Biomechanical components of the plant-insect herbivore arms race: a model test in leaf-cutter ants.

Abstract

Insects and plants have been locked in an evolutionary arms race spanning 350 million years. Insects evolved specialized tools to cut into plant tissue, and plants, to counter these attacks, developed diverse defence strategies. Much previous worked has focused on chemical defences. How can plants vary their mechanical properties to deter herbivores, and how can insects respond? We test a simple mechanical model that relates the force required to cut thin, leaf-like tissues to their mechanical properties and the geometry of the cutting tool. To remove the confounding effects of tool shape across size, we use leaf-cutter ant mandibles as a model system. Cutting forces were measured for pristine and worn mandibles that vary by one order of magnitude in size, using a custom-built fibre-optic set-up and homogeneous pseudoleaves as well as a set of plant tissues as model substrates. The results substantially support the model, enabling quantitative predictions. Fracture toughness is identified as a key mechanical defence trait for plants, cutting edge radius as the critical geometric property of the insect mandible and cutting edge wear consequently emerges as a key modulator of cutting forces, elevating it up to fivefold above a physical minimum. Thus, plants may be served by implementing strategies that maximize wear, whereas insects should seek to minimize it.