Demographic toxicological studies or life-table response experiments have been proposed as a more reliable approach for predicting pesticide impact in the field. Life-table parameters of the susceptible, propargite-resistant and reciprocal Fl hybrids of Tetranychus urticae Koch, 1836 (Prostigmata: Tetranychidae) were studied in the presence and absence of propargite residues at LC50 and LC99 of susceptible strain at Lincoln University, New Zealand. The life history data of all individuals were analyzed using the age-stage, two-sex life table. Treatment with LC50 of the susceptible strain did not affect the duration of developmental time of any strain. LC99 of the susceptible strain, however, prolonged the developmental time of the propargite-resistant strain by approximately 2 d. The intrinsic rate of increase (r(m)), R-0. and total progeny production of the propargite-resistant strain and S male x R female hybrid treated with LC50 of the susceptible strain were higher compared to that of the susceptible strain and R male x S female hybrid. Population projections were used to study the effects of relatively small differences in the life-table parameters of strains/hybrids of T. urticae. For the untreated control groups, the susceptible strain gave the highest population projection after 10 generations. In groups treated with LC50 of the susceptible strain, the projected population size showed that the number of adult females of the propargite-resistant strain superseded that of the susceptible strain. The hybrid S x R increased most from treatment with the LC50 of the susceptible strain. The differential success of different strains could, therefore, change resistance frequency throughout a growing season at a location.
