Phases of Assessment of Control Agent Impact

There are three phases in evaluating the success of a biological control program. First, before release of the agents, the status of the weed should be documented to provide a baseline for later comparison. Second, following release of the agents, establishment and spread of the agent population should be monitored. Third, once establishment has been confirmed, data on populations of the weed and agents should be compared to the pre-release baseline data to assess impact of the agents.

Baseline studies

Baseline measurements of the population density and growth characteristics of the weed are necessary for later quantitative evaluation of the effects of control agents, and to understand the biology and ecology of the weed as a basis for prioritisation of potential biological control agents.

Monitoring establishment and spread

Once an agent has been released, its establishment and spread should be monitored. Field release sites should be regularly monitored for the presence of the agent, evidence of breeding and changes in population density. Plans for determining spread away from the initial release sites should be made well in advance of their being needed.

Deciding when a released population is established depends very much on the phenology of the species. As a minimum, control agent populations should have passed through several generations and densities should be generally increasing or stable. Establishment of multivoltine species can be claimed if a population has survived and bred at a site for over twelve months. Establishment of univoltine species cannot be claimed until several years after release.

Monitoring rates of spread of agents should be done concurrently with monitoring for establishment. Fixed transects can be useful for initial indications of migration. For species with high dispersal capabilities, or once the immediate available habitat around a release site has been invaded, it is necessary to establish observation sites radiating from the release site. These sites should be visited at regular intervals and inspected for evidence of the agent. The distance of such sites from the release point will depend on the agent species.

Migration will not necessarily be uniform or linear over time. Although some species begin dispersing from the moment of release, others build up to locally high densities before dispersing long distances in search of host plants unaffected by their own species.

Evaluation of impact

Once establishment of populations of a control agent beyond the general area of its release has been confirmed, resources should be directed to measuring impact on the weed.

Studies of the effects of herbivorous insects on plants usually only consider losses to photosynthetic capacity or seed production (Lonsdale et al. 1995). It is one thing to measure the effect of a herbivorous insect on plant performance, but it is another to demonstrate that the herbivory is affecting the plant population dynamics (Crawley 1989). For example, the bud-feeding weevil Trichapion lativentre reduced seed production in the weed Sesbania punicea by more then 98%, but failed to cause a corresponding decline in the density of mature plants because the seed loss only removed plants that would have died from competition anyway (Hoffmann and Moran 1991). This example demonstrates that simple measures of damage alone may not be sufficient to indicate the success or otherwise of an agent.

The range of approaches that may be taken to determine the impact of biological control agents on the target weed includes experimental manipulation, correlation, or before-and-after photography. The principles of experimental design are applicable to some degree regardless of the approach used.

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