Assessing concrete nest boxes for cavity-nesting bees

Artificial nest boxes for solitary bees and other cavity-nesting Hymenoptera are increasingly used for a variety of purposes, including ecological research, crop pollination support and public outreach. Their attractivity and colonization success by cavity-nesting solitary bees depend on their design and placement, including hole dimensions, orientation and the neighboring habitats and available resources. While most bee nest boxes are made of wooden materials, we assessed here the suitability of perennial, concrete nest boxes for cavity-nesting bees. We carried out a three-year nesting survey of 52 custom-made nest boxes located in 11 different sites throughout France and totaling 2912 available holes of 6, 8, 10 or 12 mm in diameter. Concrete nest boxes successfully attracted reproductive females of solitary bee species and supported successful larval development until the emergence of new individuals. Preferred cavities were the smallest ones (6-8 mm), located at the lowest tested positions above ground (31-47 cm) and oriented southward. Local bee populations established in nest boxes steadily increased throughout the three successive seasons in nearly all study sites. The cavity-nesting bee communities were mostly composed of rather common and generalist species, but also comprised a foraging specialist. Additionally, two cleptoparasitic bee species were detected. All species belonged to the Megachilidae. We further discuss the effects of neighboring urban and natural habitats as potential source or sink of nesting bees, as well as opportunities of concrete nest boxes as tools for urban agriculture and more generally for the new biomimetic urban designs to restore local ecosystem services in cities.