Local heterogeneity an important driver for predation pressure of a defoliating insect in boreal forests

Plant diversity is suggested to favour predation pressure on pest insect populations across agriculture and forestry. Although plant diversity is often correlated with predator diversity, its relationship with predation pressure remains inconsistent. Plant diversity and predator activity are affected by biotic and abiotic habitat factors. Thus, habitat heterogeneity may serve as a stronger predictor of predation pressure, however, its effect on the regulation of trophic interactions, such as predation, remains unclear. In this study, we investigated the influence of local habitat heterogeneity and tree species diversity on predation on Neodiprion sertifer larvae feeding on Pinus sylvestris . We measured tree and understory diversity and structure, presence of dead wood, boulders, and ants to examine their relationships with arthropod predation pressure on N. sertifer larvae. Notably, both predation rates and ant presence did not differ between mixed and monoculture stands. Local heterogeneity directly increased ant presence, which in turn increased larval predation. Furthermore, shrub density and boulders had additional direct positive effects on larval predation. Our results show that within-stand heterogeneity factors (shrubs, boulders, herb layer characteristics) may be a stronger determinant for predation on N. sertifer than tree species diversity. These findings indicate that plant diversity effects on predator-prey interactions are strongly dependent on the context of the forests where they are measured. To mitigate pest insect populations, forest management should, therefore, not only consider increasing the number of tree species but also include other aspects of local heterogeneity.