Modelling the impacts of imports of non-native honey bees into the native Apis mellifera mellifera population in Ireland

The human-mediated import of non-native queens is threatening the unique adaptations of native honey bees in Ireland through hybridisation and introgression. To study the impacts of this import, we built a SIMplyBee stochastic model of hybridisation between a native Apis mellifera mellifera population in Ireland and a non-native (C lineage) honey bee population. The model included colony honey yield and fitness as complex polygenic traits. We modelled the effects of varying the percentage of imports, the genetic correlations between fitness in native and non-native environment, and the spatial distribution of introgression over 20 years. We measured the levels and rate of introgression and the changes in genetic means for honey yield and fitness. The results revealed that increased imports expectedly increase the rate of introgression. This introgression induced a trade-off between increased honey yield and decreased fitness. Changing the genetic correlation between fitness in native and non-native environment pointed to a larger fitness decrease with negative correlations. The study showed the spread of introgression across the whole simulated area, indicating that even colonies far from the import zones could be affected over time. Furthermore, halting imports generated a desirable direction of change, however the purging of introgressed material was very slow and varied substantially across replicates. These results underscore the risks and long-term impacts of import on complex polygenic traits. We argue that the best way forward to manage the problem of import and associated introgression is to invest in holistic selective breeding of the native honey bee population in its native environment rather than import invasive subspecies that are bred to be successful in a different climate. The study also highlights the lack of fundamental quantitative genetic parameters for complex polygenic traits of different honey bee populations in different environments.