Elevational gradients provide natural experiments for examining how variation in abiotic forces such as nutrient mineralization rates, risk of photodamge, temperature, and precipitation influence plant-insect interactions. At the Coweeta LTER site in the Southern Appalachian Mountains, we examined spatial and temporal variation in striped maple, Acer pensylvanicum, foliar quality and associated patterns in the arthropod community. Variation in herbivore densities was associated more strongly with seasonal variation in plant quality than with spatial variation in quality among three sampling sites. Leaf chewer, but not phloem feeder or arthropod predator, densities increased with elevation. Foliar quality, by our measures, decreased throughout the growing season, with decreases in nitrogen concentrations and increases in lignin concentrations. Foliar quality varied among the three sites but not systematically along the elevational gradient. We conclude that, in this system, temporal heterogeneity in plant quality is likely to be more important to insect herbivores than is spatial heterogeneity and that other factors, such as the abiotic environment and natural enemies, likely have substantial effects on herbivore density.

Induced plant responses to leaf-chewing insects have been well studied, but considerably less is known about the effects of phloem-feedings insects on induction. In a set of laboratory experiments, we examined density-dependent induction by the milkweed-oleander aphid, Aphis nerii, of putative defenses in four milkweed species (Asclepias incarnata, Asclepias syriaca, Asclepias tuberosa, and Asclepias viridis). We hypothesized that high aphid density would lead to increased cardenolide expression in species with low constitutive levels of cardenolides (e.g., A. tuberosa), but that there would be no induction in high constitutive cardenolide species (e.g., A. viridis). Based on previous studies, we did not expect cardenolide induction in A. incarnata. Contrary to our predictions, we observed feeding-induced declines of cardenolide concentrations in A. viridis. Cardenolide concentrations did not respond to aphid feeding in the other three milkweed species. Aphids also caused reductions in biomass accumulation by two of four Asclepias species, A. viridis and A. incarnata. High aphid density led to a decrease in A. viridis foliar nitrogen concentration. However, aphids had no effect on the defensive chemistry, growth, or nutritional quality of either A. syriaca or A. tuberosa. Our results highlight that congeneric plant species may respond differently to the same levels of herbivore damage.

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Maternal effects have the potential to affect population dynamics and evolution. To affect population dynamics, maternal effects must influence offspring vital rates (birth, death, or movement). Here, we explore the magnitude of nongenetic maternal influence on the vital rates of an insect herbivore and explore predictability of maternal effects with reference to published studies. We experimentally studied the effects of maternal age, host plant species (two Asclepias spp.), and density on offspring vital rates in Aphis nerii, the oleander aphid. Older mothers produced offspring that lived shorter lives, consistent with the "Lansing Effect." Older mothers also produced offspring that matured at a younger age. As maternal age increased, offspring mass at maturity decreased when mothers were on Asclepias syriaca. However, offspring mass was highest from intermediate aged mothers on A. viridis. The absence of maternal density effects seems to exclude maternal density as a potential source of delayed density dependence in A. nerii. Our results indicate that maternal effects have some influence on A. nerii vital rates. However, references to published studies suggest that only the Lansing Effect is a predictable response to maternal age in insects. Moreover, the magnitude of observed effects was generally low.