Plants in nature are constantly exposed to various stresses resulting from fluctuating environmental conditions which adversely affect their growth and development and may affect the performance of herbivores feeding on them. Drought and salinity are the most serious problems of agriculture due to climatic changes. According to global warming and drying of Urmia salty lake in Northwestern Iran which could result in salinization of agricultural lands nearby, the focus of this study was to examine the effect of two above mentioned stresses on a host plant and its pest population. Three levels of salinity (50 (low), 100 (intermediate) and 150 (high) mM NaCl) and water deficit (40–50% the amount of water given to control plants) were induced to common bean (Phaseolus vulgaris) as stress factors and the following characteristics were measured: fresh and dry weight, leaf area, chlorophyll, carotenoid, proline and protein contents and CAT, APX and GPX enzyme activities. The life table parameters of two-spotted spider mite, Tetranychus urticae Koch, reared on stress-induced plants were measured and compared with control. The high saline condition caused plant defoliation and was deleted from results. The low saline condition caused an increase in fresh weight and decrease in chlorophyll and protein contents of plant. Intermediate saline condition caused an increase in APX and CAT enzyme activities and decrease of dry weight, chlorophyll, protein and proline contents. Drought condition caused an increase of APX and GPX enzyme activities and proline content and decrease of dry weight and chlorophyll content. Analysis of life table data of T. urticae showed a significant decrease of net reproductive rate (R0), mean generation time (T) and gross reproductive rate (GRR) of mites reared on plants under both of stresses. The intrinsic rate of increase (r) and finite rate of increase (λ) decreased significantly only for mites reared on plants under intermediate saline condition.
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