Influences of plant quality and maternal environment on the performance and population dynamics of a phloem-feeding insect herbivore
Zehnder, Caralyn Beth
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An organism’s population dynamics can be influenced by its current environment, as well as the environment experienced by previous generations. Maternal effects, the influence of maternal phenotype or environment on offspring phenotype, have been implicated in the generation of population cycles. For insect herbivores, plant quality is an obvious factor through which maternal effects might operate. Aphis nerii, the milkweed-oleander aphid, is a phloem-feeding specialist of milkweed and oleander. Its host plants, species in the genus Asclepias, exhibit variation in many traits important to insect herbivores including foliar nitrogen concentrations, cardenolide concentrations and trichome densities. I first describe a set of experiments examining the effects of plant quality, insect clone and insect density on aphid vital rates (birth, death & migration). The importance of these factors was systematically assessed in both the maternal and offspring generation. While maternal effects were present, within generation effects on vital rates were much stronger and therefore more likely to impact aphid population dynamics. I next report an experimental examination of the effects of maternal age, density and host plant species on offspring vital rates. Of the three factors studied, maternal age had the largest influence on offspring vital rates. However, maternal age effects were not large in magnitude. Together, the first two studies indicate that while maternal effects are present in this system, it is unlikely that they strongly influence aphid population dynamics. I then present a density-manipulation experiment examining the effects of simulated nitrogen deposition on the interaction between aphids and their host plant. Nitrogen deposition increased plant foliar nitrogen concentrations, plant biomass and aphid per capita population growth. Nitrogen deposition caused aphid Rmax and K to increase proportionally, leading to no change in the strength of density-dependence. Finally, I discuss the effects of variation in aphid density on induced plant responses. Aphid density influenced plant chemistry and biomass, and the effect varied among plant species. However, in no case was there an increased expression of plant defensive compounds at higher aphid densities.