Colonisation Patterns of Nosema ceranae in the Azores Archipelago

SIMPLE SUMMARY: Nosema ceranae is an emergent honey bee pathogen that has now invaded most of the world. However, geographically-isolated places that are free of this pathogen may still exist, and the Azores may be one of them. Here, we used molecular tools to see whether N. ceranae has entered the Azores and how far it has colonised the archipelago. In 2014/2015 we sampled 474 colonies from eight islands, and in 2020 we re-sampled 91 colonies from four islands. Our results showed that N. ceranae was present on all islands but Santa Maria and Flores. In the 2014/2015 sampling, Pico, the island of Varroa destructor entry in the Azores, showed the greatest prevalence. Resampling in 2020 revealed that N. ceranae built-up on Terceira and São Jorge. Our findings suggest that N. ceranae colonised the archipelago recently, and it spread across the other islands. Santa Maria is also free of V. destructor, making it one of the remaining areas in the world where bees are naive to both stressors. This study will help the veterinary authority establish biosecurity rules for the movement of bees and hive products among islands to maintain the N. ceranae-free status of Santa Maria and Flores. ABSTRACT: Nosema ceranae is a highly prevalent pathogen of Apis mellifera, which is distributed worldwide. However, there may still exist isolated areas that remain free of N. ceranae. Herein, we used molecular tools to survey the Azores to detect N. ceranae and unravel its colonisation patterns. To that end, we sampled 474 colonies from eight islands in 2014/2015 and 91 from four islands in 2020. The findings revealed that N. ceranae was not only present but also the dominant species in the Azores. In 2014/2015, N. apis was rare and N. ceranae prevalence varied between 2.7% in São Jorge and 50.7% in Pico. In 2020, N. ceranae prevalence increased significantly (p < 0.001) in Terceira and São Jorge also showing higher infection levels. The spatiotemporal patterns suggest that N. ceranae colonised the archipelago recently, and it rapidly spread across other islands, where at least two independent introductions might have occurred. Flores and Santa Maria have escaped the N. ceranae invasion, and it is remarkable that Santa Maria is also free of Varroa destructor, which makes it one of the last places in Europe where the honey bee remains naive to these two major biotic stressors.