot acquire immunity to a viral disease, so they are of SIS or (more often) SI type. Very often, these diseases may be transmitted vertically as well as horizontally, by vectors as well as directly, and through the environment, although models do not generally reflect all these transmission mechanisms. Here, we shall consider an important additional complication the consequences of which have yet to be fully explored in a model, namely that both infected honey bees and their vectors may best be described using more than one infection class. For honey bees, we consider three infection classes. Covert infections occur when bees have the virus under control, such that they do not display symptoms of the disease, and are minimally or not at all affected by it. Acutely overtly infected bees often exhibit severe symptoms and have a greatly curtailed lifespan. Chronically overtly infected bees typically have milder symptoms and a moderately shortened lifespan. For the vector, we consider just two infection classes which are covert infected and overt infected as has been observed in deformed-wing virus (DWV) vectored by varroa mites. Using this structure, we explore the impact of spontaneous transition of both mites and bees from a covertly to an overtly infected state, which is also a novel element in modelling viral diseases of honey bees made possible by including the different infected classes. The dynamics of these diseases are unsurprisingly rather different from the dynamics of a standard SI or SIS disease. In this paper, we highlight how our compartmental structure for infection in honey bees and their vectors impact the disease dynamics observed, concentrating in particular on DWV vectored by varroa mites. If there is no spontaneous transition, then a basic reproduction number \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$R_0$$\end{document}R0 exists. We derive a condition for \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$R_0>1$$\end{document}R0>1 that reflects the complexities of the system, with components for vertical and for direct and vector-mediated horizontal transmission, using the directed graph of the next-generation matrix of the system. Such a condition has never previously been derived for a honey-bee–mite–virus system. When spontaneous transitions do occur, then \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$R_0$$\end{document}R0 no longer exists, but we introduce a modification of the analysis that allows us to determine whether (i) the disease remains largely covert or (ii) a substantial outbreak of overt disease occurs.

Evaluation of Dry and Wet Formulations of Oxalic Acid, Thymol, and Oregano Oil for Varroa Mite (Acari: Varroidae) Control in Honey Bee (Hymenoptera: Apidae) Colonies

The efficacy and safety of dry and wet formulations of three nonsynthetic compounds, oxalic acid (OA), thymol (T), and oregano oil (OO), for the control of Varroa destructor Anderson and Trueman infestations in honey bee (Apis mellifera Linnaeus) colonies were determined. The treatments were OA in dust, OA diluted in glycerin solvent embedded in a towel, T in dust, T in glycerin solvent and towel, OO in dry microcapsules, OO in glycerin solvent and towel, and the control. The treatments were applied weekly for 4 wk during the fall season. The rates of acaricide efficacy, weekly mite fall, bee mortality, colony survivorship, and strength, were determined for each of the treatments. All formulations, with the exemption of OO microcapsules, were effective at controlling infestations of V. destructor. The most effective formulations were T dust (96.6%), T glycerin (92.4%), and OA glycerin (79%). More than 85% of the mites were killed during the first 2 wk of treatment with T formulations, compared to less than 30% for the OA glycerin formulation. The lowest efficacy rate was for OO microcapsules (21.3%), and the only treatment that significantly increased bee mortality was OA glycerin. The rates of winter colony survival and honey bee populations were related to the varroacidal efficacy of the formulations. The implications of these findings are discussed.

