Large-scale geostatistical mapping and occupancy-abundance patterns of Cerambyx species threatening SW Spain oak forests

Wood-boring insects are considered potential contributing/inciting factors to oak decline. Cerambyx cerdo (Cc) and C. welensii (Cw) are two sympatric oak-living large sapro-xylophagous longhorn beetles with different pest/legal status, whose larvae bore into living wood of healthy/decayed trees, and whose impact has increased alarmingly in recent years. We conducted a regional-scale multi-year (2017–2020) field study to model Cc and Cw distribution and to explore species-specific occupancy-abundance patterns. Records were obtained with 1650 feeding traps placed throughout the region of Extremadura (SW Spain) (41,634 km2) in holm, cork and pyrenean oak woodlands. Catch number (a proxy of abundance) was analysed through GLMMs, LMs and geostatistical interpolation (IK algorithm) to generate catch probability maps. Catch number was extremely variable between trees (traps), stands and years (Cc: 0–252, Cw: 0–219 adults/trap) with no repulsive interspecific association at the tree scale. Explanatory factors in the models (species, sex, year and host oak) and several interactions among them significantly affected catch number. As a whole, Cw was more abundant than Cc, but catch number greatly depended on host tree (Cw: cork > holm > pyrenean oak, Cc: holm > cork > pyrenean oak). Occupancy-abundance patterns were positive with significant occupancy x species interaction. Niche breadth was more than double in Cw (Levins’ BA = 0.42) than in Cc (BA = 0.19) and niche overlap almost complete (Pianka’s O = 0.98). Our large-scale pioneer study shows that Cc and Cw are widespread in SW Spain, but with huge host-mediated intra- and interspecific geographic variation in abundance, which has critical implications in population management/control strategies.