An investigation into Varroa mite resistance in New Zealand honey bees

<p><strong>Honey bees are the most prolific pollinators in agriculture, making them critical to maintaining global food chains, and ecological biodiversity. The Varroa destructor mite poses a significant threat to western bee populations, and therefore to the future sustainability of agriculture. Current miticidal treatments are increasingly ineffective against Varroa, prompting investigation into alternative sustainable solutions to address the growing impact of Varroa to the apiculture industry. This study aims to identify resistant traits in New Zealand honey bees bred for mite resistance, with the goal of providing a more sustainable option to help mitigate Varroa’s impact. The proposed resistant strain (PRS) of honey bee investigated in this study was bred over many years for mite resistance and is purported to be resistant via a shorter bee development duration restricting a mite’s ability to reproduce. Over two seasons, experiments were conducted at Victoria University of Wellington and at a field site to measure the mite population growth and development duration of honey bees in the PRS, and in a standard Italian strain. The PRS were observed to have slower mite population growth across all sites and seasons, culminating in lower mite numbers at the conclusion of the study. Honey bee development duration varied largely, both within and between strains, resulting in no consistent difference in the development duration between strains that can be associated with the lower mite numbers. Consequently, while the proposed resistant strain does demonstrate a degree of resistance to Varroa through lower mite numbers, the evidence does not support the hypothesis that this mite resistance can be attributed to a shorter development duration in this population of bees. The lower mite population growth across all sites and years in the PRS indicates a possible resistance trait that was not identified in this investigation.</strong></p><p>The spatial distribution of Varroa mites on honey bees during the mites’ dispersal phase was investigated in a standard Italian strain and the PRS. No significant differences were observed in the spatial distribution of Varroa mites between the strains. In line with previous investigations, mites were found at higher rates on the side of the bees’ abdomen in the inter-tergite space, with the majority on the left side. Possible reasons for most of the mites being found at this location on the bee are discussed.</p><p>A second population of bees bred over many years for heightened Varroa-sensitive hygiene behaviour was examined. Variation in Varroa-sensitive hygiene behaviour, as measured by the ratio of reproducing versus non-reproducing mites, was identified in this population, with the ratio of non-reproducing mites ranging up to 100%. Investigation into the relationship between the mite non-reproduction ratio and a previously identified Varroa-sensitive hygiene (VSH)-associated genotype was limited by the small proportion of the relevant genotype in the population sampled. However, further analysis suggested a complex relationship with the VSH-associated genotype and multiple colony traits, including the mite non-reproduction ratio, that may be elucidated with further investigation. Additionally, an observation of VSH-associated behaviours in response to previously identified parasitism specific hydrocarbons (Varroa-specific compounds, VSC) found no association with mite-non reproduction. These findings emphasise the complexity of traits that attribute to mite resistance in different bee populations, and the difficulties in breeding for mite resistance. An apparent mite-resistant population was identified in the PRS but the mechanism underlying resistance was not determined. Additionally, VSH was identified in local populations, but new methods of selecting for resistant traits were unable to conclusively identify resistant colonies, though this may be due to the limitations of the study. Future research should aim to identify and characterise mite resistance traits in these populations and others, and continue to explore novel methods of trait selection to understand the underlying mechanisms of mite resistance, to support efforts toward sustainable beekeeping in New Zealand.</p>