Scientific Research on Varroa Resistance
Focusing on Varroa Sensitive Hygiene Trait
Welcome to our website dedicated to providing beekeepers, queen producers and queen breeders with valuable information about scientific research on Varroa resistance, with a specific emphasis on the Varroa Sensitive Hygiene (VSH) trait. We aim to provide clarity and guidance for beekeepers, queen producers and breeders to understand and utilize the Varroa Sensitive Hygiene (VSH) trait in New Zealand.
The Search for Varroa Resistance Traits:
The quest for Varroa resistance traits in Apis mellifera has a rich history spanning over three decades. Researchers employ various approaches to study and categorize different resistance traits, encompassing phenotype and genotype characterization, underlying mechanisms, bioassay methods, and breeding program methods.
Clarity through Comprehensive Framework:
Recent updates and reviews, such as those by Spivak and Danka (2020), Mondet et al. (2020), van Alphen and Fernhout (2020), and Leclercq (2018), have brought more clarity to the subject. Mondet et al. (2020) presented a comprehensive framework that includes a systematic global analysis of research results and definitions of Varroa resistance and tolerance traits. Standardization of definitions and testing methods is crucial for clear communication and interpretation of research results, so we follow the trait definitions in Mondet et al. (2020).
Key Distinctions for Varroa Research and Breeding Programs:
These recent reviews have emphasized important distinctions that shape Varroa research and breeding programs:
Resistance versus Tolerance Traits:
Resistance refers to the bee’s (the host's) ability to reduce the reproductive success of Varroa, keeping infestations at non-damaging levels. In contrast, tolerance is the ability of the host to reduce the impairment or harm caused by the parasite when the infestation is at a level that typically causes damage. Understanding this distinction is crucial, because the underlying causes and mechanisms differ, and this impacts the breeding programs. VSH is a resistance mechanism because it reduces the mite reproduction.
Natural Selection versus Artificial Selection:
Natural selection has resulted in "survivor" colonies that have been left untreated and but manage to coexist with Varroa mites. While these colonies showcase varying degrees of resistance traits, they may not be suitable for large-scale commercial breeding programs due to undesirable characteristics such as low bee production or frequent swarming (Mondet et al. 2020; O’Shea-Wheller et al. 2022). One of the challenges in breeding by artificial selection is introducing heritable resistance traits while still maintaining important desirable beekeeper traits such as temperament, honey production and colony size. This is why it is important ensure multi-trait breeding programs. For example, focus on your best breeders when searching for high levels of the VSH trait in your colonies and keep the focus on multi-trait selection in breeding and maintenance programs.
Breeding Challenges and Solutions
The complex social and genetic architecture of bee colonies complicates artificial selection in breeding programs. For example, some traits must be measured at the colony or caste level of biological organisation. In addition, queen bees mate on the wing with multiple drones so the paternal genetic contribution is unknown and not in the control of the breeder unless instrumental insemination or closed populations are used. However, instrumental insemination or closed bee populations can quickly lead to too much inbreeding with a reduction in genetic diversity. This is why genetic tests have been developed in New Zealand for tracking and maintaining the diversity of the critical csd alleles required for the honey-bee sex determination system which is at risk from excessive inbreeding in closed mating selection programs (Hyink et al. 2013).
There are new research trends in combining phenotypic and genotypic data to statistically infer the paternal contribution based on kinship relations, but this would be used in advanced breeding programs where both phenotype and genotype data are available (e.g., using GBLUP to improve the derivation of the Estimated Breeding Value (EBV) of an individual bee (Brascamp et al. 2018, Petersen et al. 2020)).
REFERENCES
Brascamp, E.W., Wanders, T.H.V., Wientjes, Y.C.J., Bijma, P. 2018. Prospects for genomic selection in honey-bee breeding. Proceedings of the World Congress on Genetics Applied to Livestock Production, 11.29.
