Reproductive biology of Varroa destructor in Africanized honey bees (Apis mellifera)

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.

Morphotype and haplotype identification of Varroa destructor (Acari: Varroidae), and its importance for apiculture in Nicaragua

The ectoparasite Varroa (Acari: Varroidae) is considered to be the main pest of honey bees (Apis mellifera L.) in Nicaragua. The aim of this study was to determine morphotypes and mitochondrial haplotypes of the Varroa mites, related to infestation levels in A. mellifera hives in Nicaragua in a cross-sectional study (2013-2016). Samples were taken from 34 sentinel apiaries in five geographical zones; from 713 Varroa females collected during the study, 153 were selected for measurement of various morphometric characters for further classification into morphotypes. The mitochondrial haplotype was assigned to one of the two (Korean or Japanese), using the restriction by SacI of the PCR product of a fragment of the gene CO-I. Morphometric measurement and sequencing revealed the affiliation to the species Varroa destructor with a mean (± SD) body width of 1699.1 ± 60.2 µm and a body length of 1161.1 ± 34.9 µm. Body characters were significantly different among the 34 apiaries. Varroa destructor samples were classified into four morphotypes, with no significant differences in the geographical zones. As GAGCTC SacI enzyme cleavage sequences were not observed, all PCR products were identified as V. destructor Korean haplotype. The CO-I gene nucleotide sequences from two samples V. destructor showed both 100% similarity with the Korean haplotype and 99.8% similarity to the Japanese haplotype. Although the V. destructor mite was identified as a Korean haplotype, host-parasite association in 2 decades has led into a balance without entering into severe losses in the Nicaraguan apiculture.

Genetic markers for the resistance of honey bee to Varroa destructor

In the mid-20th century, the first case of infection of European bees Apis mellifera L. with the ectoparasite mite Varroa destructor was recorded. The original host of this mite is the Asian bee Apis cerana. The mite V. destructor was widespread throughout Europe, North and South America, and Australia remained the only continent free from this parasite. Without acaricide treatment any honeybee colony dies within 1–4 years. The use of synthetic acaricides has not justified itself – they make beekeeping products unsuitable and mites develop resistance to them, which forces the use of even greater concentrations that can be toxic to the bees. Therefore, the only safe measure to combat the mite is the use of biological control methods. One of these methods is the selection of bee colonies with natural mite resistance. In this article we summarize publications devoted to the search for genetic markers associated with resistance to V. destructor. The first part discusses the basic mechanisms of bee resistance (Varroa sensitive hygienic behavior and grooming) and methods for their assessment. The second part focuses on research aimed at searching for loci and candidate genes associated with resistance to varroosis by mapping quantitative traits loci and genome-wide association studies. The third part summarizes studies of the transcriptome profile of Varroa resistant bees. The last part discusses the most likely candidate genes – potential markers for breeding Varroa resistant bees. Resistance to the mite is manifested in a variety of phenotypes and is under polygenic control. The establishing of gene pathways involved in resistance to Varroa will help create a methodological basis for the selection of Varroa resistant honeybee colonies.

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

Background

In spite of the implementation of control strategies in honey bee (Apis mellifera) keeping, the invasive parasitic mite Varroa destructor remains one of the main causes of colony losses in numerous countries. Therefore, this parasite represents a serious threat to beekeeping and agro-ecosystems that benefit from the pollination services provided by honey bees. To maintain their stocks, beekeepers have to treat their colonies with acaricides every year. Selecting lineages that are resistant to infestations is deemed to be a more sustainable approach.

Review

Over the last three decades, numerous selection programs have been initiated to improve the host–parasite relationship and to support honey bee survival in the presence of the parasite without the need for acaricide treatments. Although resistance traits have been included in the selection strategy of honey bees, it has not been possible to globally solve the V. destructor problem. In this study, we review the literature on the reasons that have potentially limited the success of such selection programs. We compile the available information to assess the relevance of selected traits and the potential environmental effects that distort trait expression and colony survival. Limitations to the implementation of these traits in the field are also discussed.

