Trait diversity and spider community composition are associated with lower herbivory in young forest plantations

• Spiders can reduce forest pests and herbivory. • Forest pest suppression by spiders could be context-dependent. • The effect of spiders can depend on pest type, herbivory, forest type, and season. • Spider diversity and community composition are crucial for forest pest suppression. Spiders constitute a numerically dominant group of generalist predators in forest ecosystems, but their biocontrol function in forest plantations is not well understood. The biocontrol potential of spiders may depend upon interaction among pest type, forest type, season, trait composition, and spider community diversity. Using a correlative approach, we addressed this gap in knowledge in young (10–15 years) oak ( Quercus spp.) and ash ( Fraxinus spp.) forest plantations. We sampled (N = 103 samples) foliage-dwelling arthropods and leaves during late spring and autumn. We then measured traits of spiders (body size and hunting strategy) and examined two indicators of herbivory (leaf damage and leaf dry biomass). In oak plantations, abundances of Hemiptera negatively correlated with functional diversity of spiders consistently during the two seasons. Abundances of caterpillars negatively correlated with abundances of ambushers and marginally with abundances of orb-web building spiders during late spring. Abundances of herbivorous Coleoptera negatively correlated with functional evenness of spiders in autumn. Moreover, herbivory negatively correlated with abundances of Space-web builders and functional diversity of spiders in spring but with mean spider body size in autumn. In ash plantations, herbivory negatively correlated with spider abundances in autumn. Our findings provide indirect evidence that foliage-dwelling spiders can be useful for the biological control of forest pests. The biocontrol potential of spiders seems to depend on both functional diversity and identity. Therefore, forest pest management should focus not only on spider community composition but also functional trait diversity of spiders.