| Tall fescue, Lolium arundinaceum (Schreb.) S. J. Darbyshire, is the most widely cultivated cool season pasture grass in the United States. Standard cultivars, however, are sometimes relatively unpalatable for livestock due to rough texture. Also, tall fescue is frequently infected with a wild endophyte fungus (Neotyphodium coenophialum) that secretes toxic ergot alkaloids that sicken livestock. Grass breeders have developed forage-type tall fescue cultivars with softer texture for improved palatability and have incorporated novel endophyte strains lacking ergot alkaloids. Such grasses are more suitable for livestock, but such changes could promote insect outbreaks and compromise pasture sustainability. This work tested that hypothesis.;The armyworm, Mythimna unipuncta (Haworth), an important pasture pest, grew equally well on fescue groups differing in texture, ecotype, and provenance, an unexpected result. Armyworms progress from obligate window feeding to obligate edge feeding. Early instars are deterred by spiny leaf margins, but they compensate by window feeding later in development compared with those feeding on smooth-edged grasses. Armyworms exhibit ontogenetic changes in mandible morphology associated with feeding mode. Early instars have dentate mandibles ideal for scraping, but later instars have chisel-like mandibles ideal for snipping leaf edges. Mandibles of 3rd instars have aspects of both types, allowing for greater behavioral plasticity in feeding mode. Armyworms feeding on smooth- and spiny-edged grass blades did not differ in susceptibility to baculovirus infection, nor was there evidence that spiny leaf edges damage the midgut's peritrophic matrix.;Communities of herbivorous and predatory arthropods differed little among pasture grasses containing wild and novel endophytes. Endophyte-free pastures supported slightly higher populations of two leafhoppers, Endria inimica and Psammotettix striatus, and slightly higher numbers or weights of root-feeding scarab larvae, Cyclocephala spp. and Popillia japonica. Growth and development of armyworm did not differ between wild endophyte and endophyte-free grass.;This research suggests that tall fescue cultivars with smooth leaf texture or novel endophytes for improved livestock performance are not markedly more susceptible to damage from insect herbivores compared with cultivars of standard texture or containing wild endophyte. Such changes are not likely to threaten pasture sustainability from the standpoint of insect pests.;KEYWORDS: Tall fescue, Endophyte, Armyworm, Insect-plant relationships, Ontogenetic change. |
