Which leaf mechanical traits correlate with insect herbivory among feeding guilds?

There is abundant evidence that leaf mechanical traits deter feeding by insect herbivores, but little is known about which particular traits contribute to defence across feeding guilds. We investigated the contribution of multiple mechanical traits from shear, punch and tear tests to herbivore deterrence across feeding guilds.

Visible damage from miners and external chewers was measured and sucker feeding density estimated in mature leaves of 20 species of forest shrubs and small trees. Cafeteria trials were undertaken using a generalist chewer (larvae of Epiphyas postvittana, Lepidoptera). Damage was compared with leaf mechanical traits and associated nutrient and chemical defence traits.

Damage by external chewers in the field and by E. postvittana correlated negatively with mechanical traits. Hierarchical partitioning analysis indicated that the strongest independent contribution to chewing damage was by the material trait of specific work to shear, with 68 % of total variance explained by the combination of specific work to shear (alone explaining 54 %) and tannin activity in a regression model. Mining damage did not correlate with mechanical traits, probably because miners can avoid tissues that generate high strength and toughness in mature leaves. Mechanical traits correlated more strongly with chewing damage in the field than chemical defences (total phenolics and tannin activity) and nutrients (nitrogen and water), but nutrients correlated strongly with diet selection in the cafeteria trial. Surprisingly, sucker feeding density correlated positively with mechanical traits and negatively with nutrients.

Mechanical traits of mature leaves influenced insect feeding guilds differentially, reflecting differences in life history and feeding modes. For external chewers, energy (work) to fracture in shearing tests, at both structural and material levels, was strongly predictive of damage. Knowing which leaf mechanical traits influence insect feeding, and in which guilds, is important to our wider understanding of plant-herbivore interactions.