Host phenology can affect leaf feeding insects through changes in foliar chemical-physical traits. Santiago's southern beeches (Nothofagus macrocarpa) form relic forest patches at the northernmost distribution range for the entire genus in America. Outbreaks of leaf feeding species in the genus Ormiscodes (Lepidoptera: Saturniidae) have the potential to defoliate extensive areas of several Nothofagus species; yet the mechanisms regulating their population dynamics remain largely unexplored. A univoltine Ormiscodes species that consumes N. macrocarpa foliage from mid October to November was studied. To investigate how host phenological variation in chemical traits relates with insect performance, larval growth and survival were determined at the time of natural hatch and on neonates whose emergence was delayed for 21, 30, and 79 days. Each time, foliar nitrogen and total phenolics concentration were quantified. Delaying egg hatch relative to plant phenology resulted in decreased larval growth and survival. Larval growth was more sensitive to changes in host quality than larval survival. The reduction of insect growth was strongly correlated with the concentration of foliar nitrogen, but not with total phenolics. These results indicate that the timing of larval emergence and foliar nitrogen concentration are key traits that may play a role in the population dynamics of this insect.