Hyink, O., Laas, F., Dearden, P.K. 2013. Genetic tests for alleles of complementary-sex-determiner to support honeybee breeding programmes. Apidologie 44:306–313. Doi: 10.1007/s13592-012-0181-6
Leclercq, G., Francis, F., Gengler, N. Blacquière, T. 2018. Bioassays to quantify hygienic behavior in honey bee (Apis mellifera L.) colonies: a review. Journal of Apicultural Research 57(5): 663-673, Doi: 10.1080/00218839.2018.1494916.
Mondet, F., Beaurepaire, A., McAfee, A., Locke, B., Alaux, C., Blanchard, S., Danka, R., Le Conte, Y. 2020. Honey bee survival mechanisms against the parasite Varroa destructor: a systematic review of phenotypic and genomic research effort. International Journal for Parasitology 50: 433–447. Doi.org/10.1016/j.ijpara.2020.03.005.
O’Shea-Wheller, T.A., Rinkevich, F. D., Danka, R. G., Simone-Finstrom, M., Tokarz, P. G., & Healy, K. B. 2022. A derived honey bee stock confers resistance to Varroa destructor and associated viral transmission. Scientific Reports 12. Doi.org/10.1038/s41598-022-08643-w.
Petersen, G.E.L., Fennessy, P.F., Van Stijn, Clarke, S.M., Dodds, K.G., Dearden, P.K. 2020a. Genotyping-by-sequencing of pooled drone DNA for the management of living honeybee (Apis mellifera) queens in commercial beekeeping operations in New Zealand. Apidologie 51:545–556. Doi: 10.1007/s13592-020-00741-w.
Spivak, M. and Danka, R. 2020. Perspectives on hygienic behaviour in Apis mellifera and other social insects. Apidologie 52. 1—16. Doi: 10.1007/s13592-020-00784-z.
van Alphen, J.J.M., Fernhout, B.J. 2020. Natural selection, selective breeding, and the evolution of resistance of honeybees (Apis mellifera) against Varroa. Zoological Letters 6:6 https://Doi.org/10.1186/s40851-020-00158-4.
Definitions for phenotypic traits for Varroa resistant bees.
This list of definitions includes the phenotypic traits that have been the most used for selective breeding programs for Varroa resistance. The definitions are extracted verbatim from Table 1 of Mondet et al. (2020a) on Page 435.
The main traits used for VSH are Mite Population Growth (MPG), Varroa Sensitive Hygiene (VSH), Mite Non-Reproduction (MNR) and Recapping (REC). Some traits have undergone redefinition as the new research results showed that they were originally misunderstood, such as SMR. Other traits are used as proxies for Varroa resistance in the sense of VSH, such as HYG and Grooming but are not as predictable as bioassays sensu VSH. The MNR trait has undergone some recent changes, but it remains to be seen if they become widely used (von Virag et al. 2022).
1. Mite population growth (MPG) — The change in the number of mites in a population over a specified time. No (or low) population growth is the basis of host resistance.
2. Varroa-sensitive hygiene (VSH) — Form of hygienic behaviour that specifically targets and removes brood infested by Varroa mites. This trait is assessed through assays that measure the removal of Varroa-parasitized brood.
3. Mite non-reproduction (MNR) — Failure of a foundress mite to produce at least one adult, mated female that will enter the colony’s mite population when the developing bee emerges from the cell as an adult bee. A foundress mite will not be successful at reproduction if she does not lay any eggs (infertile), lays only one egg, produces no male offspring, or begins laying her eggs too late in relation to the pupal development.
4. Recapping (REC) — Behavioural sequence consisting of the targeting, opening, and then recapping of brood cells, leading to the potential disruption of mite reproduction. More research is needed to confirm that this trait is totally distinct from VSH.
5. Suppressed mite reproduction (SMR) — Redefined as only cases of mite non-reproduction that are regulated by traits expressed by the brood.
6. Hygienic behaviour (HYG) — Behavioural sequence consisting of the targeting, opening and removal of diseased, injured, parasitized, or dead brood by worker bees. This trait is usually assessed using the freeze-killed brood (FKB) or pin-killed assays.
7. Grooming — Behaviour consisting of the removal of Varroa from adult bees, either by a bee infested by a mite itself (auto-grooming) or by a bee cleaning another bee (allo-grooming).