Leaf mechanics and herbivory defence: How tough tissue along the leaf body deters growing insect herbivores

Research on herbivory defence often focuses on leaf chemistry but less on how plant mechanical properties like leaf veins deter herbivores. Herbivores often eat tough, complex plant tissue, yet how mechanical properties affect feeding performance as the consumer grows is unclear. We measured the toughness and strength of five types of leaf tissue – the midrib, the secondary and marginal veins and the lamina inside (inner) and outside (outer) the marginal vein – in mature Eucalyptus viminalis and Eucalyptus ovata leaves with punch tests. Leaf veins were, on average, 6.2 times tougher than lamina. Marginal veins were uniformly strong and tough along the leaf body, while midribs were less strong and secondary veins less tough toward leaf tips. We correlated the force required to puncture leaf tissue with the feeding performance of a chewing insect herbivore (the spiny leaf insect, Extatosoma tiaratum (Phasmida)) across four instar stages to explore the role of tough leaf veins as potential feeding barriers. Larvae more often ate less tough leaf tips and tougher tissue as they grew. However, younger larvae were capable of penetrating the tough marginal vein when starved. We suggest tough leaf veins and consumer position along the leaf body influence insect herbivore feeding performance over their lifetime.