Consequences for Plantain Chemistry and Growth When Herbivores are Attacked by Predators

Abstract

The indirect effect of insect predators on the chemistry and performance of host plants of their prey is a central issue in the study of tritrophic—level interactions. Few experimental studies evaluate chemistry and performance of host plants fed upon by insect herbivores in the presence and absence of predators. To examine these relationships, we conducted a field experiment using the plant Plantago lanceolata, which contains the iridoid glycosides aucubin and catalpol. Each plot had two plants, designated the "colonization" plant upon which four insect herbivores were initially placed and the "dispersal" plant, to which the herbivores could move. We contrasted the effects of two insect herbivores, Junonia coenia (Nymphalidae), a specialist feeder, vs. Spilosoma congrua (Arctiidae), a more generalized feeder, and the effects of variation in their densities caused by two different types of insect predators, Podisus maculiventris stink bugs and Polistes fuscatus wasps, on host plant growth and phytochemistry. We contrasted these to plots without herbivores. The two herbivore species had different impacts on iridoid glycoside concentration of their host plants. Iridoid glycoside concentrations were higher in plants that had been used by the specialist J. coenia compared to those fed upon by the generalist S. congrua. In contrast to a previous study, herbivory did not induce higher iridoid glycoside concentration. The two herbivore species had similar impacts on plant mass. Predation by wasps and stink bugs had different indirect effects (via the interaction of predators and herbivores) on iridoid glycoside concentration and plant mass. Aucubin concentration was lower for plants that had had wasps than those with stink bugs. For plants with the specialist J. coenia, catalpol concentration was lower for plants that had had predators, and iridoid glycoside concentrations were lower for plants that had had wasps present than those that had stink bugs. Leaf mass was higher when predators were present than when predators were absent. For the plants with the specialist J. coenia, scape mass was higher for those that had predators present than those without predators. We discuss the potential for an indirect facultative mutualism between the host plant plantain and the insect predators, and conclude that such a mutualism is transitory, at best.