Conclusions

Improving our knowledge of the mechanisms underlying resistance to V. destructor to increase trait relevance, optimizing selection programs to reduce environmental effects, and communicating selection outcomes are all crucial to efforts aiming at establishing a balanced relationship between the invasive parasite and its new host.

Population abundance of Varroa destructor and its effects on Apis mellifera scutellata colonies in Kenya

The devastating effects of Varroa destructor Anderson and Trueman on Western honeybee colonies (Apis mellifera L.) have been well documented. Not only do these mites cause physical damage to parasitized individuals when they feed on them, but they also transmit viruses and other pathogens, weaken colonies and ultimately may cause their death. Unlike the subspecies of European origin, the honeybees of African origin suffer less from mite infestations. Absconding is one of the factors contributing to low V. destructor population in honeybee colonies as it creates a brood-free period. For a long time, researchers hypothesized that absconding was the main mechanism to control the parasite. The effects of V. destructor are well documented under temperate climatic conditions with a break during winter. Therefore, our study aimed at investigating the impact of V. destructor population growth on colony size, absconding and productivity under natural infestation levels of a tropical/subtropical climate with continuous brood production. We measured several characteristics related to the mite populations, the bee colonies and the resources of the bee colonies for a period of 8 months. The seven colonies that absconded during the study period were not influenced by densities of V. destructor. Absconding of the colonies occurred as a result of low numbers of capped brood. Mite densities were generally low throughout the study period (ranged between 26.9 and 59.8 mites per month) but were positively associated with adult bee densities. The amount of open and capped brood was positively associated with densities of V. destructor in the brood and negatively associated with denisities of V. destructor on screen boards, which appeared as extremely important factors that should be monitored regularly alongside colony stores and availability of pollen.

Natural selection, selective breeding, and the evolution of resistance of honeybees (Apis mellifera) against Varroa

We examine evidence for natural selection resulting in Apis mellifera becoming tolerant or resistant to Varroa mites in different bee populations. We discuss traits implicated in Varroa resistance and how they can be measured. We show that some of the measurements used are ambiguous, as they measure a combination of traits. In addition to behavioural traits, such as removal of infested pupae, grooming to remove mites from bees or larval odours, small colony size, frequent swarming, and smaller brood cell size may also help to reduce reproductive rates of Varroa. Finally, bees may be tolerant of high Varroa infections when they are resistant or tolerant to viruses implicated in colony collapse. We provide evidence that honeybees are an extremely outbreeding species. Mating structure is important for how natural selection operates. Evidence for successful natural selection of resistance traits against Varroa comes from South Africa and from Africanized honeybees in South America. Initially, Varroa was present in high densities and killed about 30% of the colonies, but soon after its spread, numbers per hive decreased and colonies survived without treatment. This shows that natural selection can result in resistance in large panmictic populations when a large proportion of the population survives the initial Varroa invasion. Natural selection in Europe and North America has not resulted in large-scale resistance. Upon arrival of Varroa, the frequency of traits to counter mites and associated viruses in European honey bees was low. This forced beekeepers to protect bees by chemical treatment, hampering natural selection. In a Swedish experiment on natural selection in an isolated mating population, only 7% of the colonies survived, resulting in strong inbreeding. Other experiments with untreated, surviving colonies failed because outbreeding counteracted the effects of selection. If loss of genetic variation is prevented, colony level selection in closed mating populations can proceed more easily, as natural selection is not counteracted by the dispersal of resistance genes. In large panmictic populations, selective breeding can be used to increase the level of resistance to a threshold level at which natural selection can be expected to take over.

