Leaf tissue collected from six zoysiagrass (Zoysia spp.) cultivars was analyzed for polyphenols and flavonoids by high performance liquid chromatography (HPLC). The experiment investigated the effects of sampling date and N fertilization rate on green leaf chemistry of zoysiagrass and their relationship to fall armyworm (FAW) [Spodoptera frugiperda (J. E. Smith)] resistance. Six zoysiagrasses cultivars {'Crowne' (Z. japonica Steud.), 'Palisades' (Z. japonica Steud.), 'El Toro' (Z. japonica Steud.), 'Meyer' (Z. japonica Steud.), 'Cavalier' [Z. matrella (L.) Merr.], and 'Emerald' (Z. japonica Stued. x Z. tenuifolia Willd. ex Trin.} were planted in a randomized split-plot design with 4 replicates using cultivars as the main plot and 12.2 and 48.9 kg of N ha^-1 month ^-1 [(NH₄)₂SO₄] as the split plots. Flavonoid concentration decreased as the amount of applied N increased. Flavonoid concentration was greatest in June and steadily declined throughout July and August. Two unidentified flavonoids (luteolin-glycosides) were consistently associated with FAW mortality when analyzed by stepwise regression techniques. Unidentified luteolin 3 had an inverse relationship with mortality, while unidentified luteolin 9 was positively correlated with mortality. Additional research is needed to determine if the concentration of luteolins found in resistant zoysiagrasses are sufficient to cause biocidal activity in FAW.