Abundance of Taphrorychus bicolor in beech forests: Influence of forest size and optimal conditions

The bark beetle Taphrorychus bicolor primarily inhabits Fagus sylvatica forests. Its populations increase after periods of drought. Due to the limited knowledge of factors affecting its abundance, we conducted a large-scale sampling. Beetles were captured using three Theysohn traps lured with bicolorin at 26 study sites in mature homogeneous beech forests distributed throughout the Czech Republic. The traps were checked, and beetles were sampled weekly or biweekly in 2022. Using Generalized Linear Mixed Models (GLMMs), the impact of selected environmental and management variables on the abundance of T. bicolor was analyzed. The species occurs at elevations ranging from 150 m a.s.l. to 1300 m a.s.l. throughout the Czech Republic. This represents its entire host range, e.g., beech, although in the lowest elevations it is also found on other deciduous trees. In 2022, the flight activity lasted from late April to early September, and two generations were recorded, with the filial generation starting to fly in late June. In many study sites, more than 10,000 beetles were captured per trap. The number of females trapped was higher than males, consistent with the species' polygamous strategy, where females move toward increasing concentrations of pheromones. The number of beetles in the traps was influenced by elevation, with the highest abundance observed at around 600 m a.s.l. Abundances were higher in areas with more extensive beech forest complexes. Lower beetle abundances were found in stands where the studied forest bordered with meadows, pastures, or crop fields, indicating the species' reluctance to fly outside the forest habitat. The abundance of the filial generation was higher when there was a larger area of beech forests within a 1000 m radius, suggesting a median dispersal flight distance of approximately 1 km for the filial generation.