Biological control of the Spruce Bark Beetle Ips typographus in Southern Alps

The spruce bark beetle, Ips typographus, is a destructive pest that targets Norway spruce, causing significant damage to European forests. Its infestations have intensified in the 21st century, driven by the expansion of Norway spruce beyond its natural range, poor forest management practices, and climate change. This thesis aims to identify novel strategies for biological control of the spruce bark beetle Ips typographus in the forest. Studies presented in this thesis deal with different strategies to control the spruce bark beetle with the aim to protect forest stands and reduce bark beetle population size in the field. One novel approach explored in this thesis was the use of the semiochemical push-and-pull strategy to manage beetle infestations. In a two-year field study in the South-Eastern Alps, this technique was tested following a major windthrow disturbance. The push-and-pull method involves using synthetic pheromones to attract beetles to traps ("pull") and repellent chemicals to keep them away from susceptible forest edge trees ("push"). In 2020, the study was conducted in windthrown areas, with push-and-pull, traps-only, or control protocols. In 2021, the study was repeated in clear-cut areas. Results from 2020 showed that the push-and-pull technique effectively reduced tree damage, especially in high-risk areas. However, traps alone did not prevent beetle colonization. In 2021, the push-and-pull strategy was less effective in clear-cut areas, suggesting that environmental factors and field conditions influence its success. The second study was conducted in the North-Eastern Italian Alps, treating trap logs with four biological control agents. Log treated with the predator-attracting lure Thanasiwit showed the most promise, reducing beetle offspring by 25.8%. Other treatments, such as the entomopathogenic fungi Beauveria bassiana and Metarhizium anisopliae, and the bacterium Bacillus thuringiensis, showed only minor reductions in offspring production, none of which were statistically significant. The success of Thanasiwit may be attributed to enhanced predation through higher attraction of volatile cues. However intraspecific competition could have contributed to this significant reduction. Though further optimization is needed to improve the efficacy of biological control agents in the field. The thesis explored the use of synthetic pheromone dispensers to induce intraspecific competition among beetles by artificially increasing colonization densities on trap logs. This method reduced the number of surviving adult beetles by 45.12%, with high-density conditions negatively impacting beetle reproduction and fitness. This strategy, which forces beetles to compete for resources, could be a viable population control method. The last study examined the effects of elevation and aspect on beetle winter survival and diapause in the northeastern Italian Alps. Results indicated higher beetle densities at lower elevations before winter, but no significant differences in density after winter. Survival rates were highest at higher elevations, likely due to lower pre-imaginal stage presence and reduced population densities. Solar radiation exposure on tree stems did not affect survival rates. Warmer winters driven by climate change could further increase beetle survival and reduce the impact of chilling temperatures. However, mild winter temperatures may also deplete beetles’ fat reserves, leading to higher mortality. The study also observed a shift in the sex ratio after diapause, which may affect reproductive success in the following season. The thesis underscores the need for integrated management strategies that combine biological control strategies with climate-aware forest management to mitigate the future risks posed by I. typographus in a changing climate