Early-phase colonization by introduced sculptured resin bee (Hymenoptera, Megachilidae, Megachile sculpturalis) revealed by local floral resource variability

There is a growing interest to document and better understand patterns and processes involved in non-native bee introductions and subsequent colonization of new areas worldwide. We studied the spread of the East Asian bee Megachile sculpturalis in Serbia and southeastern Europe; the bee was earlier established in the USA (since 1994) and western Europe (since 2008). Its establishment in Serbia remained dubious throughout most of 2017–2019, following the first detection. We herewith report about its establishment and spreading, that was corroborated in 2019 under specific circumstances. Owing to the exceptionally poor blooming of Styphnolobium japonicum, we recorded high activity density of M. sculpturalis concentrated on a scarce key food resource. We presented a novel quantitative approach for the improved early detection of M. sculpturalis, based on the interplay between the bee's local occurence pattern and dynamics of key food-plant(s) availability. The approach seems particularly effective, during the early-phase colonization, at initially low population density of introduced bees. We addressed the importance of integration of the genuine plant usage patterns with context-specific bee assessment options, for establishing the effective monitoring. Yet, no comprehensive approach exists for the effective quantification of its wider spatial trends, various interactions and their effects; hence, further research is needed for evaluation of bee's invasiveness potential. Our improved understanding of M. sculpturalis local dynamics and detectability triggered an extension of the study scope to the regional-scale context: the early colonization of SE-Europe. With a more insightful interpretation of M. sculpturalis introduction into Serbia, we suggest that a diffusive mode of spreading now seems the more plausible scenario (than the seemingly more likely long-distance jump). The two-scale study outcomes further advanced the evidence base and relevant efforts for the development of standardized monitoring protocols.