We treated two citrus cultivars with a complete fertilizer diluted to 25, 100, 200, or 400 ppm N to test whether increasing fertilizer concentration alters root and leaf chemistry and decreases resistance of citrus to root-feeding larvae of Diaprepes abbreviatus L. Roots and leaves of better-nourished ‘sour orange’ (Citrus aurantium L.) had larger amounts of total proteins and increased activities of enzymes associated with resistance than did plants given 25 ppm N. The fertilizer effect was less consistent for ‘Swingle citrumelo’ (C. paradise Macf. × Poncirus trifoliate L.), which has greater resistance to D. abbreviatus. Herbivory increased root protein content and peroxidase but decreased activities of chitinase and β-1,3-glucanase, which are enzymes associated with resistance to microbial pathogens. When significant, the effect of root herbivory on enzyme activities in leaves was opposite the effects on roots. Fertilizer and herbivory rarely interacted, indicating enzyme induction was not a function of nutrient supply. Fertilizer did not affect total phenolics in roots of either citrus, but root herbivory increased levels in ‘sour orange’. Despite elevated levels of putative defense proteins, ‘sour orange’ given ≥100 ppm N produced 50% greater total larval mass per pot than did plants given 25 ppm N. Fertilizer concentration did not affect mass of larvae on ‘Swingle citrumelo’ roots and did not affect larval mortality for either citrus cultivar. Our results concerning a root herbivore are consistent with the body of studies of folivores that have demonstrated that increased fertilizer has no effect or increases herbivore performance.
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