Effects of predation risk and plant resistance on Manduca sexta caterpillar feeding behaviour and physiology

Because predation risk typically alters how prey forage for food, interactions between predation risk and food availability are commonly found. Less is known about how host plant quality and predation risk interact to affect prey behaviour and physiology. Using the caterpillar, Manduca sexta, and its predator, Podisus maculiventris, the effects of predation risk and host plant quality on caterpillar feeding, growth, assimilation efficiency, and resting metabolic rate were tested. Overall, caterpillars on low-resistance tomato plants (jasmonate-insensitive) gained 14% more mass than caterpillars on high-resistance plants (wild-type tomato). On low-resistance plants, the presence of predators caused caterpillars to eat 32% less, but they gained the same mass as unthreatened caterpillars (i.e. a 19% increase in assimilation efficiency). In addition, caterpillars showed a 17% increase in resting metabolic rate in the presence of predators. On high-resistance plants, predation risk caused a decrease in feeding, but did not alter assimilation efficiency or resting metabolic rate. The reduction in physiological responses to predation risk on high- versus low-resistance plants demonstrates a tradeoff between the ability to respond to predation risk and the ability to grow, especially on well-defended plants.