Using a process-based model to analyse compensatory growth in response to defoliation: Simulating herbivory by a biological control agent

The weevil Cleopus japonicus Wingelműller has been identified as a biological control agent for the highly invasive weed species Buddleia davidii. To study the potential effect of C. japonicus on the growth of this prolific plant, browsing was simulated on field grown plants over the course of a year, using four artificial defoliation levels (0, 33%, 66% and 100%). A simple process-based model was fitted to measurements to identify compensatory mechanisms induced by defoliation and to quantify their influence on above-ground plant biomass (Wp) and the ratio of leaf to total biomass (Wl/Wp). The method outlined in this paper provides a framework for quantifying the net growth impact of feeding by folivorous biological control agents on weeds. This method also provides a means of understanding critical levels of defoliation needed to achieve target levels of weed suppression. Results showed relative values of Wp for treatments in which 33%, 66% and 100% of leaf area had been removed, were 0.61, 0.44 and 0.08, respectively, compared to the undefoliated control. Defoliation intensity was positively related to light use efficiency (ε), daily allocation of biomass to leaves (γ) and specific leaf area, and negatively related to rates of natural leaf loss. Model results show that defoliation induced increases in ε and γ to be the most effective means of compensating for removed leaf area in the two treatments with highest levels of defoliation.