Assessing virulence of Varroa destructor mites from different honey bee management regimes

The mite Varroa destructor is an important honey bee parasite that causes substantial losses of honey bee colonies worldwide. Evolutionary theory suggests that the high densities at which honey bees are managed in large-scale beekeeping settings will likely select for mites with greater growth and virulence, thereby potentially explaining the major damage done by these mites. We tested this hypothesis by collecting mites from feral bee colonies, "lightly" managed colonies (those from small-scale sedentary operations), and "heavily" managed colonies (those from large-scale operations that move thousands of colonies across the US on a yearly basis). We established 8 apiaries, each consisting of 11 colonies from a standardized lightly managed bee background that were cleared of mites, and artificially infested each apiary with controlled numbers of mites from feral, lightly managed, or heavily managed bees or left uninoculated as negative control. We monitored the colonies for more than 2 years for mite levels, colony strength (adult bee population, brood coverage, and honey storage), and survival. As predicted by evolutionary theory, we found that colonies inoculated with mites from managed backgrounds had increased V. destructor mite levels relative to those with mites from feral colonies or negative controls. However, we did not see a difference between heavily and lightly managed colonies, and these higher mite burdens did not translate into greater virulence, as measured by reductions in colony strength and survival. Our results suggest that human management of honey bee colonies may favor the increased population growth rate of V. destructor, but that a range of potential confounders (including viral infections and genotype-by-genotype interactions) likely contribute to the relationship between mite reproduction and virulence.

Low fertility, fecundity and numbers of mated female offspring explain the lower reproductive success of the parasitic mite Varroa destructor in African honeybees

Although Varroa destructor is the most serious ecto-parasite to the honeybee, Apis mellifera L., some honeybee populations such as Apis mellifera scutellata in Kenya can survive mite infestations without treatment. Previously, we reported that grooming behaviour could be a potential tolerant mechanism expressed by this honeybee subspecies towards mite infestation. However, both hygienic and grooming behaviours could not explain the lower mite-infestation levels recorded in these colonies. Here, we investigated the involvement of other potential resistant mechanisms including suppression of mite reproduction in worker brood cells of A. m. scutellata to explain the low mite numbers in their colonies. High infertility rates (26-27%) and percentages of unmated female offspring (39-58%) as well as low fecundity (1.7-2.2, average offspring produced) were identified as key parameters that seem to interact with one another during different seasons to suppress mite reproduction in A. m. scutellata colonies. We also identified offspring mortality in both sexes and absence of male offspring as key factors accounting for the low numbers of mated daughter mites produced in A. m. scutellata colonies. These results suggest that reduced mite reproductive success could explain the slow mite population growth in A. m. scutellata colonies.

Behavioral Modulation of Infestation by Varroa destructor in Bee Colonies. Implications for Colony Stability

Colony Collapse Disorder (CCD) has become a global problem for beekeepers and for the crops that depend on bee pollination. While many factors are known to increase the risk of colony collapse, the ectoparasitic mite Varroa destructor is considered to be the most serious one. Although this mite is unlikely to cause the collapse of hives itself, it is the vector for many viral diseases which are among the likely causes for Colony Collapse Disorder. The effects of V. destructor infestation differ from one part of the world to another, with greater morbidity and higher colony losses in European honey bees (EHB) in Europe, Asia and North America. Although this mite has been present in Brazil for many years, there have been no reports of colony losses amongst Africanized Honey Bees (AHB). Studies carried out in Mexico have highlighted different behavioral responses by the AHB to the presence of the mite, notably as far as grooming and hygienic behavior are concerned. Could these explain why the AHB are less susceptible to Colony Collapse Disorder? In order to answer this question, we have developed a mathematical model of the infestation dynamics to analyze the role of resistance behavior by bees in the overall health of the colony, and as a consequence, its ability to face epidemiological challenges.

Factors influencing the prevalence and infestation levels of Varroa destructor in honeybee colonies in two highland agro-ecological zones of Uganda

Varroa mites are ecto-parasites of honeybees and are a threat to the beekeeping industry. We identified the haplotype of Varroa mites and evaluated potential factors that influence their prevalence and infestation levels in the eastern and western highland agro-ecological zones of Uganda. This was done by collecting samples of adult worker bees between December 2014 and September 2015 in two sampling moments. Samples of bees were screened for Varroa using the ethanol wash method and the mites were identified by molecular techniques. All DNA sequences obtained from sampled mite populations in the two zones were 100 % identical to the Korean Haplotype (AF106899). Mean mite prevalence in the apiaries was 40 and 53 % for the western and eastern zones, respectively, during the first sampling. Over the second sampling, mean mite prevalence increased considerably in the western (59 %) but not in the eastern (51 %) zone. Factors that were associated with Varroa mite infestation levels include altitude, nature of apiary slope and apiary management practices during the first sampling. Our results further showed that Varroa mites were spreading from lower to higher elevations. Feral colonies were also infested with Varroa mites at infestation levels not significantly different from those in managed colonies. Colony productivity and strength were not correlated to mite infestation levels. We recommend a long-term Varroa mite monitoring strategy in areas of varying landscape and land use factors for a clear understanding of possible changes in mite infestation levels among African honeybees for informed decision making.

