Posted by Hester Williams @HesterW123
Classical biological control, i.e. the introduction and release of non-native insects, mites, or pathogens to give permanent control, is the predominant method in invasive plant biocontrol. A successful biological control programme eventually reduces, or in some cases removes, the need other methods of control for an invasive species that is growing prolifically in the absence of its natural enemies. The benefit-to-cost ratio of successful control can be very high, especially when earlier successes in one country form the basis for repeating the introductions elsewhere.
New Zealand has a long history in biological control of invasive plants and is one of five countries that are very active in this field. In a recent analysis on the benefits of biocontrol in New Zealand it was found that 30 % of releases (of those that could be assessed because of sufficient passage of time) resulted in significant beneficial effects. Cases with negligible benefit (36%) included agents that failed to establish, and cases with minimal benefit (33%) included some cases where predation reduced the realized benefit of established agents.
An important component of all biocontrol projects is Post-Release Evaluation, the process of assessing how successful the projects have been and to understand why they succeed, fail or achieve intermediate results, and to determine and evaluate any non-target effects. Such information would not only provide better justification for biocontrol funding, but would also inform the agent selection process for subsequent projects, assist in the improvement of pre-release screening, help to increase establishment success and provide gateways to integrating biocontrol with other management practices.
Identifying biocontrol successes
Remarkable successes have been achieved through biocontrol projects, including the
control of St John’s wort (Hypericum perforatum) which used to be one of the worst four weeds in New Zealand. This plant displaced pasture in the dry high country and poisoned stock. Two beetles that defoliate the plant and a midge that stunts growth by deforming the plant were released as part of the biocontrol programme. The lesser St John’s wort beetle was the first to be released in 1943, while the greater St John’s wort beetle and the gall midge were released about 20 years later. All three agents established and today the plant has declined to the point where it is no longer considered a problem. A recent economic analysis has estimated that the Net Present Value of introducing the beetles is between $140 and $1490 million over 70 years, a benefit to cost ratio of 10:1 and 100:1 respectively. A remarkable return on investment!
Identifying biocontrol failures
Biocontrol programmes of course do not always result in successes, and failures are inevitable. Failures include inability of released agent populations to establish, or underperformance of agents. For example, here in New Zealand, the heather beetle (Lochmaea suturalis), has underperformed as a biocontrol agent when compared with the damage it does to native heather in Europe. Post-release evaluation studies have indicated that the smaller body size of beetles in NZ, probably mostly due to a severe founder effect, resulted in higher winter mortalities and therefore underperformance of the beetle in NZ. In 2014, an effort to genetically rescue the NZ population was undertaken; more beetles were collected from Scotland and mated with New Zealand beetles. These new genetic lines of beetles were released in November 2014 and currently post-release evaluation studies are underway to confirm establishment. Future studies will compare the performance of the new and original populations. This project represents a novel approach to explore the possibility of enhancing the performance of already established biocontrol agents so that they can better adapt to the local conditions and more effectively control the target weed.
Identifying Non-target effects of biocontrol
- Direct risk to non-target plant species (usually closely related species).
The case of the weevil Rhinocyllus conicus is particularly well known. First introduced from France to North America in 1968 to control invasive musk thistle (Carduus nutans), then widely distributed in the United States, this seed predator utilizes at least 22 native species of Cirsium in North America, including some species of conservation concern that have been shown to be seed-limited. Additional studies have found that the observed level of seed predation by the weevil to not be sufficient to limit seedling recruitment.
- Indirect non-target eﬀects, for example, via interactions in food webs.
A highly host plant–speciﬁc weed biocontrol agent, the tephritid fly, Mesoclanis polana, introduced into Australia to control bitou bush, is associated with declines of local insect communities. As the agent shares natural enemies (predators and parasitoids) with seed herbivore species from native plants, a study implicated locally signiﬁcant competition causing negative eﬀects on indigenous seed feeding insects.
- Conflicts of interest.
The proposed biological control programme for saltcedars (Tamarix spp.) in North America is associated with concern about the wellbeing of an endangered species—the south-western willow flycatcher. Originally this bird species nested in indigenous riparian vegetation. Many western riparian areas are now dominated by introduced invasive saltcedars, which the south-western willow flycatchers are now using for nesting.
Post Release Evaluation – necessary but expensive
Post release evaluation is expensive, and requires long-term funding commitments and community support. Modern biocontrol practices recognize the need for post-release evaluation of biocontrol programmes, but in the past it has been seen as ‘an expensive luxury’! This is because evaluation is often perceived as basic research with no additional benefits to the community and the funding agencies. When a project has clearly been highly successful it is unappealing to channel further resources into a former problem when so many others still require attention. Likewise, if a project appears to have failed there is little incentive to spend precious resources documenting this in more detail.
The cost of undertaking Post Release Evaluation studies has to date proven to be a major obstacle both in New Zealand and worldwide. In 2015, the National Biocontrol Collective (NBC), the major funder of the development and release of new weed biocontrol agents in New Zealand, accepted a National Assessment Protocol developed by Landcare Research to ensure some level of assessment is undertaken in biocontrol projects in New Zealand. The protocol outlines minimum standards plus further options where additional resources are available.
Role of Post-Graduate Research Programmes
This is where Universities and their post-graduate research programmes can and are making major contributions, as evaluation studies are often incorporated into their research programmes. We as post graduate students (low-paid but reasonably intelligent – or just amazing supervisors?) can indulge in detailed population and ecosystem level studies – the ultimate goal of post release evaluation.
My PHD study focusses on the dynamics of small populations. Many species benefit from the presence of conspecifics but at low population densities the fitness level of individuals in the population decrease. This phenomenon is known as the Allee effect. The Allee effect can drive very small populations to extinction and can play a major role in the establishment and spread of biocontrol agent populations.
I am studying how population size, dispersal and host patch connectivity interact with the Allee effect and how this influences the establishment and persistence of the leaf feeding beetle, Neolema ogloblini, a biocontrol agent for Tradescantia fluminensis. As mentioned in the introduction paragraph, 36 % of the biocontrol programmes in NZ has negligible benefit, in many cases because of establishment failure. My studies will help us to understand why some releases of biocontrol agents result in successful establishment and why others fail to do so.
Hester Williams is a PhD candidate in the School of Biological Sciences, University of Auckland and is stationed with the Landcare Research Biocontrol team in Lincoln, Canterbury. She is interested in invasion processes of both insect and plant species. Hester is supervised by Darren Ward (Landcare Research/University of Auckland) and Eckehard Brockerhoff (Scion), with Mandy Barron (Landcare Research) as advisor. Her studies are supported by a joint Ministry for Primary Industries – University of Auckland scholarship. The project is an integral part of an MBIE program “A Toolkit for the Urban Battlefield” led by Scion.