A COMPARISON OF ACUTE MORTALITY AND POPULATION-GROWTH RATE AS END-POINTS OF TOXICOLOGICAL EFFECT

The objective of this study was to determine how closely acute (72-hr) lethal concentration estimates developed from probit analysis compare d to the demographic toxicological endpoints, net reproductive rate (R -o), the intrinsic rate of increase (r(m)), and realized fecundity (U- x), in terms of predicting effects of pesticides on populations, Letha l and sublethal effects of the insecticide imidacloprid on the arthrop od Acyrthosiphon pisum Harris (pea aphid) were determined for populati ons exposed to foliar-sprayed broad bean Vicia faba L. (variety Banner ), An examination of R-o indicated that sublethal effects were occurri ng that reduced reproduction, However, by looking at the mean number o f offspring produced per surviving female and U-x, it was determined t hat the reduction in R-o was entirely due to acute mortality and a red uction in life span, Also, exposure to increasing concentrations of im idacloprid did not cause a shift in either the day of initial reproduc tion or the day of peak reproduction, Therefore, this pesticide caused no sublethal effects on reproduction and, as such, a lethal concentra tion estimate should have been a good predictor of effect at the popul ation level. However, the 72-hr lethal concentration estimate was not a good predictor of effect of this pesticide on population growth, Pop ulations exposed to the 72-hr LC60 were able to maintain rates of popu lation increase (r(m) = 0.224) similar to those of the control (r(m) = 0.295), The data indicate that the reason for the discrepancy between acute lethal concentration estimates and population growth was that s urviving individuals were able to sustain heightened rates of reproduc tion following acute exposure to imidacloprid, The ability of survivin g individuals to maintain these high reproductive rates allowed them t o compensate for losses and act as reservoirs for future reproduction, It is not possible, using acute mortality estimates alone, to predict this ''reservoir effect,'' and therefore not possible to predict how a population's growth rate will respond or change based on this endpoi nt, Thus this would suggest that the assessment of a xenobiotic based solely on acute mortality estimates will lead to flawed conclusions ab out a population's exposure response. (C) 1997 Academic Press.