Biocontrol effectiveness when invasive plants defend via tolerance: a theoretical model

Background/Question/Methods: Between the two recognized categories of plant defense strategies, resistance and tolerance; tolerance is critical in determining the impact of defoliation by insect herbivores. The significance of tolerance as a defense strategy is further augmented in the context of biocontrol management of invasive plants as it is typically based on insect herbivory. Moreover, invasive plants with long-term belowground biomass are plants that can tolerate high amounts of aboveground defoliation or biomass loss. We developed a theoretical model for an invasive plant with long-term stored biomass and seasonal growth and its specialist insect herbivore using three state variables: aboveground biomass, belowground biomass and herbivore population density. The theoretical model analyses how three closely associated traits—(i) belowground biomass allocation to roots, (ii) spring utilization of stored reserves, and (iii) post-herbivory regrowth capacity—modulate the persistence of the plant population under different intensities of herbivory. 

Results/Conclusions: Model results highlighted the critical role played by stored belowground biomass in determining tolerance to aboveground herbivory by insects. Low belowground biomass allocation resulted in the extinction of both the herbivore and plant population, while stable coexistence of plants at low manageable biomass, along with its specialist insect, required a moderate amount of post-herbivory belowground allocation to roots. High values of belowground allocation and stored reserve utilization typically resulted in sustained cycles of the herbivore and plant populations. In the context of invasive plants with belowground stored biomass, model findings suggest that biocontrol agents (i.e.specialist insect herbivores) causing periodic short-term defoliation are incapable of extirpating plants that can efficiently allocate biomass belowground. Our model thus combines, for the first time, three key traits attributable to plants with long-term belowground storage (belowground biomass allocation, spring utilization of stored reserves and post-herbivory regrowth capacity) into a consideration of the effects of herbivory and offers a fresh perspective on how variation in these three key traits can influence the success of biocontrol programs.