DNA metabarcoding reveals a constancy in the foraging strategy of bumblebees in grasslands with high plant diversity after strong reduction of colony’s workforce.. Foraging for pollen in reduced bumblebee populations

1. Among pollinators, bees are particularly dependent on flower resources for feeding their larvae. However, during their life cycle, social bees might undergo workforce losses because workers could be overpowered by environmental stressors.2. In this study, commercial colonies of the bumblebee Bombus terrestris were experimentally manipulated by removing half of the workers, in order to investigate consequent changes in foraging strategies and bumblebee-plant networks. Before and after the manipulation, pollen pellets were collected from the individual workers after their foraging trips. The manipulated colonies were compared with untreated ones in the same period. 3. DNA metabarcoding on pollen pellet samples was performed by targeting the ITS2 region using High Throughput Sequencing (HTS). A ROC approach (Receiver Operating Characteristic) was used for the filtering of sequencing reads and taxonomical assignments were based on a molecular reference dataset of the plant species found at the studied area. Molecular data revealed that bumblebees were feeding on 34 plant taxa and that the majority of the pollen pellets were polyfloral. Despite this high diversity, only minor changes in the diet breadth of the bumblebees or in the feeding networks were found after the workforce removal. 4. The maintenance of foraging strategy in the treated colonies indicates that bees are not adapting to a new condition. Thus, they may not compensate the lack of incoming resources when the population is reduced. DNA metabarcoding provided insights on the constancy in foraging performed by social pollinators even at reduced densities, as they may not adapt their resource acquisition to the over-stressing environment they live in.