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Performance of the specialist herbivore Spodoptera pectinicomis (Hampson) was studied when fed the floating aquatic plant waterlettuce, Pistia stratiotes L. (Araceae). Plants were either collected from 6 populations in southern Florida or from plants grown with low or high fertilizer levels. Consumption of leaves with increasing toughness resulted in increased larval mortality (>80%); most mortality occurred during the first 2 instars. Larvae compensated for low-nitrogen leaves by increasing fresh weight consumption 3-fold. Both developmental time and biomass gain were effected by the source of the plants but these performance parameters were not directly related to either leaf toughness or nitrogen levels. These results are useful in understanding the adaptations and limitations of specialist herbivores to low-nutrient foods. Additionally, they assist in the selection of suitable release sites for this weed biological control agent. Finally, they improve our mass-rearing techniques for augmentative releases of this biological control species.