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

Abstract 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 ( W p ) and the ratio of leaf to total biomass ( W l / W p ). 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 W p 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 ( e ), 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 e and γ to be the most effective means of compensating for removed leaf area in the two treatments with highest levels of defoliation.