COMPARISON OF 2 POPULATION-LEVEL ECOTOXICOLOGICAL END-POINTS - THE INTRINSIC (R(M)) AND INSTANTANEOUS (R(I)) RATES OF INCREASE

The instantaneous rate of increase (r(i)) was compared to the intrinsi c rate of increase (r(m)) as an alternative population-level ecotoxico logical endpoint. The terrestrial arthropod Acyrthosiphon pisum Harris , the pea aphid, was used as the model species and the new nicotinergi c insecticide, imidacloprid, as the xenobiotic. In the first experimen t, r(m) was generated each day from a life table for A. pisum neonates exposed to nontreated (control) potted broad bean, Vicia faba L. (var iety Banner), and compared to determine the earliest r(m) which was no t significantly different from the final r(m). The r(m) for neonates i ntroduced to nontreated plants was significantly different on days 6 a nd 7 but not significantly different between 8 to 40 d postintroductio n. A second experiment examined the change in r(i) over time for neona tes introduced at birth to nontreated broad bean and censused at 8, 11 , 13, 15, 17, 19, and 21 d postintroduction. The r(i) for neonate popu lations introduced to nontreated broad bean peaked on days 11 and 19. The r(i) on nontreated beans was significantly different (p < 0.05) fr om r(m) on days 8 and 15 but not significantly different at 11, 13, 17 , 19, and 21 d. A third experiment was conducted comparing r(m) and r( i) following 1 1 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 r(i) and r(m) 11 d after the start of the study, su ggesting that r(i) can be used as a predictive measure in place of r(m ). There was a negative relationship, y = 0.32 - 0.43x (r(2) = 0.88, n = 21), between r(i) and exposure to imidacloprid-treated broad bean, suggesting that simple regressions may be utilized as tools in the ass essment of xenobiotics on population growth rates.