Effects of Nosema ceranae and thiametoxam in Apis mellifera: A comparative study in Africanized and Carniolan honey bees

Multiple stressors, such as chemicals and pathogens, are likely to be detrimental for the health and lifespan of Apis mellifera, a bee species frequently exposed to both factors in the field and inside hives. The main objective of the present study was to evaluate comparatively the health of Carniolan and Africanized honey bees (AHB) co-exposed to thiamethoxam and Nosema ceranae (N. ceranae) spores. Newly-emerged worker honey bees were exposed solely with different sublethal doses of thiamethoxam (2% and 0.2% of LD50 for AHB), which could be consumed by bees under field conditions. Toxicity tests for the Carniolan bees were performed, and the LD50 of thiamethoxam for Carniolan honey bees was 7.86 ng bee(-1). Immunohistological analyses were also performed to detect cell death in the midgut of thiamethoxam and/or N. ceranae treated bees. Thiamethoxam exposure had no negative impact on Nosema development in experimental conditions, but it clearly inhibited cell death in the midgut of thiamethoxam and Nosema-exposed bees, as demonstrated by immunohistochemical data. Indeed, thiamethoxam exposure only had a minor synergistic toxic effect on midgut tissue when applied as a low dose simultaneously with N. ceranae to AHB and Carniolan honey bees, in comparison with the effect caused by both stressors separately. Our data provides insights into the effects of the neonicotenoid thiamethoxam on the AHB and Carniolan honey bee life span, as well as the effects of simultaneous application of thiamethoxam and N. ceranae spores to honey bees.

Impact of Varroa destructor on honeybee (Apis mellifera scutellata) colony development in South Africa

The devastating effects of Varroa destructor Anderson & Trueman on European honeybee colonies (Apis mellifera L.) have been well documented. Not only do these mites cause physical damage to parasitised individuals when they feed on them, they also transmit viruses and other pathogens, weaken colonies and can ultimately cause their death. Nevertheless, not all honeybee colonies are doomed once Varroa mites become established. Some populations, such as the savannah honeybee, A. m. scutellata, have become tolerant after the introduction of the parasite and are able to withstand the presence of these mites without the need for acaricides. In this study, we measured daily Varroa mite fall, Varroa infestation rates of adult honeybees and worker brood, and total Varroa population size in acaricide treated and untreated honeybee colonies. In addition, honeybee colony development was compared between these groups in order to measure the cost incurred by Varroa mites to their hosts. Daily Varroa mite fall decreased over the experimental period with different dynamics in treated and untreated colonies. Varroa infestation rates in treated adult honeybees and brood were lower than in untreated colonies, but not significantly so. Thus, indicating a minimal benefit of treatment thereby suggesting that A. m. scutellata have the ability to maintain mite populations at low levels. We obtained baseline data on Varroa population dynamics in a tolerant honeybee over the winter period. Varroa mites appeared to have a low impact on this honeybee population, given that colony development was similar in the treated and untreated colonies.

Spread and strain determination of Varroa destructor (Acari: Varroidae) in Madagascar since its first report in 2010

