There has been great speculation on the consequences of resource quality, as determined by plant nutritional and defensive traits, in regulating population of insect herbivores. Although density dependence has been well studied, the mechanisms that generate the strength of density dependence are not well understood. Nutritional and defensive traits are plastic and can be induced as herbivore density increases. Induced nutrients can increase the plant quality whereas induced defenses can decrease quality to the herbivore, and therefore effects on density dependence can be hard to predict as both traits are induced following herbivore feeding. We investigated whether plant resistance affects (i) aphid performance in terms of fecundity and weight, and (ii) aphid population growth and density-dependent processes on Macrosiphum euphorbiae. We manipulated plant resistance using three tomato lines that vary in the expression of jasmonic acid, a phytohormone that mediates resistance to insects.
Results/Conclusions
We found that highly resistant plants decreased aphid fecundity and weight. Aphids feeding on plants with low resistance showed higher per capita growth rate which translated into higher aphid density. In contrast, on high resistance plants aphids showed lower population growth and lower densities. Regulation of aphid population growth was strongly density-dependent on plants with low resistance, weaker but still significant on plants with intermediate levels of resistance, and non-existent on highly resistant plants, which were regulated by density-independent mechanisms. Therefore, plant resistance has the potential to determine the strength of density dependence in aphid populations. These results suggest that both nutritional and defensive traits might be involved in aphid population regulation as induced changes are expected to occur on low resistance and intermediate resistance plants, remaining unaltered on high resistance plants.