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 PO4
3−, 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 PO4
3−, 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 moderatesalinity 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.