The instantaneous rate of increase (ri) was compared to the intrinsic rate of increase (rm) as an alternative population‐level ecotoxicological endpoint. The terrestrial arthropod Acyrthosiphon pisum Harris, the pea aphid, was used as the model species and the new nicotinergic insecticide, imidacloprid, as the xenobiotic. In the first experiment, rm was generated each day from a life table for A. pisum neonates exposed to nontreated (control) potted broad bean, Vicia faba L. (variety Banner), and compared to determine the earliest rm which was not significantly different from the final rm. The rm for neonates introduced to nontreated plants was significantly different on days 6 and 7 but not significantly different between 8 to 40 d postintroduction. A second experiment examined the change in ri over time for neonates introduced at birth to nontreated broad bean and censused at 8, 11, 13, 15, 17, 19, and 21 d postintroduction. The ri for neonate populations introduced to nontreated broad bean peaked on days 11 and 19. The ri on nontreated beans was significantly different (p < 0.05) from rm on days 8 and 15 but not significantly different at 11, 13, 17, 19, and 21 d. A third experiment was conducted comparing rm and ri following 11 d exposure to a range of imidacloprid concentrations applied to broad bean. There was a high correlation (r = 0.91, n = 21, p < 0.01) between ri and rm 11 d after the start of the study, suggesting that ri can be used as a predictive measure in place of rm. There was a negative relationship, y = 0.32 – 0.43x (r2 = 0.88, n = 21), between ri and exposure to imidacloprid‐treated broad bean, suggesting that simple regressions may be utilized as tools in the assessment of xenobiotics on population growth rates.