Embed Size px x x x x Abstract The rich variety of native honeybee subspecies and ecotypes in Europe oers a good geneticresource for selection towards Varroa resistance. There are some examples of mite resistance that have de-veloped as a consequence of natural selection in wild and managed European populations. However, mostcolonies are influenced by selective breeding and are intensively managed, including the regular use of miti-cides. We describe all characters used in European breeding programs to test for Varroa resistance. Some ofthem e.
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Landesbetrieb Landwirtschaft Hessen, Bieneninstitut, Erlenstr. Abstract The rich variety of native honeybee subspecies and ecotypes in Europe oers a good genetic resource for selection towards Varroa resistance. There are some examples of mite resistance that have developed as a consequence of natural selection in wild and managed European populations. However, most colonies are influenced by selective breeding and are intensively managed, including the regular use of miticides.
We describe all characters used in European breeding programs to test for Varroa resistance. Some of them e. Survival tests of pre-selected breeder colonies and drone selection under infestation pressure are new attempts to strengthen eects of natural selection within selective breeding programs.
Some perspectives for future breeding activities are discussed. Europe has a high diversity of climatic regions and natural biotopes. As honeybees are endemic to most of regions in Europe, they have evolved into numerous natural subspecies and ecotypes Ruttner, with dierent adaptive capacities.
This diversity oers rich potential genetic resources for selection on mite resistance. Wild honeybee populations under natural selection are very rare, not only as a consequence of Varroa infestation but also due to a lack of natural habitats in densely populated areas with intensive use of farmland and forests. Today, nearly the entire honeybee population of Europe is managed by the beekeeping industry, which involves regular chemical treatments for diseases and mites that can con-.
Breeding and selection techniques have a long tradition in European countries and are widely used with varying degrees of rigor. Starting at the end of the 19th century, beekeepers transported queens and colonies across the natural ranges of dierent subspecies and ecotypes.
This practice led to hybridisation followed by changes in the spread and frequency of certain genotypes. While the economically important subspecies A. Corresponding author: R. Bchler, ralph. Breeding programs are well established in many European countries, based on performance tests, statistical data analysis, and the mass propagation and controlled mating of.
The strong impact of selective breeding on the population is evident due to the significant reduction of genetic variability in European compared to wild African colonies Moritz et al. In the majority of selection and breeding programs, economic traits such as honey productivity and colony strength together with traits desirable for modern beekeeping such as gentle temper and low swarming tendency have been of predominant importance.
In contrast, disease resistance, viability, and adaptation to local conditions were considered less important, as deficiencies in these characters could often be compensated by pharmaceuticals, artificial feeding, and other management techniques.
Since Varroa destructor began spreading throughout Europe, the beekeeping industry has had to face a new situation. The regular use of chemical treatments has been accompanied by several disadvantages, such as high costs and labor, residues in bee products, and the selection of mites resistant to acaricides.
However, repeated high colony losses due to varroosis could not be prevented. Consequently, research on mite resistance of honey bees started in the s and continues to receive a large amount of scientific interest and practical attention in Europe. We consider resistance in honey bees as the ability of a bee population to survive without therapeutic treatments in a given environment and management system.
Resistance is therefore not an absolute trait, but rather has to be viewed as the result of successful interactions in a specific environment. High levels of resistance occur in some untreated European bee populations. However, as most colonies are under strong influence of modern beekeeping management, which includes regular use of acaricides and requeening with selected stock, Varroa resistance should be implemented on a broader scale in selective breeding activities.
This implementation depends on having suitable test characters and an eective coordination of breeding programs. Comparative tests between European bee strains and Primorski lines from the US allow for an evaluation of the European breeding eorts. Similar observations were reported by Seeley from a small bee population in the Arnot Forest in the northeastern United States. When Varroa mites first invaded France in the s, feral and untreated colonies were destroyed by the mites.
The first observations of natural VSB were made in in western France, near Le Mans, where feral and untreated colonies seemed to survive the mite infestation for a few years. In , 10 out of 12 of such untreated colonies were still surviving 5 of them survived for more than 11 years. At that time, 82 colonies that were untreated for at least 2 years were collected to characterize their survival without Varroa control.
The colonies were managed only to monitor their survival. They were allowed to swarm and replace their queens naturally. On average, the survival of those colonies was 7. Varroa populations were estimated between VSB and Varroa susceptible control colonies by counting natural mite mortality using a screened bottom board to collect the mites see Fries et al.
Traps were left in place constantly, and the mites falling down on the bottom board were counted one to three times a week depending on the amount of brood in the colonies.
In Avignon, from April to June , natural mite mortality was observed in 12 VSB colonies and 16 control Varroa susceptible colonies. The number of mites collected in VSB colonies was three times lower than in control colonies, which was statistically significant Le Conte et al.
The infestation of the VSB colonies from July to September, as measured by mite fall, decreased in both groups. After that period, Varroa populations increased more rapidly in control colonies than in VSB colonies. The VSB maintained lower. Figure 1. Number of mites collected per month on the bottom board mean S. Mites were not counted between October and December Varroa populations year round, suggesting that VSB colonies had developed mechanisms to inhibit the growth of Varroa populations.
