Weeds - Phase 1 - Research Highlights

Branched Broomrape and Siam Weed

What the report is about

This report details the methods used for predicting the cost of the national eradication of branched broomrape (Orobanche ramosa) and Siam weed (Chromolaena odorata). The report also provides estimates of the costs of such eradication programs over various time frames.

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Who is the report targeted at?

The report is targeted at policy makers to help inform the cost, length and feasibility of future investments aimed at eradicating branched broomrape and Siam weed.

Where are the relevant industries located in Australia?

The research was specifically focussed on Siam weed in Queensland and branched broomrape in South Australia. While the research is relevant to policy makers and land managers in these states in particular, it is also of interest to federal administrators involved in national cost-sharing arrangements for weed eradication. The results of the research have been formulated in terms of national eradication campaigns for these weeds.

Background

If an eradication program is to be successful it is essential that sufficient funding is available for completion of the program. In Australia, national cost-shared weed eradication programs have usually been started with poor, if any, estimates of anticipated program duration and hence the amount of resources that would be required to achieve eradication. This is understandable to a point, because during the early stages of an incursion there will be uncertainties about its extent and the critical biological attributes of the target. But periodic reviews of these programs have been carried out in an ad hoc manner, in circumstances of very high uncertainty about possible duration and thus resource requirements.

Aims/objectives

The objective of this project was to develop estimates of the duration and total cost of the national cost-shared eradication programs for branched broomrape (Orobanche ramosa) and Siam weed (Chromolaena odorata).

Methods used

Researchers used two different approaches for the eradication programs. For branched broomrape they acquired historical data in order to build an economic model designed to provide estimates of program duration and the resources required. For Siam weed researchers adopted the slightly different approach of estimating the resources required in order to achieve eradication within different time frames, based on combinations of various rates of progression and reversion.

For the branched broomrape program the researchers were able to establish relationships that describe the rates of progression from active to monitoring status for individual infestations (over time) and rates of reversion from monitoring to active status, as well as relationships that predict rates of detection of newly infested area.

In both cases the researchers used a stochastic dynamic model that employs key relationships to predict how long it will be until completion of the eradication programs that included:

• detection of new infested area
• rates of progression to monitoring status and reversion to active status for branched broomrape, and
• rates of progression to monitoring status and reversion to active status for Siam weed

Data was also acquired on program expenditure and its allocation between different activities; for example, searching, control and administration.

Results/key findings

The effort to achieve weed eradication consists of the search effort required to define the extent of an incursion plus the search and control effort required to prevent reproduction until extirpation occurs over the entire infested area. Researchers have estimated the total costs of the programs until their predicted completion. These costs have been apportioned between different activities, such as searching, weed control, administration, research and communication.

Given the 2008 levels of investment used, the model predicted it would take on average an additional 73 years to eradicate branched broomrape in South Australia at an additional cost (net present value) of A$67.9 million. For the final 20 or so years, however, fewer than 10 hectares of infested area might remain, so there could be scope to shorten the program's duration considerably through application of expensive methods such as fumigation. Estimates of program costs varied between A$63 million and A$75 million, with control and searching being the largest components-at 53.6 per cent and 23.8 per cent respectively.

In no case was eradication possible in 20 years or less for Siam weed. In order for eradication to be achieved in 25 years, the rate of progression from active to monitoring status needs to be consistently high (more than 0.8 of active infestations should progress to monitoring stage within 12 months) and reversion coefficients of less than -0.275 should be achieved and maintained. Since 2004 the rate of progression in the Siam weed eradication program has been considerably lower (range 0.118-0.136). In the same period the reversion coefficient has varied between -0.082 and -0.253. These figures suggest that a longer eradication time-frame is more suitable. It should also be noted that for the purposes of this analysis it was assumed that there were no further detections of infestation.

These figures suggest that, unless major improvements are effected in the management of this weed-a recent substantial increase in program investment might help in this regard-a longer time frame is more suitable. Note that in this analysis researchers assumed that no further infested area is detected, which is probably unrealistic.

Assuming a 50-year time frame for eradication, the total cost (present value) of eradicating Siam weed was predicted to be A$10.1 million. The largest component of this cost was searching (65.2 per cent), consisting of ground searching (57.8 per cent) and helicopter searching (7.4 per cent). Administration accounted for the second-largest component (23.8 per cent). The total costs of eradication were not much different when shorter time frames were considered-ranging from A$9.6 million for a 40-year program to A$8.2 million for a 25-year program.

Implications for relevant stakeholders

The research has implications for governments, industry stakeholders, and land managers as it can act as a guide to future investment decisions.

However, the researchers believe the estimates for both program duration and cost are conservative because they are not anticipating major increases in the total infested area. Significant increases in newly detected areas would obviously extend the program and give rise to substantial additional costs.

Recommendations

Through the application of a simple stochastic dynamic model researchers have been able to make predictions of the duration and cost of two ongoing national cost-shared eradication programs. The work can be relatively easily extended to other weed eradication targets, essentially requiring simple modifications in programming to reflect the maximum seed persistence expected for the target in question.

This has proved a fertile area for investigation, and there are still a number of aspects that remain to be explored. The most important of these is the definition of parameter combinations that would allow eradication within defined time frames and the linkage of these combinations to on-ground practice. In the absence of cost-effective tactics for directly depleting the soil seed bank, every effort should be made to ensure that plants do not achieve reproductive status.