Honey bee genetic resistance outperforms a cold-storage induced halt in brood production to control mites and viruses

Varroa destructor mites seriously threaten honey bees by spreading viruses like Deformed Wing Virus (DWV) and contributing to global colony losses. Growing resistance to widely-applied miticides highlights the urgent need for sustainable mite control methods. This study evaluated the impact of a cold storage strategy to decrease bee brood production, and increase mite treatment efficacy, in a commercial Italian bee stock and mite-resistant Russian and Pol-line bee stocks, from prior to cold storage in August until the start of the commercial pollination season the following February. For each year of two years, thirty new bee colonies (10 colonies per stock) were either placed in cold storage (5 °C, darkness, 18 days) starting mid August or left outdoors, and all hives subsequently treated with a thymol-based varroacide. Colony brood area, adult bee mass, hive weight, internal temperature and CO₂ levels were monitored during the experiments using periodic hive assessments as well as sensors. Honey bee workers were sampled at different points and evaluated for bee health biomarker gene expression (vitellogenin) as well as virus levels of DWV-A and DWV-B. Cold storage effectively halted brood production but differences in brood levels between groups disappeared within two months, with no long-term impact on population size, mite levels, virus loads, or daily hive weight change. Bee stock was the dominant factor influencing outcomes: mite susceptible Italian colonies had higher mite densities, higher DWV loads, lower vitellogenin expression and higher rates of hive weight loss than Russian or Pol-line colonies. In this study mite-resistant honey bee stocks offered more effective control, reducing mite loads by over 65% compared to the susceptible stock, across both years and both treatment groups of the study, and they have the potential to support honey bee health by reducing reliance on chemical treatments in beehives.