DEMOGRAPHIC MODELS INFORM SELECTION OF BIOCONTROL AGENTS FOR GARLIC MUSTARD (ALLIARIA PETIOLATA)

Abstract

Nonindigenous invasive plants pose a major threat to natural communities worldwide. Biological control of weeds via selected introduction of their natural enemies can affect control over large spatial areas but also risk nontarget effects. To maximize effectiveness while minimizing risk, weed biocontrol programs should introduce the minimum number of host-specific natural enemies necessary to control an invasive nonindigenous plant. 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 reduction of seed output 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. If demographic models can successfully predict biocontrol agent impact on invasive plant populations, a continued dialogue and collaboration between empirical and theoretical approaches may be the key to the development of successful biocontrol tactics for plant invaders in the future.

Testing the mutualism disruption hypothesis: physiological mechanisms for invasion of intact perennial plant communities

“…In garlic mustard-invaded sites, the continuous presence of garlic mustard rosettes and/or adults, its high seedling and rosette mortality throughout the year (Davis et al 2006), and a 2-month period of intense leaf litter input as adults senesce (Anderson et al 1996, Cantor et al 2011) likely contribute to the year-round release of allelochemicals into the soil. Given the significant physiological suppression shown by our short-term pulse experiments, we anticipate that longer-term exposure of AMF-dependent plants to garlic mustard allelochemicals could affect carbon storage and resource allocation.…”

Theoretical contributions to biological control success

“…Bieb.) Cavara & Grande) (Brassicaceae) (Davis et al 2006;Evans et al 2012). One study, involving two parasitoids for control of mango mealy bug (Rastrococcus invadens Williams) (Hemiptera: Pseudococcidae), was designed to be prospective but became retrospective when biological control success was achieved faster than the system could be modeled (Godfray and Waage 1991).…”

A seasonal, density‐dependent model for the management of an invasive weed

“…Biocontrol agents for garlic mustard include monophagous Ceutorhynchus weevils (Blossey et al 2001, Davis et al 2006, Gerber et al 2009), most prominently C. scrobicollis. The effects of C. scrobicollis are modeled as a reduction in new seeds (by a proportion w s ) and second-year rosettes (by a proportion w r ) during season 4.…”

“…The effects of C. scrobicollis are modeled as a reduction in new seeds (by a proportion w s ) and second-year rosettes (by a proportion w r ) during season 4. We assume a linear relationship between these two effects with the ranges in Davis et al (2006) (9-43% rosette mortality, 11-49% seed reduction; w r ¼ 0.895w s À 0.008):…”