Relationship between tree-related microhabitats and the captures obtained using different trap methods for large threatened saproxylic beetles

Osmoderma eremita and Cerambyx cerdo are saproxylic beetle species, included in the IUCN Red List and in the EU/92 Habitats Directive. Their occurrence has been recorded, through appropriate traps, in several localities in Italy, including urban and peri-urban parks, mostly associated with veteran trees. In this study, traps were tested over 17 veteran oak trees in the Castelporziano Presidential Estate (Latium, Rome province, central Italy) and the abundance of sampled saproxylic beetles was related to the growing stock volume (GSV) and the abundance of microhabitats. Moreover, we compared three different trapping methods: a trap designed to monitor C. cerdo (CC), and two traps designed for monitoring O. eremita, i.e., the Black Cross Window Trap (BCWT) and the Black Bottle Traps (BBT). We found that larger trees hosted a great variety of microhabitats, in particular cavities, which abundance was correlated with the number of specimens sampled, especially O. eremita species. Both classic traps (i.e., CC traps and BCWT) and the herein-introduced BBT variants were effective for capturing large saproxylic beetles. In this work, we underline the importance of the preservation of veteran trees to the occurrence of threatened saproxylic beetles, and we contributed to (i) extending general knowledge of the habitat preferences of saproxylic beetles and (ii) improving new cost-effective trapping system variants.