The most visible symptoms of emerald ash borer (EAB) (Argilus planipennis Fairmaire) infestation do not usually appear until six years after the borer’s arrival, by which time the prognosis is so grim that many communities have resorted to either heavy chemical use that only slows mortality, or clear-cutting the entire ash tree population. We utilized leaf-level spectroscopy for early detection of invasive pest-related stress, focusing on EAB as it reaches the edges of the geographic range for green and white ash trees (Fraxinus pennsylvanica and Fraxinus americana) in North America. Over the course of two full growing seasons, we sampled trees in three study areas with EAB infestation in Johnson County, Kansas, and two sample groups without infestation in Riley County, Kansas. Our method utilizes field spectrometer readings for reflectance, along with lab spectrophotometry for estimation of leaf chlorophyll and carotenoid content at several points during the growing season. Results show significant differences between pigment ratios and hyperspectral indicators between infested and non-infested ash trees, although the extent of the separation varies across the foliar season. This work has the potential to make stress diagnosis more effective, thereby improving response, and decreasing both chemical application and plant loss.