The Australian Government predicted that 50-60% of beekeepers, mostly recreational and small commercial, would leave the industry should varroa become established in Australian honey bee colonies, and that there would be a small decrease (less than 5%) in the number of hives kept by large commercial beekeepers. Based on data from the United States and Europe they predicted that 95—100% of the feral population would die within four years of an area getting varroa. The difference in the rate of colony death between feral and managed populations is due to managed hives being treated for varroa.

Resistance means the bee population keeps mite numbers low. Tolerance means the mite population may be higher, but the population does not suffer ill effects — they can tolerate mite infestations. These terms are sometimes used interchangeably and varroa surviving populations are not always categorised into one or the other.

Overseas, some beekeepers choose not to treat for varroa and instead use the Darwinian natural selection approach. They accept there will be colony losses without treatment, but stock that survive will likely have genetic traits for survival with varroa, and treatment may not be required in the future.

Resistant populations: Several naturally surviving populations have been documented around the world.

Naturally varroa surviving populations have been documented around the globe. African populations tend to withstand varroa better than European stock, but few populations in Africa have been investigated by scientists. In part, survival in Africa is due to the shorter development time of African subspecies, though other traits are involved (Table 1).

Table 1. Traits present in different naturally resistant populations around the world.

*No Deformed wing virus present (Roberts et al. 2020)
#Hygienic behaviour includes varroa sensitive hygiene.
Source: Locke 2016, Le Conte et al. 2020, Mondet et al. 2020a, Grindrod and Martin 2021, Hawkins and Martin 2021, Locke et al. 2021.

Viruses

While varroa causes damage to honey bees by feeding on their fat bodies and depleting nutrients, another major impact of mite feeding is the vectoring of viruses. Deformed wing virus (DWV) is the major virus associated with varroa. Some surviving populations show tolerance to DWV and acute bee paralysis virus (ABPV) where survival may largely be driven by virus tolerance and colonies can survive with high virus numbers.

Traits

A trait consistently seen in surviving populations is recapping. Worker bees partially remove the wax capping of suspected mite-infested brood, then proceed to remove the infected pupae (hygienic behaviour) which results in the death of immature mites. Alternatively, they may replace the cap and the immature mites die, reducing mite population growth. Studies have suggested that recapping is not the primary mechanism for resistance but may be an indicator of varroa sensitive hygiene — workers recognise signals that a brood cell is infested with varroa and remove the brood — a trait that has been found to contribute to the survival of some populations.

Other traits 

Reduced mite fertility and grooming (removal/damage of mite by workers on themselves or others) are found in only some populations.

Traits undesirable to commercial beekeeping

Other traits, including small colonies, low honey production and high swarming, that have been associated with some naturally surviving populations would not be popular with many commercial beekeepers. Smaller colonies tend to produce less honey, and low honey production is likely a by-product of smaller colonies rather than this contributing to survival itself. Small colonies are likely a result of frequent swarming.

Swarming helps bee colonies to withstand varroa for two reasons:

• The new colony starts with only the mites that are on the bees in the swarm.
• Both the new and old colonies experience a break in the brood cycle, forcing the mites onto the adult bees.

So, you want to try natural selection – how do you get started?

Natural resistance is difficult to achieve without everyone in the area being on board with the objectives. If beekeepers are allowing varroa to kill their hives then this will impact upon beekeepers in the surrounding area. It seems that natural selection is not feasible at this time.

When naturally resistant populations are moved elsewhere it is often reported that they succumb to varroa. This could be due to a variety of reasons:

• The bees are not actually resistant.
• Exposure to new/different viruses and viral strains or other pathogens overwhelms them.
• Exposure to untreated collapsing colonies can overwhelm breeding colonies with mites. While they are resistant, they cannot cope with an influx of hundreds or thousands of mites at once.

The aim of any varroa resistance breeding program should be that colonies can be moved anywhere and survive and thrive.

If natural resistance mechanisms exist in Australia this should be observed in the feral population, which will remain untreated. If feral bees continue to survive for more than three years after an area has been infested, this could indicate that the population is resistant.

Australian managed and feral populations are genetically different, but there is some degree of mixing. So, if the feral population is tolerant then resistance genes could also be present in the managed population. Feral populations will eventually start to recover, but whether beekeepers will wish to incorporate these genes into breeding programs will be determined by the traits that the regenerated population possess.

More information

• Darwinian Natural Selection Approach to beekeeping – Tom Seeley

Acknowledgements

• Holmes, Gerdts, Grassl, Mikeheyev, Roberts, Remnant, Chapman (2024) Resilient beekeeping in the face of Varroa. AgriFutures Australia.
• Plan Bee (National Honey Bee Genetic Improvement Program) is supported by funding from the Australian Government Department of Agriculture, Fisheries and Forestry as part of its Rural Research and Development for Profit program. The project is further supported by AgriFutures Australia, the Department of Regional NSW, University of Sydney, University of New England Animal Genetics and Breeding Unit, Better Bees WA Inc, Wheen Bee Foundation, Costa Group, Olam, Beechworth Honey, Monson’s Honey and Pollination, South Pacific Seeds, Australian Queen Bee Breeders Association, Australian Honey Bee Industry Council, and commercial beekeepers.
• Blacquire et al (2019) Darwinian black box selection for resistance to settled invasive Varroa destructor parasites in honey bees. Biological Invasions 21: 2519-2528
• Department of Agriculture, Fisheries and Forestry (2011) A honey bee industry and pollination continuity strategy should Varroa become established in Australia. Canberra.
• Grindrod, Martin (2021) Parallel evolution of Varroa resistance in honey bees: a common mechanism across continents? Proceedings of the Royal Society B 288: 20211375
• Hawkins, Martin (2021) Elevated recapping behaviour and reduced Varroa destructor reproduction in natural Varroa resistant Apis mellifera honey bees from the UK. Apidologie 52: 647-657
• Le Conte et al (2020) Geographical distribution and selection of European honey bees resistant to Varroa destructor. Insects 11: 873
• Locke (2015) Natural Varroa mite-surviving Apis mellifera honeybee populations. Apidologie 47: 467-482
• Locke et al (2021) Adapted tolerance to virus infection in four geographically distinct Varroa destructor-resistant honeybee populations. Scientific Reports 11: 12359
• Martin et al (2019) Varroa destructor reproduction and cell re-capping in mite-resistant Apis mellifera populations. Apidologie 51: 369-381
• Mondet et al (2020) Honey bee survival mechanisms against the parasite Varroa destructor: a systematic review of phenotypic and genomic research efforts. International Journal for Parasitology 50: 433-447
• van Alphen and Fernhout (2020) Natural selection, selective breeding, and the evolution of resistance of honeybees (Apis mellifera) against Varroa. Zoological Letters 6:6
• This article was peer-reviewed by Emily Remnant and Elizabeth Frost.
• Main image: Resistant populations.png; Several naturally surviving populations have been documented around the world.