Cabomba Eradication Programme

The Decision to Eradicate

The decision to eradicate cabomba was based on:

• The potential impacts of cabomba.
• The successful eradication of cabomba from Marlow Lagoon using chemicals in 2002.
• The limited extent of the infestation (approximately one hectare).

The Taskforce

When cabomba was discovered in Darwin River in 2004, a task force was formed to direct, coordinate and oversee the cabomba eradication program. Other stakeholder groups, including AFANT, Local Government and the NT Environment Centre were also consulted. The taskforce consisted of:

Agency	Business Unit	Role
NRETAS	Biodiversity Conservation Unit	Monitoring riparian zone, aquatic fauna and flora.
	Weeds Management Branch	Survey, control, monitoring, landholder liaison
	Advisory and Regulatory Services	Water quality monitoring
	Marketing and Communications	Media management/contact/public awareness
	Environment and Heritage	Environmental monitoring and advice
DPIFM	Horticulture	Industry liaison
	Aquatic Pest Management Group	Industry Liaison
	Fisheries and Aquaculture	Industry Liaison
DHCS	Environmental Health	Drinking water quality monitoring,
PAW	Power Water	Domestic water supply, dam quarantine

The Traditional Owners of the Area, the Kungarakany People, were also represented on the taskforce to oversee the protection of registered sacred sites.

Based on the success to date of the eradication plan, the intergovernmental/community taskforce has been disbanded and been replaced by the Cabomba Community Reference Group. All key stakeholders will continue to be involved in the management process.

Quarantine Areas

If cabomba spreads into other water bodies, the process of eradication will be more expensive and difficult. To minimise the chance of cabomba being spread beyond its current distribution, the infested section of Darwin River was placed under quarantine on 9 November 2004. The quarantine order will remain in place until 8 November 2011, or until it is revoked or extended by the Minister for Natural Resources, Environment and Heritage.

The area quarantined comprises the section of Darwin River between Cox Peninsula Rd and Leonino Rd. This prohibits the movement of people or any object, including boats, vehicles and fishing equipment, into or out of this section of river and the five metres of land adjacent to the water’s edge, unless an appropriate permit from the Department of Natural Resources, Environment, the Arts, and Sport (NRETAS) has been obtained. Vehicles are not to pass over causeways at Old Bynoe Road or Reedbeds Road if the river is flowing over these causeways.

Non-compliance to this order is an offence with a maximum penalty of $50,000 for individuals and $250,000 for body corporate.

• Darwin River Quarantine (pdf 131Kb)

Every effort is made to restrict spread of cabomba by non-human means. Water from infested billabongs is restricted by installing floating booms at each end of the billabongs during the dry season. Unfortunately, these are ineffective during high wet season flows. Control of feral animals such as pigs is also important.

Awareness

It is important that as many people as possible are aware of cabomba, its potential impacts, and the fact that an eradication program is in process. This is vital for three reasons:

• Some people may still have cabomba growing in aquariums, unaware of the threat that the plant poses or its associated status as a weed.
• People need to be aware that Darwin River is quarantined, especially those living in the area. Further spread could have devastating implications.
• Improved awareness increases our chances of locating unknown or new infestations before they spread further and become too big to eradicate.

Awareness has been promoted by conducting information days at the river, installing quarantine signs at access points along the river, posting notices in local shops, placing adverts in local newspapers, distributing information into the area by mail, developing a brochure for wider distribution including bait and tackle shops, and targeting specific groups that spend time at freshwater bodies, including Kakadu and NT park rangers, water monitoring workers, medical entomology staff, Charles Darwin University researchers, Landcare groups, other nature-based clubs and fishermen.

Surveillance

Early detection of any new infestations is a vital part of the eradication program. For this reason, NRETAS Weeds Management Branch regularly monitor a number of water bodies in the region. These include:

• Uninfested parts of the Darwin River,
• Sections of the Blackmore River,
• Berry Springs Nature Reserve,
• Howard Springs Nature Reserve,
• McMinns Lagoon,
• Fairway Waters,
• Girraween Lagoon,
• Darwin River Dam,
• Manton Dam,
• Marlow Lagoon,
• Knuckey Lagoon, and
• Numerous other water bodies throughout the Darwin, Palmerston, and Outer Darwin areas.

