Insect Outbreaks, Host-Pathogen Interactions, and Induced Plant Defenses
Abstract
Ecologists have long debated whether insect-herbivore population dynamics are driven by "top-down" forces, such as predators and pathogens, or "bottom-up" forces, such as induced plant defenses. In the case of outbreaking forest insects, theoretical ecologists have argued that top-down forces are most important, because mathematical models that incorporate only predators and pathogens explain time series of outbreaks from the field. Proponents of bottom-up forces concede that induced plant defenses often have only weak direct effects, but nevertheless argue that the models that do not consider plant quality are inaccurate because they do not include interactions between induced defenses and insect pathogens. Such interactions reduce average infection risk in the laboratory, and should therefore reduce infection rates during outbreaks in nature, when induced defenses are highest, but data from the field have shown that infection rates instead peak during outbreaks. Here we reconcile models, experiments, and field data, first by using a field experiment to show that, in the outbreaking North American gypsy moth, induced plant defenses affect pathogen transmission mainly by reducing variability in infection risk rather than average infection risk, which has the effect of increasing infection rates during outbreaks, not reducing them. We then show that this reduction in variability has important implications for insect population dynamics, because models in which variability in pathogen infection risk is determined by induced plant defenses produce realistic outbreaks, whereas standard models, when accurately parameterized, instead predict unrealistic stable population dynamics. Our work makes clear that top-down and bottom-up forces may interact in complex ways, contrary to traditional either-or viewpoints, and suggests that induction of plant defenses may augment the effectiveness of baculoviruses in reducing the damage caused by forest pests.
Document Details
- Document Type
- Technical Report
- Publication Date
- Sep 30, 2009
- Accession Number
- ADA519351
Entities
People
- Bret D. Elderd
- Brian J. Rehill
- Greg Dwyer