Importance of Population Structure at the Time of Toxicant Exposure

Populations in nature often consist of a mixture of stages and ages, yet toxicological studies even demographic studies, usually evaluate one starting life stage. In this study it was asked whether the starting age/stage structure of a population at the time of initial pesticide exposure influenced the impact that pesticides have on population growth rates. This question was answered by exposing differently structured populations of two terrestrial arthropod species, the two-spotted spider mite,Tetranychus urticae(Koch), and the pea aphid,Acrythosiphon pisum(Harris), to pesticides. The three structured populations tested were (1) eggs or neonates forA. pisumandT. urticae,respectively, (2) stable age distribution, and (3) young adult females only. Instantaneous rates of population increase (ri) for the three structured populations were determined over time without exposure to pesticides (control) and after exposure to pesticides. Populations ofT. urticaewere exposed to 100 ppm of the pesticide dicofol; populations ofA. pisumwere exposed to 200 ppm Neemix. Therifor the three control populations ofT. urticaeandA. pisumconverged in a closed system 16 and 17 days after the start of the study, respectively. Unlike the control populations, theriof the three treated populations did not converge by Day 16 for the mite species or Day 17 for the aphid species after exposure to pesticides. Growth rates of populations started as eggs (mites) or neonates (aphids) remained significantly lower than those of the adult or mixed-age populations (P≤0.05). Acute mortality data indicated that exposure to 100 ppm dicofol was equivalent to the LC21for the egg stage, the LC59for immatures, and the LC69for adultT. urticae. Thus, even though the egg stage was the least susceptible stage ofT. urticae, populations started as eggs were significantly more susceptible than populations started as the stable age distribution or as adults. It was concluded that the initial structure of a population does have an influence on the impact that pesticides will have on populations and that age/stage structure should be given serious consideration when evaluating toxicant effects.