Stand diversification increasingly emerges as a promising means for improving the multi-functionality and sustainability of management in plantation forests. Increasing tree species richness might potentially also benefit natural enemies, which can substantially contribute to sustainable forest management via top-down control of forest pests. However, there is little empirical evidence on how tree species richness affects the diversity and abundance of predators, as the majority of analyses to date have rarely gone beyond comparisons of monocultures and two species mixtures. Here, we analyzed the performance of spiders as important generalist predators in a tree diversity experiment that uses four of the economically most important broadleaved and coniferous tree species in Europe. We tested the extent to which tree species richness and the identity of the planted tree species affect the abundance, biomass, species richness and functional diversity of spiders. Whereas tree species richness in general had no significant effect, tree species identity strongly affected spider biomass and abundance—with a particularly strong negative effect of the non-native Douglas fir (Pseudotsuga menziesii (Mirb.) Franco). Our results indicate that increasing tree species richness does not necessarily promote characteristics of natural enemy assemblages relevant for pest control in forests and thus not all functions that may be important in a multi-functional management context. Rather, tree species composition and identity will often be of crucial importance in determining forest ecosystem functions and services. The fact that the severe impact of Douglas fir persisted even in diversified tree species mixtures suggests that stand-level predator efficiency can be reduced for tree species growing adjacent to or in mixture with this species. This calls for a more thorough examination of the ecological consequences of the increasing use of this species in forestry across Europe, in particular considering that climate change may increase the potential of pest outbreaks and thus the need for adequate control in the next decades.