Effective Reduction in Natural Enemy Catches in Pheromone Traps Intended for Monitoring Orthotomicus erosus (Coleoptera, Curculionidae)

Infestations have persisted following a sudden and intense outbreak of the bark beetle Orthotomicus erosus along the Croatian coast, necessitating a continuous battle against this pest. A recommended protective action is the utilization of pheromone traps for population surveillance. Previous monitoring efforts have recorded an exceptionally high capture rate of natural enemies using pheromone traps; these traps inadvertently prevented natural enemies from fulfilling their essential role in controlling bark beetle populations. To address and significantly diminish instances of this unintended capture, our study designed a modification to the Theysohn-type pheromone trap by integrating a metal mesh within the trapping container. An experimental setup was established in Marjan Forest Park, situated on a peninsula bordered by the sea on three sides and partly by the city of Split. For monitoring purposes, unmodified standard pheromone traps were deployed at the onset of a significant O. erosus outbreak in Croatia in 2018. Catch data from 2020 to 2022 show a marked decrease in the bark beetle population, indicating a shift toward a latent phase. In 2022, modified traps were integrated into the existing monitoring setup, consisting of 10 pairs, to evaluate whether modifications to the traps could significantly reduce the capture of the bark beetle’s natural enemies, specifically Temnoscheila caerulea, Thanasimus formicarius, and Aulonium ruficorne. The objective is to offer recommendations for forestry practices on employing pheromone traps with minimal disturbance to the ecological equilibrium. Our findings indicate that the modifications to the traps markedly decreased the capture of natural predators, particularly T. caerulea, which was the predominant predatory insect found in the traps. Simultaneously, captures of the target species, all bark beetles in the trap, were marginally reduced. This decrease in the capture rates of the target bark beetle species, O. erosus, is not considered problematic when pheromone traps are utilized primarily for monitoring purposes. The modifications to the traps significantly reduced the capture of common bark beetle predators, thereby facilitating a more balanced strategy in forest protection efforts.