Selective Breeding for Low and High Varroa destructor Growth in Honey Bee (Apis mellifera) Colonies: Initial Results of Two Generations

Simple Summary

The mite Varroa destructor is considered the most damaging parasite of honey bees worldwide. Beekeepers use synthetic chemical products to control mite infestations in colonies, but the parasites soon develop resistance to them, which compromises their control. One alternative control strategy is the development of Varroa-resistant honey bees. Therefore, a breeding program was initiated to select for lower and higher rates of Varroa-population growth (LVG and HVG, respectively) and deformed wing virus (DWV) levels, which is transmitted by the mites. After two years of bidirectional selection, LVG colonies had Varroa population increases over the summer of 1.7 fold compared to 9.6 fold for HVG colonies. Additionally, HVG colonies had higher mite infestation rates in adult bees compared to LVG colonies. DWV presence and levels were higher in HVG colonies than in LVG colonies and winter mortality rates were 26% and 14% for the HVG and LVG bee types, respectively. The results of this study thus far indicate that selection for LVG may result in colonies with lower Varroa infestation rates, lower prevalence, and levels of DWV and higher colony winter survivorship. Future work will focus on determining mechanisms responsible for genetic differences and in identifying genes associated with Varroa-resistance in honey bees.

Abstract

After two years of bidirectional selection for low and high rates of Varroa destructor population growth (LVG and HVG, respectively) in honey bee (Apis mellifera) colonies in Ontario, Canada, significant differences between the two genotypes were observed. LVG colonies had V. destructor population increases over the summer of 1.7 fold compared to 9.6 fold for HVG colonies by Generation 2. Additionally, HVG colonies had significantly higher mite infestation rates in adult bees compared to LVG colonies for both selected generations. DWV prevalence and levels were significantly higher in HVG colonies than in LVG colonies in Generation 1 but not in Generation 2. Winter mortality rates of Generation 1 colonies were significantly different at 26% and 14% for the HVG and LVG genotypes, respectively. The results of this study thus far indicate that selection for LVG may result in colonies with lower V. destructor infestation rates, lower prevalence, and levels of DWV and higher colony winter survivorship. Future work will focus on determining what mechanisms are responsible for the genotypic differences, estimating genetic parameters, and molecular analyses of the genotypes to identify candidate genes associated with resistance to V. destructor and DWV that could potentially be used for marker-assisted selection.

Interaction of Varroa destructor and Sublethal Clothianidin Doses during the Larval Stage on Subsequent Adult Honey Bee (Apis mellifera L.) Health, Cellular Immunity, Deformed Wing Virus Levels and Differential Gene Expression

Honeybees (Apis mellifera L.) are exposed to many parasites, but little is known about interactions with abiotic stressors on their health, particularly when affected as larvae. Larvae were exposed singly and in combination to the parasitic mite Varroa destructor and three sublethal doses of the neonicotinoid insecticide clothianidin to evaluate their effects on survivorship, weight, haemocyte counts, deformed wing virus (DWV) levels and gene expression of the adult bees that subsequently developed. Clothianidin significantly reduced bee weight at the highest dose and was associated with an increase in haemocyte counts at the lowest dose, whereas V. destructor parasitism increased DWV levels, reduced bee emergence, lowered weight and reduced haemocyte counts. An interaction between the two stressors was observed for weight at emergence. Among the differentially expressed genes (DEGs), V. destructor infestation resulted in broader down-regulatory effects related to immunity that was often shared with the combined stressors, while clothianidin resulted in a broader up-regulatory effect more related to central metabolic pathways that was often shared with the combined stressors. Parasites and abiotic stressors can have complex interactions, including additive effects on reduced weight, number of up-regulated DEGs and biological pathways associated with metabolism.

Interaction of field realistic doses of clothianidin and Varroa destructor parasitism on adult honey bee (Apis mellifera L.) health and neural gene expression, and antagonistic effects on differentially expressed genes

