The plant stress hypothesis posits that a herbivore's reproductive success increases when it feeds on stressed plants, while the plant vigor hypothesis predicts that a herbivore preferentially feeds on more vigorous plants. We examined these opposing hypotheses by growing spider mites (Tetranychus urticae) on the leaves of stressed and healthy (vigorous) cucumber plants. Host plants were grown under controlled conditions at low, moderate, and high concentrations of NaCl (to induce salinity stress), at low, moderate, and high fertilizer concentrations (to support growth), and without these additions (control). The effects of these treatments were evaluated by measuring fresh and dry plant biomass, carotenoid and chlorophyll content, antioxidant enzyme activity, and concentrations of PO43-, K+, and Na+ in plant tissues. The addition of low concentrations of fertilizer increased dry mass, protein, and carotenoid content relative to controls, suggesting a beneficial effect on plants. The highest NaCl treatment (2560 mg L-1) resulted in increased Na+ and protein content relative to control plants, as well as reduced PO43-, K+, and chlorophyll levels and reduced catalase and ascorbate peroxidase enzyme activity levels. Analysis of life table data of T. urticae mites raised on leaves from the aforementioned plant groups showed the intrinsic rate of increase (r) for mites was 0.167 day-1 in control specimens, 0.125 day-1 for mites reared on plants treated with a moderate concentration of fertilizer (10 mL L-1), and was highest (0.241 day-1) on plants grown under moderate salinity conditions (1920 mg L-1 NaCl). Reproductive success of T. urticae did not differ on plants watered with a moderate concentration of NaCl or a high concentration of fertilizer. The moderately-stressed plants formed a favorable environment for the development and reproduction of spider mites, supporting the plant stress hypothesis.
Keywords: Fertilizer; Intrinsic rate of increase; NaCl; Plant vigor; Salinity; Stress.