Do enemies of herbivores influence plant growth and chemistry? Evidence from a seminatural experiment

It is predicted that enemies of insect herbivores may influence the effects of herbivores on their host plants by affecting the choice of plant genotypes. To examine the effect of predators, we conducted two experiments, each with a different caterpillar species (Junonia coenia and Pyrrharctia isabella). Under seminatural conditions, we provided a choice between two genotypes of plantain (Plantago lanceolata) with different levels of iridoid glycosides and used Podisus maculiventris stinkbugs as predators. There were four treatments:no herbivores and no predators, low density of herbivores and no predators, high density of herbivores and no predators, and high density of herbivores plus predators. The caterpillars had little effect on plant growth but did influence the iridoid glycoside concentration. For the Junonia experiment, the concentration of iridoid glycosides was less for plots with a low density of caterpillars (with no predators) compared to the other treatments of caterpillar density. In the Pyrrharctia experiment, catalpol was induced by a high density of caterpillars (with no predators). There were no increases in total iridoid glycosides associated with either herbivore species. The presence of predators had no effect on plant growth or total iridoid glycoside pattern. The lack of effect by predators seems to reflect the relatively large variation in iridoid glycoside concentration among leaf ages, and the herbivores ability to respond to that variation, such that the difference in iridoid glycoside concentrations in the plant genotypes was less important.