While many studies have examined the effects of neonicotinoid insecticides and the parasitic mite Varroa destructor on honey bees (Apis mellifera), more information on the combined effects of such stressors on gene expression, including neural related genes, and their impact on biological pathways is needed. This study analyzed the effects of field realistic concentrations of the neonicotinoid clothianidin on adult bees infested and not infested with V. destructor over 21 consecutive days and then determined bee survivorship, weight, deformed wing virus (DWV) levels and gene expression. V. destructor parasitism with or without clothianidin exposure was significantly associated with decreased survivorship, weight loss and higher DWV levels, while clothianidin exposure was only associated with higher levels of DWV. Expression analysis of the neural genes AmNlg-1, BlCh and AmAChE-2 showed that V. destructor caused a significant down-regulation of all of them, whereas clothianidin caused a significant down-regulation of only AmNrx-1 and BlCh. An interaction was only detected for AmNrx-1 expression. RNAseq analysis showed that clothianidin exposure resulted in 6.5 times more up-regulated differentially expressed genes (DEGs) than V. destructor alone and 123 times more than clothianidin combined with V. destructor. Similar results were obtained with down-regulated DEGs, except for a higher number of DEGs shared between V. destructor and the combined stressors. KEGG (Kyoto Encyclopedia of Genes and Genomes) biological pathway analysis of the DEGs showed that the stressor linked to the highest number of KEGG pathways was clothianidin, followed by V. destructor, and then considerably fewer number of KEGG pathways with the combined stressors. The reduced numbers of DEGs and KEGG pathways associated with the DEGs for the combined stressors compared to the stressors alone indicates that the interaction of the stressors is not additive or synergistic, but antagonistic. The possible implications of the antagonistic effect on the number of DEGs are discussed.

Multiple transmission routes sustain high prevalence of a virulent parasite in a butterfly host

Understanding factors that allow highly virulent parasites to reach high infection prevalence in host populations is important for managing infection risks to human and wildlife health. Multiple transmission routes have been proposed as one mechanism by which virulent pathogens can achieve high prevalence, underscoring the need to investigate this hypothesis through an integrated modelling-empirical framework. Here, we examine a harmful specialist protozoan infecting monarch butterflies that commonly reaches high prevalence (50–100%) in resident populations. We integrate field and modelling work to show that a combination of three empirically-supported transmission routes (vertical, adult transfer and environmental transmission) can produce and sustain high infection prevalence in this system. Although horizontal transmission is necessary for parasite invasion, most new infections post-establishment arise from vertical transmission. Our study predicts that multiple transmission routes, coupled with high parasite virulence, can reduce resident host abundance by up to 50%, suggesting that the protozoan could contribute to declines of North American monarchs.

Impact of Varroa destructor and deformed wing virus on emergence, cellular immunity, wing integrity and survivorship of Africanized honey bees in Mexico

The ectoparasitic mite Varroa destructor is the primary health problem of honey bees (Apis mellifera) worldwide. Africanized honey bees in Brazil have demonstrated tolerance to the mite, but there is controversy about the degree of mite tolerance of Africanized bees in other countries. This study was conducted to quantify the effect of V. destructor parasitism on emergence, hemocyte concentration, wing integrity and longevity of Africanized honey bees in Mexico. Africanized bee brood were artificially infested with V. destructor mites and held in an incubator until emergence as adults and compared to non-infested controls. Deformed wing virus (DWV) presence was determined in the mites used to infest the bees. After emergence, the bees were maintained in an incubator to determine survivorship. The percentage of worker bees that emerged from parasitized cells (69%) was significantly lower than that of bees emerged from non-infested cells (96%). Newly-emerged parasitized bees had a significantly lower concentration of hemocytes in the hemolymph than non-parasitized bees. Additionally, the proportion of bees with deformed wings that emerged from V. destructor-parasitized cells was significantly higher (54%) than that of the control group (0%). The mean survival time of bees that emerged from infested and non-infested cells was 8.5 ± 0.3 and 14.4 ± 0.4 days, respectively, and the difference was significant. We conclude that V. destructor parasitism and DWV infections kill, cause deformities and inhibit cellular immunity in developing Africanized honey bees, and significantly reduce the lifespan of adult bees in Mexico. These results suggest that the tolerance of Africanized bees to V. destructor is related to adult bee mechanisms.

