Strong teeth are needed to eat tough food: Caterpillar mandible durability in response to plant feeding

Plants, especially grasses, accumulate large amounts of silicon dioxide in their leaves.  Silica deposition increases the strength and plant rigidity, but it is also a critical component of defense against both abiotic and biotic stresses including herbivores.  This research determined the effect of plant silica deposition on insect herbivory and its inducibility.  We used corn, rice and the fall armyworm (FAW), Spodoptera frugiperda (Lepidoptera: Noctuidae) as a model system.  The inducibility of silica was tested by exposing the plants to herbivory for 24 hours and measuring the Si accumulation in the new regrowth tissues using the Molybdenum yellow method.  The effect of silica on FAW mandible durability was evaluated by examining the mandibles of caterpillars fed on corn, rice and artificial diet using backscatter X-ray spectroscopy.  Our results show that FAW herbivory induces silica accumulation in rice plants by ~37.2 % compared with undamaged controls.  Furthermore, feeding on plants with high silica content destroys most of the mineralized area of the FAW mandibles (mainly composed of Zn, Na and Cl).  We conclude that silica accumulation is an inducible plant defense mechanism that causes wear on the insect mandibles and likely hampers their ability to feed on plant tissues.