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Wild solitary bees and their use of bee hotels in southwest Spain

There is an increasing interest in preserving and, if possible, increasing wild bee populations as evidenced by increasing investigations into providing supplemental nesting resources, commonly called bee hotels. The study presented here was carried out in 2017 and 2018 with two objectives: a) to understand the role that insect refuges could have on beneficial arthropod fauna, especially bees, and b) to evaluate different materials and which species used them. We present the preliminary results of three constructed refuges in Seville, Spain: Hymenoptera visited the refuges most frequently (88.7% of the visitors), of which the social wasps (Polistes dominula (Christ)) were most common, followed by bees. Bees were observed visiting bamboo canes, Arundo canes, drilled logs, and grooved boards. Drilled logs were the most used material (31.5 and 37.6% occupied in 2017 and 2018, respectively), followed by bamboo canes (14.1 and 17.4% of occupied in 2017 and 2018, respectively). For drilled logs, holes of 4.9–6.5 mm (2017) and 7.0–9.2 mm (2018) were preferred, whilst diameters of 2.6/2.7–4.9/5.0 mm (both 2017 and 2018) were preferred for bamboo canes. For grooved boards, holes of 5.0 mm (only 2018) were preferred. The bee species most frequently nesting in bamboo canes were Ceratina cucurbitina (Rossi) and Ceratina dentiventris Gerstacker, whereas in drilled logs Hoplitis lepeletieri (Pérez) was most common, but Hoplitis adunca (Panzer) was also recorded. Their abundance throughout the study period varied between species, and their role in biodiversity and sustaining wild flora is discussed.