Effects of sublethal doses of clothianidin and/or V. destructor on honey bee (Apis mellifera) self-grooming behavior and associated gene expression

Little is known about the combined effects of stressors on social immunity of honey bees (Apis mellifera) and related gene expression. The interaction between sublethal doses of a neurotoxin, clothianidin, and the ectoparasite, Varroa destructor, was examined by measuring differentially expressed genes (DEGs) in brains, deformed wing virus (DWV) and the proportion and intensity of self-grooming. Evidence for an interaction was observed between the stressors in a reduction in the proportion of intense groomers. Only the lowest dose of clothianidin alone reduced the proportion of self-groomers and increased DWV levels. V. destructor shared a higher proportion of DEGs with the combined stressors compared to clothianidin, indicating that the effects of V. destructor were more pervasive than those of clothianidin when they were combined. The number of up-regulated DEGs were reduced with the combined stressors compared to clothianidin alone, suggesting an interference with the impacts of clothianidin. Clothianidin and V. destructor affected DEGs from different biological pathways but shared impacts on pathways related to neurodegenerative disorders, like Alzheimer's, which could be related to neurological dysfunction and may explain their negative impacts on grooming. This study shows that the combination of clothianidin and V. destructor resulted in a complex and non-additive interaction.

Continuous release of oregano oil effectively and safely controls Varroa destructor infestations in honey bee colonies in a northern climate

The ectoparasitic mite Varroa destructor is responsible for the death of millions of honey bee (Apis mellifera) colonies worldwide. Testing potential miticide compounds with different delivery methods that effectively control V. destructor and have low toxicity for honey bees is crucial to manage this parasite in hives. We determined the varroacide efficacy of three natural compounds delivered to hives with three application methods over a 4-week period. Oxalic acid in a sucrose solution was applied impregnated in cardboard (T1). A mixture of oregano and clove oils in an ethanol-gelatin solution was applied impregnated in absorbent pads (T2). Oregano oil alone was delivered using electric vaporizers (T3) to test the hypothesis that continuous release of miticides increases the varroacidal efficacy of essential oils. The varroa mite control rates for treatments T1-T3 were 76.5 ± 7.11, 57.8 ± 12.79 and 97.4 ± 0.68%, respectively, and there were no differences for bee mortality between control and treatments 1 and 3. Additionally, most mites were killed in the first 2 weeks in T3 colonies compared to the last 2 weeks in colonies of the other treatments. These results demonstrate the importance of continuously releasing natural miticides to achieve safe and high rates of mite control in hives. They also show that oregano oil may be an effective miticide against V. destructor infestations in colonies.

Differential Gene Expression Associated with Honey Bee Grooming Behavior in Response to Varroa Mites

Honey bee (Apis mellifera) grooming behavior is an important mechanism of resistance against the parasitic mite Varroa destructor. This research was conducted to study associations between grooming behavior and the expression of selected immune, neural, detoxification, developmental and health-related genes. Individual bees tested in a laboratory assay for various levels of grooming behavior in response to V. destructor were also analyzed for gene expression. Intense groomers (IG) were most efficient in that they needed significantly less time to start grooming and fewer grooming attempts to successfully remove mites from their bodies than did light groomers (LG). In addition, the relative abundance of the neurexin-1 mRNA, was significantly higher in IG than in LG, no groomers (NG) or control (bees without mite). The abundance of poly U binding factor kd 68 and cytochrome p450 mRNAs were significantly higher in IG than in control bees. The abundance of hymenoptaecin mRNA was significantly higher in IG than in NG, but it was not different from that of control bees. The abundance of vitellogenin mRNA was not changed by grooming activity. However, the abundance of blue cheese mRNA was significantly reduced in IG compared to LG or NG, but not to control bees. Efficient removal of mites by IG correlated with different gene expression patterns in bees. These results suggest that the level of grooming behavior may be related to the expression pattern of vital honey bee genes. Neurexin-1, in particular, might be useful as a bio-marker for behavioral traits in bees.