Technical Abstract: Susceptibility of the armyworm, Mythimna unipuncta (Haworth), to infection by a baculovirus isolated from a Kentucky armyworm population was compared on two suspected progenitors of tall fescue, Festuca mairei and Festuca arundinacea subsp. fenas, with spiny leaf margins intact or removed to test whether leaf spines abrade or puncture the peritrophic matrix (PM) and facilitate passage of virions to infection sites in midgut tissue. Edge spines had no effect on percent infection or days to death of newly molted 5th instars fed spiny- or smooth-edged F. mairei leaf blades for 24 h before and after droplet feeding doses of 108 or 109 OBs per ml. Fifth instars fed spiny- or smooth-edged grass blades had similarly undamaged PMs when viewed by scanning electron microscope. Fourth instars fed virus-treated F. arundinacea subsp. fenas leaf blades with spiny edges intact or removed did not differ in proportion infected or days to death. The food bolus moves in a liquid medium within the PM, and frass dissections showed that edge spines were often located inside a food bolus separated from the PM by non-spiny plant material rather than contiguous with the PM; therefore, friction of edge spines with the PM may have been low. These results suggest that armyworms will not be less susceptible to baculovirus infection when feeding on tall fescue cultivars with smooth leaf edges planted for improved livestock performance compared with those feeding on standard spiny-edged cultivars. This is the first study to investigate the effects of a natural physical structure on disruption of the PM and infection by a baculovirus.