Control Options

1. Physical/Mechanical - Physical and mechanical removal is only suitable for small infestations because stem fragmentation is common, and if left behind, fragments will simply develop into new plants, either at the same location or at a different location downstream.
2. Draining - Draining the river and drying plants out is not feasible given the amount of water that would need to be moved (over 100 MegaLitres) and the presence of freshwater springs.
3. Biological - There are currently no biological controls for cabomba, and although biological control is being investigated, the timeline on introducing a biocontrol agent, should one be found, is at least 10 years.
4. Liming - Adding lime could be used to control cabomba, however initial tests are not promising, and again would only be a suitable option for small infestations.
5. Salt – Salt could be used for small infestations. However, there may be off-target impacts to this method because salt-tolerance levels may be lower for surrounding species compared to cabomba.
6. Shading - Shading the cabomba with black plastic has been researched and results are encouraging. Results suggest that a reduction in light of at least 90% must be achieved to kill plants. Shading plants has been shown to hinder growth and flower and seed production. Shading was successfully used, in combination with herbicide application (see below), to tackle the Darwin River infestations.
7. Herbicide - Herbicide control (the use of herbicides) has proven to be the most feasible control option . There are no registered herbicides for use on Cabomba in Australia. Currently the Weed Management Branch of NRETAS applies to the Australian Pesticides and Veterinary Medicines Authority (AVPMA) to use 2,4-D n butyl ester to control cabomba in Darwin River under an off-label permit. The same chemical was used to eradicate cabomba from Marlow Lagoon.

The eradication program carried out at Marlow Lagoon between 1996 and 2002 implemented a wide range of control methods, including draining the lagoon, manually and mechanically removing plants, shading with plastic and covering with carpet. All of these methods failed, so in 2002, herbicide was used. After one treatment of the affected area with the herbicide 2,4-D n butyl ester, all the cabomba was killed and it has never returned. Of the available options listed above, four have been successfully used at Darwin River and are intended for use into the future. These are: chemical, shading, and a combination of draining and physical removal.

Chemical

Agricrop Rubbervine Spray, with the active constituent 2,4-D n butyl ester, is a commercial herbicide is registered by AVPMA.  This Authority is tasked with ensuring that products are safe and effective when used as directed and do not have an adverse effect on the environment or human health. Importantly 2,4-D n butyl ester does not accumulate in the food chain. In 2003 the APVMA deregistered all products containing 2,4-D butyl ester in Australia.

2,4-D n butyl ester is listed in the 1996 Australian Drinking water Guidelines .  These guidelines state that 2,4-D n butyl ester should not be detected in drinking water and if present, would not be a health concern unless the concentration exceeded 0.03mg/L.  Regular water monitoring of the affected area has shown, where detectable, concentrations are well below the stipulated level.

Submerged nozzles are used to apply a mixture of Agricrop Rubbervine Spray and diatomaceous earth. This light silica soil absorbs the herbicide and makes it less mobile. This method of application ensures that the control is as targeted as possible and the chance of herbicide spread is minimised. Herbicide is primarily applied in the Dry season when water is clear and water flow is minimal. A high degree of sub-surface visibility is essential for targeted application and minimal flow means the herbicide will remain in the required area for as long as possible.

2,4-D n butyl ester has been applied to all four known cabomba infestations with a high degree of success. Small remnant populations of cabomba are now only present in one of the four originally infested areas.

Herbicide application at Darwin River using hand-held wand (jpg 47Kb)

 

Careful Planning for Herbicide Use

Darwin River is an important area for many reasons. The river runs through a rural residential area, it provides the necessary water resources for several aquaculture developments and it connects to Darwin River Dam, a major water supply area for Darwin. Additionally, the River is highly valued from cultural and environment perspectives.

