Importance of population structure at the time of toxicant exposure

Populations in nature often consist of a mixture of stages and ages, yet to xicological studies even demographic studies, usually evaluate one starting life stage, In this study it was asked whether the starting age/stage stru cture of a population at the time of initial pesticide exposure influenced the impact that pesticides have on population growth rates. This question w as answered by exposing differently structured populations of two terrestri al arthropod species, the two-spotted spider mite, Tetranychus urticae (Koc h), and the pea aphid, Acrythosiphon pisum (Harris), to pesticides. The thr ee structured populations tested were (1) eggs or neonates for A. pisum and T. urticae, respectively, (2) stable age distribution, and (3) young adult females only. Instantaneous rates of population increase (r(i)) for the th ree structured populations mere determined over time without exposure to pe sticides (control) and after exposure to pesticides. Populations of T, urti cae were exposed to 100 ppm of the pesticide dicofol; populations of A. pis um were exposed to 200 ppm Neemix, The ri for the three control populations of T, urticae and A, pisum converged in a closed system 16 and 17 days aft er the start of the study, respectively, Unlike the control populations, th e ri of the three treated populations did not converge by Day 16 for the mi te species or Day 17 for the aphid species after exposure to pesticides. Gr owth rates of populations started as eggs (mites) or neonates (aphids) rema ined significantly lower than those of the adult or mixed-age populations ( P less than or equal to 0.05), Acute mortality data indicated that exposure to 100 ppm dicofol was equivalent to the LC,, for the egg stage, the LC,, for immatures, and the LC,, for adult T, urticae, Thus, even though the egg stage was the least susceptible stage of T, urticae, populations started a s eggs were significantly more susceptible than populations started as the stable age distribution or as adults. It was concluded that the initial str ucture of a population does have an influence on the impact that pesticides mill have on populations and that age/stage structure should be given seri ous consideration when evaluating toxicant effects. (C) 1999 Academic Press .