All organisms, including animals and plants, are faced with a seemingly fundamental trade-off of whether to grow or defend in that behaviors and processes associated with growth are typically at odds with optimal defensive strategies against enemies. In herbivores, this trade-off is manifested in the behavioral decision of whether to forage freely for plants and risk being eaten by a predator or, alternatively, to remain vigilant against predators by lowering activity rates (and thus food intake) but suffer the consequences of reduced growth. We used a tri-trophic system consisting of tomato plants, caterpillars (the Solanaceae-specialist, Manduca sexta, and the generalist, Trichoplusia ni), and the predaceous stinkbug Podisus maculiventris to test for such a trade-off in leaf-chewing insects. Our prior work in this system demonstrated that caterpillars feed less in the presence of stinkbugs and thus we predicted that such foraging decisions have costs for larval growth rates and development. We used predators with their mouthparts surgically altered to prevent killing and measured leaf damage, caterpillar weight gain, and frass production in predator-free and predation-risk environments. These three response variables were then used to assess caterpillar consumption, growth, and digestive efficiency in the presence and absence of predators.
Results/Conclusions
Both caterpillar species consumed less food in the presence of stinkbug predators (30-45% reduction in leaf area damaged), demonstrating that these insects perceive enemies in their environment and respond according to theoretical expectations of reduced foraging activity. The generalist herbivore, T. ni, suffered adverse growth penalties from this strategy with ~25% reduction in relative growth rate and declines in their digestive efficiency. Surprisingly, the specialist, M. sexta, did not experience similar costs, with larval growth rates remaining approximately constant in the presence and absence of predators, despite M. sexta consuming significantly less leaf tissue under predation-risk. M. sexta are able to do this by increasing their efficiency of conversion of digested food by 38% in the presence of stinkbugs. Thus, M. sexta larvae can apparently both grow and defend by enhancing their digestive capabilities when faced with predators, and therefore are able to (at least temporarily) circumvent the growth-defense trade-off.
