Biocontrol of Weeds

Population Dynamical Approaches to Biocontrol

The focus of this project is to address a very general question: For a host with a given set of life history features, what types of pathogens and/or herbivores are likely to be most effective (in terms of initial rates of spread, ability to persist, and effects on host population size)? This will involve identifying important host (e.g. longevity, dispersal mode), pathogen and insect herbivore (e.g. transmission mode, stage-specificity of attack) life history features that (i) can distinguish qualitative classes of weeds and biocontrol agents, and (ii) are likely to be important in terms of their impact on rates of spread and persistence. The goal is to develop general models to investigate the dynamical consequences of interactions between different classes of host and biocontrol agent life histories, and use these to derive conditions for invasion and spread, and relative magnitude of host impact which might (particularly if results from the empirical model systems validate this approach) then be used as a general tool for focusing on target types of agents to be used for particular pest species.

Ultimately, the success of biocontrol programs depends not only on what happens within single populations, but on regional (metapopulation) persistence. In such situations, how the agent is deployed (e.g. the number and distribution of initial release sites) may influence overall dynamics and rates of spread (and therefore impacts on the host). In turn, these may determine the rate at which resistance evolves in the target host. We have developed biologically realistic, spatially explicit models that can be easily modified to investigate general questions, including: (i) Do optimal release strategies (e.g. few sites, much inoculum through to multiple sites, little inoculum) vary for target hosts and biocontrol agents according to their life histories?; (ii) Given particular life histories and dispersal scales, how do different deployment strategies influence long-term persistence and overall impact?; (iii) How does the rate at which host resistance evolves depend on the spatial scales at which hosts and biocontrol agents interact, and on the severity of fitness effects (e.g. is the optimal biocontrol agent always the one with the greatest immediate impact on host fitness)?

The overall objectives of this project are to (i) increase short-term efficiency of biological control programs; (ii) minimize possibility of evolution of resistance in target species; and (iii) develop general guidelines for matching the ecologies of hosts and potential biocontrol agents. Despite some positive changes in the last decade or two, biological control is still a very empirical process in which there is relatively little investigation of why programs succeed or fail. Existing accumulated knowledge of host-pathogen systems will be used to devise release and management strategies that should maximize the positive effects of control while considering the possibility of resistance changes.

Scientific Staff: Burdon, Thrall