Spines Direct Insect Herbivores Away From Meristems and May Protect Grasses

Abstract

Many grasses have unidirectional hairs and spines that make it easy for insects to move from the leaf bases to leaf tips but hard to move in the other direction. This observation led to the hypothesis that these traits may direct small arthropod herbivores away from grass meristems which are located at the base of leaves and plants. I tested this hypothesis for two common grasses that grow at the Center for Ecological Research. For Andropogon virginicus, leaf tips received more chewing damage than bases. Grasshoppers landed on leaf bases but quickly oriented upwards. Leaves with more spines had less damage although areas in the field with more herbivores had plants with more damage and more spines. New leaves on plants that had been experimentally clipped had more spines although there was no evidence that neighbors also responded. For Phragmites australis, tips received more damage than bases although I found no evidence for an association between density of spines and damage. These results suggest that unidirectional hairs can protect some grasses from small chewing insects and motivate future work examining these traits in economically important grasses such as rice.