Experimental evidence of reduced Ips typographus damage in mixed spruce plantations

Due to ongoing climate change, outbreaks of the spruce bark beetle Ips typographus have caused massive tree mortality across Europe. As the forests most often damaged are pure spruce plantations, the question has arisen as to whether increasing tree species diversity could improve the resistance of spruce forests to the damage caused by these bark beetles. We took advantage of a spruce bark beetle infestation in a tree diversity experiment in Germany, where spruce trees were planted in pure or mixed plots, with one to three other tree species in a substitutive design, to test this hypothesis of associational resistance. Using aerial images, we retrospectively counted the number of spruces killed in all pure and mixed spruce plots from 2019 to 2023, and monitored new colonization by I. typographus in 2024 using pheromone traps. Bark beetle damage decreased significantly when the proportion of spruce trees in mixed plots was lower, a consequence of greater tree species richness. The damage and colonisation by bark beetles decreased even more when taller heterospecific neighbours, in particular Douglas firs, overshadowed the spruces, which probably reduced their visual and chemical apparency. This associational resistance likely stems from a combination of reduced host tree availability and release of non-host volatiles, causing disruption to the localization of the hosts by the beetles. These results confirm that mixing tree species can help prevent forest insect damage, and give an insight into the species composition of more resistant mixed spruce plantations, particularly with the association of other fast-growing species.