VARIATION IN PLANT QUALITY AND THE DYNAMICS OF INSECT HERBIVORE POPULATIONS by SANDRA ELIZABETH HELMS SPIRES

1. Previous studies have demonstrated that phenotypic traits of plants have the potential to affect interactions between herbivores and their natural enemies. Consequently, the impact of natural enemies on herbivore vital rates and population dynamics may vary among plant species. This study was designed to investigate the potential for density-dependent parasitism of an aphid herbivore feeding on six different host plant species. 2. Population densities of the aphid Aphis nerii B de F (Homoptera: Aphididae) and its parasitoid Lysiphlebus testaceipes Cresson (Hymenoptera: Braconidae) were recorded within a single growing season on six different species of milkweed in the genus Asclepias L. (Asclepiadaceae). Asclepias species are known to vary in their quality as food for herbivores. Although data on plant quality were not available in this study, population data were analyzed to determine the effects of different Asclepias species on rates of parasitism and aphid population growth. 3. Parasitism rates of A. nerii varied among Asclepias species but were temporally density dependent over at least some range of aphid density on all plant species. Aphid population growth rates also varied among Asclepias species, and declined with an increase in the maximum parasitism rates among plant species. However, in no case was density-dependent parasitism sufficient to prevent exponential population growth of aphids within the growing season. The results serve to emphasize that, if natural enemies are to regulate herbivore populations, density-dependent mortality is a necessary, but not sufficient, condition for