Alligator Weed: A Financial Snapshot
Alligator weed beginning to show severe moth damage in 1991.
Alligator weed (Alternanthera philoxeroides) is a tough plant to control. Herbicides can be used to some extent, but the structure of this plant, with its many nodes, means translocation can be poor. Manual/physical control can provide short-term relief in some situations, but the plant quickly regrows from any small fragments left behind. Fragments are also easily dispersed, particularly during floods, but also by birds, boats and humans. For these reasons, biocontrol was an obvious choice for alligator weed, particularly in the north, where warm temperatures create near perfect growing conditions.
Snapping at the heels of biocontrol programmes for alligator weed in North America and Australia, the first agent, a beetle (Agasicles hygrophila), was released here in 1982. A moth (Arcola malloi) was released soon after, and both agents established. Another beetle (Disonycha argentinensis) was also released but failed to establish. The alligator weed beetle and moth provide excellent control on static water bodies like lakes. However, they are less effective on flowing water (especially if it floods frequently), in colder areas, or where the weed invades dry land. The project is therefore categorised as a ‘partial success’, but recently an economic study has shown that a little biocontrol can go a long way.
Fortunately detailed information on alligator weed control costs and the impact of the biocontrol agents was published back in the 1980s. This enabled Simon Fowler to estimate that the biocontrol agents are saving around $505,000 per year in Auckland and Northland. These savings are ongoing and sustainable, with no further input required. Simon also prepared an estimate of the cost (at 2014 rates) of the New Zealand biocontrol programme, which came to $479,000. The figures were adjusted to present-day-value figures (using Treasury’s recommended 8% discount rate). The resulting benefit to cost ratio turns out to be an impressive 101:1. “This means for every dollar invested in alligator weed biocontrol there has been $101 worth of benefits,” explained Simon.
To test the validity of the economic analysis, Simon ran some scenarios to see how robust the figures were. In this sensitivity analysis discount rates of 6% and 10% were used, as well as scenarios where biocontrol costs were increased by 25% or benefits were reduced by 50%. Under all scenarios the results stood up well, with the overall benefit to cost ratios never dropping below 50:1, suggesting that the conclusion of substantial economic benefits arising from the biocontrol programme was robust.
“What was particularly unusual is that valuable information was also published in the 1980s study about the cost of controlling replacement weeds that can move in once alligator weed is controlled,” said Simon. Using this data we can calculate that replacement weeds are potentially reducing the effectiveness of alligator weed biocontrol by nearly 70%. However, this suggestion needs further investigation. It is not known exactly which weed species are replacing alligator weed, how easy or expensive they are to control, and if they could be better managed, for example through the development of biocontrol.
Since alligator weed control in the north is still estimated to be costing land managers $6.47 million per year in New Zealand, work to determine if other biocontrol agents could potentially increase the level of control is continuing. A thrips (Amynothrips andersoni) and a gall midge (Clinodiplosis alternantherae) have been rejected as insufficiently host specific. Testing of a stem/root galling fly (Ophiomyia marellii) has shown it has a clear preference for alligator weed but may also attack A. denticulata, a recent introduction to New Zealand, albeit to a lesser extent. The likely impact of the fly is being studied to help inform whether a case could be made that damage to A. denticulata might be acceptable. However, the economic study suggests it might be worth releasing the fly even if it is only able to increase the existing level of alligator weed control by a modest amount. A foliage-feeding beetle (Systena nitendula) is another potential agent that could be considered in the future.
Despite biocontrol agents only having a minimal impact on the overall control costs (a saving of 7.2%), the economic analysis has shown that the overall benefits are substantial. “The financial benefits well outweigh the costs of establishing the programme,” said Simon, “and this doesn’t include the flow-on effects for the environment, such as improved diversity and ecosystem functioning, that accompany the removal of invasive monocultures.” This study also once again emphasises the importance of monitoring the success of agents post-release.
This project was funded by the Ministry of Business, Innovation and Employment as part of Landcare Research’s Beating Weeds programme.