To adequately plan for Cabomba eradication, all potential issues associated with herbicide use needed to be identified, then monitored for during the management process. It was essential to involve the community and all relevant stakeholders throughout the entire management process. Potential issues and the identified management actions are outlined below:

1. Contamination of river water - Some landholders in the area use river water for drinking, washing and irrigation. Possible contamination during herbicide application meant that an alternative water supply needed to be secured for these people and as a result alternative water sources were established where necessary. Continuous water monitoring has never indicated chemical levels that would preclude human consumption, as determined by the Australian Drinking water Guidelines. Vigilant water quality monitoring has continued throughout the project.
2. Possible contamination of estuary water – A similar situation existed downstream where aquaculture farms pump estuary water into their ponds. As a precautionary measure, a bund wall was constructed in 2004 below the treated area to block the flow of water. Water quality in the estuary was monitored by the Advisory and Regulatory Services Division of NRETAS. The effect of dilution meant that the concentrations of 2,4-D n butyl ester in the estuary was very low, generally below detection. The bund wall was removed when wet season water flow became too high. Throughout 2006 and 2007 herbicide was applied in small, fortnightly doses without a bund wall in place. Water quality was monitored carefully so that the application amount could be reduced if unacceptable levels were detected downstream from sample sites.
3. Possible contamination of groundwater - 12 weeks after the initial herbicide treatment in 2004, 12 bores were tested for contamination. In all samples, neither 2,4-D n butyl ester nor its breakdown products were detected. The level of detection for these products is well below the acceptable levels of consumption as outlined in the Australian drinking water guidelines. Testing in 2006, 2007 and 2008 reconfirmed the lack of contamination.
4. Possible impacts of chemicals on surrounding fauna. Since the first herbicide applications in 2004, the effect of 2,4-D n butyl ester on wildlife has been monitored by the Biodiversity Conservation Unit of NRETAS. It was concluded that herbicide application has had minimal impact on wildlife. No fish kills were observed during or after applications and incidental wildlife sightings during surveys supported this conclusion. Water monitors, water dragons, barramundi and rainbow fish were regularly observed at previously treated sites. Crocodiles and turtles have also been observed in treated water bodies, and this continues.

Wherever possible herbicide is used in conjunction with other methods.

Shades installed at Darwin River (jpg 98Kb)

Shades installed at Darwin River (jpg 70Kb)

Shading

Shading is primarily designed to stop flowering and seed production, but it can also result in plant mortality. The shades consist of 2 layers of black builders plastic that are held in place on top of the water by ropes along the edge of the billabong. Weed-matting extends to 3 m vertically into the water around the infestation.

Shading has successfully reduced flowering and seeding of plants. The effect of shades on plant biomass was difficult to assess due to poor visibility. From the evidence available, it seemed that biomass was reduced at deeper and poorly established plots, but not at shallow, well established plots (i.e. where underwater leaves reach the surface). Unfortunately, shading is labour-intensive and not feasible over large areas.

Draining and Physical Removal

Draining and physical removal is only suitable for very small infestations. A small (20m x 8m) backwash billabong isolated from the main river during the dry season was drained and weeded in July 2005. Cabomba was discovered re-establishing in the billabong in October. Shades were installed after this date. In 2006, a combination of draining, weeding and shading was successfully used and as a result plants have not been detected at this site since 2005.

Results

The initial result of chemical control on cabomba infestations in 2004 was encouraging. By the time follow-up treatments were completed in December 2004, not a single mature plant could be seen. However by May 2005, it was estimated that 50% of the treated area was again infested with cabomba.

Regeneration of cabomba infestations is most likely to have occurred from seed. Cabomba was not known to produce seed in Australia until seedlings were found in a household pond in Pine Creek in December 2004. Investigations in 2005 confirmed that cabomba was producing viable seed at Darwin River. Given this information, it is vital that seed production is stopped at Darwin River and research into seed biology continues. It is also possible that chemical application was less successful at Darwin River (compared to Marlow Lagoon) because of the River's depth. This makes it difficult to treat the entire plant with herbicide and some shoots may have survived. This problem can be addressed by increasing the number of follow-up applications to ensure that all shoots are treated with chemical. The chemical control programs in 2006, 2007 and 2008 ran during the dry season each year to ensure adequate follow up.

As of September 2008 cabomba has not been recorded at any site upstream of Loklandji billabong for 2 years. Within Loklandji billabong cabomba continues to germinate at a number of sites. Survey and control continues at all sites as seed longevity is unknown.

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