Submitted to: Ecological Applications
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/26/2006
Publication Date: 12/15/2006
Citation: Davis, A.S., Landis, D.A., Nuzzo, V., Blossey, B., Gerber, E., Hinz, H. 2006. Demographic models inform selection of biocontrol agents for garlic mustard. Ecological Applications. 16:2399-2410. Interpretive Summary: Garlic mustard (Alliaria petiolata) is one of the most troublesome plant invaders of temperate forests in North America. The large area and sensitive ecosystems occupied by garlic mustard make herbicide control of this species difficult, if not impossible. Biological control of garlic mustard is a promising alternative, with four species of Ceutorhynchus weevils that show strong host-specificity for garlic mustard currently in quarantine in the U.S. Biocontrol agents can be very effective in reducing invasive plant populations, but there is also the potential for non-target effects on native species and habitats. To reduce the possibility of non-target effects, we used a computer simulation of weevil effects on garlic mustard population dynamics to determine the minimum number of agents necessary to control garlic mustard in different parts of its range. This is the first time models have been used to guide plant biocontrol efforts prior to agent release. Our model showed that the combined action of two of the four agents would be sufficient to reduce populations of garlic mustard across its entire range, and one of the agents would be sufficient in areas where garlic mustard populations were growing slowly.
Technical Abstract: To reduce the possibility for non-target effects, biological weed control programs should select and introduce the minimum number of host specific natural enemies necessary to control an invasive non-indigenous plant. However, selection of the best agent (or agent combination) is no easy task and depends on the ability to forecast the anticipated impact of each herbivore species on host plant demography. We used elasticity analysis of a matrix model to help inform biocontrol agent selection for garlic mustard (Alliaria petiolata (M. Bieb.) Cavara and Grande). The Eurasian biennial A. petiolata is considered one of the most problematic invaders of temperate forests in North America. Four weevil species in the genus Ceutorhynchus (Coleoptera: Curculionidae) are currently considered potential biocontrol agents. These species attack rosettes (C. scrobicollis), stems (C. roberti, C. alliariae), and seeds (C. constrictus) of A. petiolata. Elasticity analyses using A. petiolata demographic parameters from North America indicated that changes in the rosette to flowering plant transition and changes in fecundity consistently had the greatest impact on population growth rate. These results suggest that attack by the rosette-feeder C. scrobicollis, which reduces overwintering survival, and seed or stem feeders that reduce seed output should be particularly effective. Model outcomes differed greatly as A. petiolata demographic parameters were varied within ranges observed in North America, indicating that successful control of A. petiolata populations may occur under some, but not all, conditions. Using these a priori analyses we predict (1) rosette mortality and reductions of inputs to seedbanks will be the most important factors determining A. petiolata demography; (2) the root-crown feeder C. scrobicollis will have the most significant impact on A. petiolata demography; (3) releases of single control agents are unlikely to control A. petiolata across its full range of demographic variability; (4) combinations of agents that simultaneously reduce rosette survival and seed production will be required to suppress the most vigorous A. petiolata populations. These predictions can be tested using established long-term monitoring sites coupled with a designed release program to foster a continued dialogue between empirical and theoretical approaches to develop successful biocontrol tactics for plant invaders.