A picky generalist: nesting females of Pseudaugochlora graminea (Halictidae) are highly specialised in an urban area

Abstract

Urban areas act as important shelters to support bee populations but some guilds, such as generalist species, are more benefited compared to others. We aimed to investigate if the floral preferences of a polylectic species would change throughout the years and if gardening practices as mowing could affect the females' floral preferences and, thus, the bee-plant interaction networks in an urban area within a medium-sized city in Brazil. Nesting females of Pseudaugochlora graminea (Fabricius) (Halictidae) were collected and had the pollen grains attached to their scopae removed, acetolysed and identified in two different periods: March 2019 and March 2022. Only five plant species were identified in samples from 2019 whilst 13 species were detected in 2022, in spite of mowing and plant replacements that occurred throughout this interval of three years. Among the floral sources observed, both exotic and native species were visited for pollen and nectar collection by the females. Despite being a generalist species, all interaction networks between P. graminea females and plant species were highly specialised. Females concentrated their pollen gathering on few plant species, which were represented by less than 10 individuals each at the study area. Mowing lead to the extirpation of the third most important pollen source in 2019 but plant replacements may have favoured females by some nectar sources seedlings. Our findings demonstrate that gardening practices impacted the floral choices by females of a generalist bee species and, consequently, their food webs.