A 2021 Annual Review of Entomology synthesis (Jactel, Moreira and Castagneyrol) consolidated more than 600 case studies showing that insect herbivory is, on average, lower in mixed than in pure forest stands — associational resistance — while proving the effect contingent on herbivore diet breadth and tree species composition. The review's explicitly unresolved agenda comprises the conditions under which neighbourhood-induced changes in tree defence traits prevail, the relative importance of genotypic versus species-level tree diversity, the role of volatile-mediated tree-to-tree communication, the mediation of diversity effects by microbial multitrophic interactions, the interaction of climate and drought with diversity effects, the resistance of mixed forests to non-native invasive pests, and the optimal spatial arrangement of tree species in mixed stands.

Background

Forest entomology has long asked whether trees growing among other species suffer less damage from insect herbivores than trees in single-species (pure) stands. 'Associational resistance' (AR) names the pattern in which a focal plant experiences less herbivory when its neighbours belong to different species; the opposite pattern, 'associational susceptibility' (AS), also occurs. The dominant explanatory mechanisms are: (1) resource concentration and host apparency — specialist herbivores, which feed on one or a few related host species (generalists feed broadly), have more difficulty finding and remaining on hosts diluted among non-host neighbours, an effect tied to how conspicuous a host tree is to searching herbivores; (2) top-down control — mixed stands support more diverse and stable communities of natural enemies (predators and parasitoids); and (3) trait-mediated effects, in which the neighbourhood changes the focal tree's food quality or defences, including physical defences such as leaf toughness, chemical defences such as phenolics and terpenes, induced resistance, tolerance, and indirect defences such as extrafloral nectar and herbivore-induced volatiles. 'Volatile organic compounds' (VOCs) are airborne chemicals emitted by plants; damaged trees emit specific blends that neighbouring plants can perceive, sometimes 'priming' them (preparing defensive responses that are only fully deployed upon later attack) or directly inducing defences — so-called plant-to-plant communication. In a quantitative synthesis of more than 600 case studies, Jactel, Moreira and Castagneyrol (2021) confirmed that insect herbivory is on average lower in mixed than in pure forest stands, but showed the effect is contingent on herbivore diet breadth and tree species composition: tree species diversity mainly reduced damage by specialist herbivores, and most strongly in mixtures of phylogenetically distant tree species (for example conifers mixed with broadleaves). Their review, the field-defining statement of this research area, then enumerated the discipline's major unresolved uncertainties — the facets listed in the problem statement — which together define when and why diversification suppresses, or fails to suppress, forest insect pests. The practical stake is large: mixed-species stands and plantations are widely promoted for pest suppression and climate adaptation, yet current knowledge supports only an average-effect recommendation, not conditional, system-specific guidance on composition, genetics, and spatial design.

Problem Statement

The overarching objective, as set by Jactel et al. (2021), is to establish the conditions under which tree diversity reduces (or increases) insect herbivory in forests and the mechanisms responsible, so that associational effects can be predicted rather than described case by case. As of 2026, the unresolved facets of this objective are: (i) the conditions under which trait-mediated associational effects prevail — that is, when do neighbourhood-induced changes in tree defence traits (physical defences, secondary metabolites, induced resistance, tolerance, nutritional quality, and indirect defences such as extrafloral nectar and herbivore-induced volatiles) actually drive the observed change in herbivory, given that results so far are inconsistent; (ii) the relative importance and interactive effects of intraspecific (among-genotype) versus interspecific (species-level) tree diversity in driving associational resistance, hypothesised to favour species-diversity effects but scarcely measured in forests, with genotypic effects potentially substantial for generalist herbivores; (iii) whether VOC-mediated communication between trees — damaged emitter trees priming or inducing defences in receiver trees — is weak enough between heterospecifics to produce associational susceptibility in mixed forests, for which empirical evidence in forest systems remains very scarce; (iv) the role of multitrophic interactions with microorganisms (leaf and soil bacteria, mycorrhizal and saprophytic fungi, and pathogens), including the ecological consequences of cross-talk between anti-herbivore and anti-pathogen defence signalling pathways, in mediating tree diversity effects on insect herbivores; (v) how macroclimatic gradients (latitude, temperature, precipitation) and local microclimatic conditions such as drought interact with tree diversity to shape associational effects, given evidence that drought can reverse the direction of associational resistance or susceptibility depending on the tree–herbivore system; (vi) the mechanisms and generality of the resistance of mixed versus pure forests to non-native invasive insect herbivores — the pattern-level question (are mixed forests more resistant?) having now been answered meta-analytically in the affirmative for temperate Europe by Jactel et al. (2026), leaving the underlying mechanisms and biome- and guild-level generality open; and (vii) the optimal spatial arrangement of tree species within mixed stands (intimate tree-to-tree mixtures, row-wise patterns, or mixed hedgerows surrounding pure stands) for maximizing pest suppression while accommodating silvicultural and economic constraints. These facets are the review's own enumerated unresolved future directions (its sections 4.1–4.3 and 4.5–4.7, together with the spatial-arrangement discussion in section 5.3); each carries its own open question contributing to the overarching objective stated above.

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