Plant responses to herbivores and their effects on pest outbreaks

Background/Question/Methods

Insect pest outbreaks, sudden increases in herbivore densities, are detrimental to natural and agricultural plant populations. Plants respond to herbivores with different defensive strategies such as resistance and tolerance. Plant resistance, a bottom-up control strategy where plants reduce the survival or fecundity of the herbivores, has been an important focus of studies and mathematical models. However, plants also exhibit tolerance to low levels of herbivory, where damaged plants have the same fitness/yield as undamaged plants. In an extreme case of tolerance known as overcompensation, damaged plants have a higher fitness/yield than undamaged plants. Tolerance and overcompensation can have immediate effects on yield and herbivore density, but the long term implications are unknown. The increase in plant biomass could increase herbivore density, suggesting an overall reduction in plant fitness and requiring an increase in human-mediated control mechanisms, such as pesticide use. We have added tolerance and overcompensation to mathematical models that focused on the effects of inducible plant resistance and food limitation on herbivore population dynamics. Using these models, we quantify the effects of tolerance and overcompensation on herbivore population dynamics and determine the efficacy of using plant varieties with various defensive traits to control pest populations while maintaining yield.

Results/Conclusions

Our results reveal a complex interaction between the effects of food limitation, plant resistance, and plant tolerance on herbivore dynamics. When food limitation or the combination of food limitation and inducible resistance cause herbivore outbreaks, tolerance and overcompensation may either exaggerate or dampen these outbreaks, depending on properties of both the plant and the herbivore. Tolerance and overcompensation need to be incorporated into the bottom-up framework of plant-herbivore interactions to better predict and mitigate pest outbreaks. Our work suggests that different combinations of defensive strategies will be optimal for pest management under different conditions, and cautions against presuming that heightened tolerance or overcompensation in crop plants will necessarily result in greater yield.