Varroa destructor is a major pest in world beekeeping. It was first detected in Madagascar in 2010 on the endemic honeybee Apis mellifera unicolor. To evaluate V. destructor spread dynamics in Madagascar a global survey was conducted in 2011-2012. A total of 695 colonies from 30 districts were inspected for the presence of mites. 2 years after its introduction, nine districts were found infested. Varroa destructor spread was relatively slow compared to other countries with a maximum progression of 40 km per year, the five newly infested districts being located next to the first infested ones. The incidence of mite infestation was also investigated by monitoring 73 colonies from five apiaries during 1 year (2011-2012). Sixty percent of local colony mortality was recorded after 1 year of survey. Varroa destructor strain determination was done by partial sequencing of the cytochrome oxidase I gene of 13 phoretic mites sampled in five districts. A single V. destructor mitochondrial haplotype was detected, the Korean type, also present in the closest African countries. A global pathogen survey was also conducted on the colonies inspected for mite presence. The greater wax moth, Galleria mellonella has been found in all colonies all over the country. Two other pathogens and morphological abnormalities in workers, such as deformed wings, were found associated with only V. destructor presence. A prevention management plan must be implemented to delimit mite spread across the island.

The ectoparasite mite Varroa destructor Anderson and Trueman in southeastern Brazil apiaries: effects of the hygienic behavior of Africanized honey bees on infestation rates

In Brazil, the ectoparasitic mite of bees Varroa destructor Anderson and Trueman (Acari: Varroidae) remains at low levels of infestation causing no major damage. However, with the introduction and possible dominance of a new haplotype (K) of the mite, usually found in areas with high infestation rates (IR), it is necessary to monitor and select beehives that are resistant to the pest in order to avoid future problems. Several factors are listed as potentially being responsible for the dynamics of mite infestation, among which hygienic behavior (HB) stands out. In this context we sought to evaluate the HB of Africanized honey bees Apis mellifera L. (Hymenoptera: Apidae) compared with the mite IR in apiaries of two municipalities of southeastern Brazil (Taubaté and Viçosa). For the municipality of Taubaté, the average IR was 4.9% (3.4 to 5.8%), while the HB averaged 98.6% (96 to 100%). In Viçosa, the average mite IR was found to be 10.0% (5.4 to 21.0%) with an average value for HB of 57.7% (0 to 79.0%). Results from this research show that IR and HB were negatively correlated (R = -0.9627, P<0.01), suggesting that hives with higher HB have lower IR.

Bees brought to their knees: microbes affecting honey bee health

The biology and health of the honey bee Apis mellifera has been of interest to human societies for centuries. Research on honey bee health is surging, in part due to new tools and the arrival of colony-collapse disorder (CCD), an unsolved decline in bees from parts of the United States, Europe, and Asia. Although a clear understanding of what causes CCD has yet to emerge, these efforts have led to new microbial discoveries and avenues to improve our understanding of bees and the challenges they face. Here we review the known honey bee microbes and highlight areas of both active and lagging research. Detailed studies of honey bee-pathogen dynamics will help efforts to keep this important pollinator healthy and will give general insights into both beneficial and harmful microbes confronting insect colonies.

Development of a user-friendly delivery method for the fungus Metarhizium anisopliae to control the ectoparasitic mite Varroa destructor in honey bee, Apis mellifera, colonies

A user-friendly method to deliver Metarhizium spores to honey bee colonies for control of Varroa mites was developed and tested. Patty blend formulations protected the fungal spores at brood nest temperatures and served as an improved delivery system of the fungus to bee hives. Field trials conducted in 2006 in Texas using freshly harvested spores indicated that patty blend formulations of 10 g of conidia per hive (applied twice) significantly reduced the numbers of mites per adult bee, mites in sealed brood cells, and residual mites at the end of the 47-day experimental period. Colony development in terms of adult bee populations and brood production also improved. Field trials conducted in 2007 in Florida using less virulent spores produced mixed results. Patty blends of 10 g of conidia per hive (applied twice) were less successful in significantly reducing the number of mites per adult bee. However, hive survivorship and colony strength were improved, and the numbers of residual mites were significantly reduced at the end of the 42-day experimental period. The overall results from 2003 to 2008 field trials indicated that it was critical to have fungal spores with good germination, pathogenicity and virulence. We determined that fungal spores (1 × 10(10) viable spores per gram) with 98% germination and high pathogenicity (95% mite mortality at day 7) provided successful control of mite populations in established honey bee colonies at 10 g of conidia per hive (applied twice). Overall, microbial control of Varroa mite with M. anisopliae is feasible and could be a useful component of an integrated pest management program.