Pollen load analysis reveals feeding preference of bee pollinators of Dictamnus albus

Dictamnus albus L., commonly known as dittany, is a long-lived perennial herb belonging to the family Rutaceae; its flowers produce abundant pollen and nectar and are visited by several pollinating insects. Observations on insect behaviour while foraging on D. albus flowers showed that medium to large bees have the highest percentage of contacts with receptive stigmas, i.e. they show the highest probability of acting as valuable pollinators. However, this can be proved through the analysis of the pollen present on their body. We separately collected the pollen sparse on insect body (head, thorax and abdomen) and the pollen actively collected in specialized structures (corbiculae, scopae), in order to assess the fidelity of each visitor taxon, determined as the mean percentage of specific (D. albus) pollen. In addition, we performed a more precise analysis of the pollen load of the main pollinators of D. albus: Habropoda tarsata, Bombus spp. and Megachilidae spp.. For these insect taxa we identified pollen grains at the genus or family level, to infer the levels of competition or synergy among co-flowering species, and to obtain additional information on insect feeding habits. Most visitors showed a high fidelity to D. albus in the study area, meaning that it is a good resource of rewards, and the potentially best pollinators of dittany tend to prefer its flowers compared to co-flowering species, indicating a low competition during full blooming. From the insect perspective, different plants may be source of different rewards at a same time. For example, dittany is mainly visited for nectar by most pollinators but other flowering species are visited for pollen or nectar as well. This study allows us to explore the interactions among species in a complex ecological context and to develop specific conservation actions at both the plant and insect levels.