The independent and interacting effects of predators and plant resistance on aphid movement and performance

There has always been a debate about the relative roles of different ecological forces, top-down and bottom-up, in determining herbivore performance, population dynamics and community structure. But the interactive effects of plant quality and predation on patterns of insect movement have not been well-studied. We determined how consumptive and non-consumptive effects of predators (Hippodamia convergens) and plant resistance alter the performance and dispersal of aphids (Macrosiphum euphorbiae) on tomato plants (Solanum lycopersicum). We manipulated plant resistance using three tomato lines that vary in the expression of the jasmonate pathway that mediates resistance to insects. The effects of predators were measured on the three tomato lines by comparing the impact of ‘no predators’, ‘lethal’ predators that both kill and scare prey and ‘risk’ predators whose mandibles were impaired to prevent killing. Aphids produced more offspring, had a higher population growth and dispersed more when feeding on low resistant plants and in the presence of predators. Predators and plant resistance interacted to effect aphid dispersal rate; in the presence of predators, aphids feeding on low resistant plants dispersed more than when feeding on high resistant plants. The number of offspring and movement results are at least partially due to the non-consumptive component of the predator as the predation risk treatment affected offspring production and dispersal rate. Thus, plant resistance and natural enemies have independent and interacting effects on aphid movement and performance and have the potential to influence patterns of distribution which could influence aphid densities, plant damage, or transmission of disease.