Not all woods are equal: local, rather than landscape, factors are important to conserve a xylosaprophagous hoverfly

Among the many forest-dwelling invertebrates, saproxylic species play a key role in decomposition and nutrient cycling. Several saproxylic insects are threatened with extinction and depend on strict microhabitat features. Hoverflies play a key role in pollination and can be used as bioindicators of habitat conditions. Mallota fuciformis is a rarely observed saproxylic hoverfly internationally recognised as a species of conservation interest. We sampled Mallota fuciformis in oak-hornbeam stands and alluvial woodland with a standardised observation protocol of adults on flowering Prunus trees. This new sampling method proved to be efficient and non-invasive, eliminating the need to collect specimens of this rare species. Overall, 48 out of 86 sites investigated were found to be positive for the presence of the species, significantly increasing knowledge about its distribution in the region. We tested several environmental predictors to explain M. fuciformis detection, but only three were significant for the model: the presence of overmature Quercus trees, the presence of alluvial plants and the current size of the forest. Other predictors considered as proxies of landscape and past condition of the woodland showed no effect. These data confirmed the importance of the quality of forest woods (mainly the presence of large oak trees that ensure larvae development) to allow the presence of this saproxylic species.In the present study, we investigate the distribution and habitat requirements of M. fuciformis in order to find forest features that could be beneficial also for other, more threatened saproxylic species. This research underlines the importance of veteran trees and their microhabitats in the conservation of saproxylic species of hoverflies. We also provide an effective ad hoc protocol to monitor the target hoverfly species, which can be implemented in the assessment of rare species.