Mite non reproduction (MNR) or suppressed mite reproduction (SMR) is a suite of traits, and factors, that work together to limit successful mite reproduction. Some bee specific traits can be selected individually, such as Varroa sensitive hygiene (VSH),  recapping (REC), and hygienic behaviour (HYB) (Figure 1). However, by focussing on singular traits, we fail to recognise and promote all the mechanisms that honey bees have to keep mites from reproducing in their colonies. MNR also includes mite specific factors and traits that limit reproduction (e.g. infertile males). When quantifying MNR it’s important to remember that this trait is a product of the actions of both the bees (suppressed mite reproduction) and the mites themselves (inability to reproduce).

Figure 1. Varroa resistance traits

MNR also includes:

• The inability of mites to find brood hosts of the appropriate age, likely due to changes in brood signalling.
• The mite getting caught in the silk or dying due to movements of the larva during pupation.
• Prevention or delay of mite oogenesis (egg development) by lack of cue from brood.
• Death or infertility of the male, resulting in the production of unmated females that shortens mite reproductive lifespan.
• Age of the mite — either very young or very old.
• Effects of the quality and age of the bee that the mite spends the dispersal period on.

The terminology used to describe mechanisms and factors driving varroa resistance in honey bees has evolved over time as our understanding of the mechanisms and factors has developed. Varroa sensitive hygiene (VSH) is often used as a broad label for varroa resistance, it is one trait that contributes to MNR.

In the early days of exploring varroa resistance mechanisms in honey bees, suppressed mite reproduction (SMR) was coined and described as varroa resistant colonies having a high proportion of non-reproductive mites, which consequently slowed mite population growth.  It was later determined that the high proportion of non-reproductive mites found in colonies was because hygienic bees were targeting cells with reproductive mites, skewing the analysis. Researchers determined that VSH was driving varroa resistance, and consequently, the term VSH was used. But it has since been discovered that it is not the only trait that is involved in MNR.

Although MNR continues to be referred to as VSH in many breeding programs, as the industry’s understanding of varroa resistance mechanisms develops, so does the language. VSH should only be used in cases where VSH is specifically tested for.

MNR is quantified by removing purple-eyed pupae and determining the proportion of infested cells that have not produced at least one viable adult daughter. A minimum of 35 infested cells should be inspected. This means that significantly more effort must be invested to assess the best colonies. This can be solved by manually infesting colonies with varroa, but this requires a population with high mite loads to be maintained, which presents a biosecurity risk, and it is difficult to keep these colonies alive.

VSH

VSH occurs when 15-18 day old worker bees respond to chemical cues produced by the developing bee, the reproductive mite and associated offspring, or both. Bees investigate the wax capping of a suspected mite-infested brood and remove it, before removing the pupae, which results in immature mites dying.

Selecting for VSH involves examining purple eye or later aged pupae for the removal of mites. Natural infestation and reproduction can be measured. Brood cells can be manually infested with a single mite or highly infested brood frames — where infestation levels have been quantified — can be placed in the colony, and the frames removed for quantification after a set period. Pupae are removed from the cell and the pupae and cell are examined under a dissection microscope for foundress mites and associated offspring.

Currently there is no standardised methodology for VSH phenotyping. The process is laborious and there is variable evidence for a reduction in infestation levels and increased colony survival.

More information

• Video: Breeding and testing Varroa resistant bees
• AgriFutures on Air – Varroa Expert Interview Series: Topic 4 – Breeding for varroa resistance

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.
• Eynard et al. (2020) Descriptive analysis of the Varroa non-reproduction trait in honey bee colonies and association with other traits related to Varroa resistance. Insects 11: 492
• Guichard et al. (2020) Advances and perspectives in selecting reistance traits against the parasitic mite Varroa destructor in honey bees. Genetics Selection Evolution 52: 71
• Mondet et al. (2020) Evaluation of suppressed mite reproductive (SMR) reveals potential for varroa resistance in European honey bees (Apis mellifera L.). Insects 11: 595
• This article was peer reviewed by John Roberts and Michael Holmes.