Various hypotheses could explain this phenomenon. First, the bees may have developed behavioral or physiological resistance against the mite.
Martin et al. Thus, VSB could be more ecient in their ability to detect and destroy the mite from workers through grooming behavior, similar to the behavior of Apis cerana Peng et al.
They may also be more ecient in removing mite-infested pupae from the cells as reported in the MN Hygienic stock e. Indeed, preliminary observations show that VSB are able to detect and remove mite-infested pupae from cells Anderson and Le Conte, pers. Interestingly, gene-expression analysis of the VSB shows over-expression of a set of genes related to responsiveness to olfactory stimuli compared with Varroa susceptible bee colonies Navajas et al.
Harbo and Harris b suggested only a few genes to be involved in VSH behavior, which. The genes involved in olfaction in the VSB, VSH, and MN Hygienic stocks are potentially very interesting to characterize as they may be involved in the ability of these bees to detect and remove and thus resist Varroa.
Other characters such as the inhibition of Varroa reproduction, the duration of the post-capping stage, and thermoregulatory abilities may also contribute to the survival of the VSB. Moreover, an average of Fries et al. Dierential virulence of the mite also can be hypothesized to explain the survival of VSB.
After the first years of Varroa invasion in France, most of the untreated colonies were found dead with plenty of mites trapped in entire frames of dead sealed brood. Individual fitness of a mite in those cells was therefore. As a less-virulent parasite such that it would not kill the host and would have an increased individual fitness , the hypothesis of sub-populations of mites with dierent levels of virulence was tested using mitochondrial and nuclear microsatellites markers Navajas et al.
The structure of Varroa population in Europe was found to be an invasive clone Solignac et al. Therefore, it is unlikely that sub-populations of less-virulent mites could explain VSB, or if they are, virulence would be due to a limited number of genes as it is the case with Varroa populations that are resistant to the acaricide fluvalinate-tau Milani, ; Liu et al.
Therefore, survival of VSB could be due to a higher tolerance of the bees to those viruses. However, the VSB did not survive longer compared to control bees when injected with the two viruses Le Conte, personal communication.
This suggests that the VSB have fewer viruses because they have fewer mites to transmit virus in the bee population. Nevertheless, it is reasonable to suggest that honeybee resistance, Varroa virulence, and virus prevalence are constantly under selective pressure, and that natural selection favors a co-evolution that secures the survival of both the host and parasites.
The presence of a specific pathogen of the Varroa mite, such as fungus, cannot be ruled out and should be investigated as a factor contributing to the survival of VSB. Also, the eect of the environment and apicultural methods on the survival of VSB cannot be excluded. The areas where the experiments were done are outside Frances major agricultural zone and are very favorable to the development of honeybee colonies. The colonies were manipulated only if necessary and were not moved or managed as professional beekeeping would recommend.
This approach, called the Bond test Live and let die! Kefuss et al. The resistance of those bees was compared with 12 A. Only A. They hybridized with local bee populations and most of the hybrids survived mite infestations indicating a genetic control of the resistance. Since , Kefuss et al. From this naturally surviving stock, they subsequently select their breeder colonies for economical traits. The best colonies are then tested for hygienic behavior using a freeze-killed brood assay and for Varroa infestation.
Their colony losses are comparable to other beekeepers of the region that still treat their hives with acaricides. In Gotland, an island of the Baltic Sea, Fries et al. Five of the colonies survived over 5 years. Later, Fries and Bommarco compared the surviving colonies to Varroa susceptible bees parasitized by mites from a dierent source. It was hypothesized that the dierences in mite populations may have been due to the smaller brood areas in the Bond colonies and dierences in mite distribution, with more mites present on adults rather than the brood in the Bond colonies.
Even if those surviving colonies may not be interesting for professional beekeeping because of their lower honey production due to the cost of the tolerance Le Conte et al. The selection for mite resistance in treated populations has to rely on indirect selection characters, because the direct trait of survivability cannot be studied when the colonies are influenced by medical treatments.
Starting about 25 years ago, much research in European institutes focused on the identification of suitable selection characters Bchler, b. In addition to the biological relevance correlation with the direct selection goal , the heritability and the practicability of testing under field conditions were deemed to be of major importance in the implementation of such characters in breeding programs. In the following, we briefly summarize the research on characters that have been used at least temporarily in European breeding projects for increased mite resistance.
Mite population development The eect of varoosis on the colony level depends on the infestation level Garrido and Bchler, unpubl.
Therefore, slow and limited mite population growth is a fundamental criterion of resistant stock that can be used as a character for selective breeding on mite resistance, even if the underlying behavioral and physiological causes remain unknown. Dierent methods to accurately estimate the Varroa infestation level are well documented Fuchs, ; Rademacher, ; Calatayud and Verdu, ; Garza and Wilson, ; Branco et al.
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