Extraordinary incidence and persistence of feral honey bees in urban environments of Belgrade (Serbia)

Abstract

In the modern world managed bee species are necessary for the stability and sustainability of food production, of which the western honey bee (Apis mellifera) is the best known and the single most important species (Kevan, 2007). Wild honey bees have largely become extinct throughout Europe since the 1980s, following the introduction of the ectoparasitic mite Varroa destructor and various pathogens associated with it (Crane, 1999). The honey bee has been the subject of extensive research around the world for decades, particularly in terms of preserving the health and stability of their colonies under these new pressures. In that period, the importance of rare, accidentally surviving wild and feral colonies of honey bees was largely neglected. However, several recent studies (e.g., Le Conte et al., 2007) report on the growing number of colonies that have successfully survived the Varroa mite infestation without usual chemical treatments. Unlike in other studies on unmanaged colonies, we presented a special case of the free-living population of honey bee in a large and highly populated urban environment of Belgrade, the capital of Serbia (Southeast Europe). During the period 2011–2017, we collected a large set of observational data (>1,300 records) from the apparently dense wild/feral honey bee population. Large share of these records can not be spatially related to the existence of managed apiaries, which are very rare in the core of downtown Belgrade. We hypothesized that numerous unmanaged bee colonies and the high frequency of swarms indicate a stable, self-sustaining wild population, and not the regular influx of swarms from the managed apiaries situated more peripherally. Also, we elaborated on various plausible explanations for this extraordinary finding. By geospatial analysis of different categories of occurrence (colonies and swarms observed in different habitats and micro-situations) in relation to the parameters of urban environments suitability for honey bees, we evaluated the patterns of distribution and long-term survival of feral colonies under urban conditions. The results indicate an extremely high and stable population density and unusually long persistence of feral colonies, which is a rare phenomenon on a global scale (Bila Dubaić et al., 2021). Previous isolated cases of unmanaged colony survival (unknown in urban conditions) triggered great scientific attention: understanding the factors behind these survival cases may contribute to the efforts on improving the managed honey bee health. Therefore, our results contribute to ongoing initiatives to support naturally selected resistance mechanisms against Varroa mites, and hopefully reduce the currently growing incidence of colony die-offs. The collecting of data was based on opportunistic citizen science, which we hereby evaluate as a highly prospective approach in the study of wild/feral bees in urban areas. We believe that specifically designed citizen science projects, based on our experiences, could be implemented with a great success in other countries with similar highly urbanised circumstances; in particular, investigations should be focused on environments with comparable potentials for the unnoticed existence of unmanaged honey bees (Bila Dubaić et al., 2021). Despite the growing global trend of urban beekeeping, little is known about the impact of urbanization on the genetic diversity of honey bees. In particular, we investigated and performed genetic analysis of 82 individual bee genomes in a portion of a sample of feral and managed honey bee colonies distributed throughout this large city, including highly urbanised areas. We established the existence of a high genetic differentiation between these two groups. Also, the comparison of mitochondrial and microsatellite DNA of bees sampled in Belgrade and in rural parts of Serbia showed that colonies of wild/feral bees have different patterns of genetic diversity. It can be concluded that urbanisation can be a positive driver of the genetic diversity of wild honey bees nesting in a highly urbanised and densely populated areas.