1- Pest regulation is an important ecosystem service provided by biodiversity, as plants growing in species-rich communities often experience associational resistance to herbivores. However, little is known about the respective influence of the quantity and identity of associated species on herbivory in focal plants. 2- Using a meta-analysis to compare insect herbivory in pure and mixed forests, we specifically tested the effects of the relative abundance of focal tree species and of phylogenetic distance between focal and associated tree species on the magnitude of associational resistance. 3- Overall, insect herbivory was significantly lower in mixed forests, but the outcome varied greatly depending on the phylogenetic relatedness among tree species and the degree of herbivore feeding specialization. 4- Specialist herbivore damage or abundance was positively related to relative abundance of their host trees, regardless of the phylogenetic distance between host and associated tree species. 5- By contrast, tree diversity triggered associational resistance to generalist herbivores only when tree mixtures included tree species phylogenetically distant to the focal species. 6- Synthesis and applications. Our study demonstrates that the establishment of mixed forests per se is not sufficient to convey associational resistance to herbivores if the identity of tree species associated in mixtures is not taken into account. As a general rule, mixing phylogenetically more distinct tree species, such as mixtures of conifers and broadleaved trees, results in more effective reduction in herbivore damage.