Viruses are common in honey bee colonies but tend to exist as covert infections with only sporadic disease outbreaks. The emergence and global spread of varroa mites has drastically changed the impact of viruses by creating a new transmission pathway and exacerbating virus replication in colonies.

Three virus complexes in particular have become serious pathogens in association with varroa: Deformed wing virus A and B (DWV-A and DWV-B), the Acute, Israeli and Kashmir bee paralysis virus (ABPV, IAPV and KBV), and Slow bee paralysis virus strains (SBPV) (de Miranda et al. 2010, McMenamin and Genersch 2015). These viruses are vectored by varroa as they parasitise bee pupae, significantly increasing brood mortality or reducing the lifespan of emerging bees.

There is strong evidence that DWV is currently absent from Australia. Large-scale virus surveys of honey bees in Australia have never detected DWV within Australia (Roberts et al. 2017, Taylor 2022). Two overseas studies reported DWV in limited samples imported from Australia (Singh et al. 2010, Wilfert et al. 2016) but may have been contaminated post import. It has been suggested that DWV exists covertly in all honey bee populations, but only becomes detectable when vectored by varroa (Martin et al. 2012, Mondet et al. 2014). This seems unlikely to be the case for Australia because DWV has been detected in other varroa-free bee populations. Thus it is extremely important to keep Australia DWV free by preventing further incursions of varroa and mitigating the risk of introducing viruses through imported queens or bee semen. The impact of varroa on Australian honey bees will be greatly reduced without the harmful effects of DWV.

Varroa is expected to exacerbate the endemic viruses IAPV, Sacbrood virus (SBV) and Black queen cell virus (BQCV)  but not establish ongoing associations. These are faster replicating viruses and are unlikely to sustain chronic viral loads in colonies infected by varroa, as they lead to high larval or pupal mortality which is incompatible with varroa’s life cycle. Slower replicating viruses that allow adult bees to emerge, such as DWV, have established chronic infections overseas, but it is unclear what long term associations might emerge in Australia.

Another potential outcome from varroa establishment could be a delayed emergence of DWV or another slow replicating virus in Australia, most likely from a subsequent incursion or perhaps from the current viral landscape. Regular monitoring of virus dynamics before and as varroa spreads will be critical for managing this ongoing threat.

Table 1. Honey bee viruses known to be vectored by varroa

More information

VIDEO: Dr Jamie Ellis – Bee viruses and what to do about them

FACTSHEET: AgriFutures Australia Resilient Beekeeping Bee Health Factsheet – https://honeybee.org.au/wp-content/uploads/2024/05/Resilient-Beekeeping-Bee-Health-Factsheet.pdf

Acknowledgements:

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• This article was peer-reviewed by Nadine Chapman and Emily Noordyke.