Armyworm feeding plasticity trumps structural defenses of fescue pasture grasses

The true armyworm, Pseudaletia unipuncta (Haworth), develops equally well on fescue, Lolium spp., pasture grass cultivars having spiny or smooth-edged leaf blades, but behavioral mechanisms allowing for this plasticity were unknown. Second or third instars were fed grass blades of either type, and their behavior, especially edge feeding versus adaxial window feeding, was observed. Larvae were observed biting spiny edges but failing to remove leaf tissue. They compensated by more window feeding on spiny edged grass blades, and more edge feeding on smooth-edged blades. Although excision of the spiny edges resulted in increased edge feeding, larval growth rate did not increase with spine excision. In paired choice tests, smooth edged blades were preferred. Second instar caterpillars initially window feed but later switch to edge feeding as they grow; that shift occurs earlier on smooth edged grass. Because of this feeding plasticity, structural modification of fescue leaf edges for improved palatability to livestock is unlikely to increase or decrease pasture grass resistance to armyworms.