Weed Control Using Biocontrol Agents

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 What is Biocontrol?
Ragwort Flea Beetle1

Biocontrol or biological control is the use of one or more host specific organisms, such as an insect or plant disease, to control weeds. Once introduced and established, populations of these biocontrol organisms build up to very high levels, due to the abundance of the weed, and eventually their attack results in the decline in weed biomass, reproduction and/or populations.

Biocontrol is risky, it is important the agents do not become pests themselves. So before exotic agents can be released into Australia they are extensively tested to make sure they are host specific i.e. only attack the weed species. Once approved, they are bred up, usually by government agencies in laboratory and greenhouse conditions, where they can be nurtured and monitored. Not all agents tested are successful.

Gorse Spider Mite3The next step is to establish nursery sites in areas where the target weeds are causing problems. There is no guarantee that exotic agents will be able to cope with Australian and/or Tasmanian conditions. If the agents become well established, it is then possible to use these nursery sites to supply other sites and over time they should be self-sustaining.

What can Biocontrol Do?

Other control methods, such as chemicals and manual removal, need continual reapplication. There are also some sites that are difficult to get to or that chemicals are unsuitable. Biocontrol agents, once established, are self-sustaining and should not need reapplication. This reduces the costs and effort required to control weeds in the long term. Although initial costs for testing and breeding are expensive (undertaken by government agencies and research institutions), once established in the environment the costs to the land holder are negligible.

Biocontrol agents are also natural and do not rely on chemicals that may adversely impact the environment. Once established they are able to spread, until they’re stopped by physical, environmental or chemical barriers, just as they would in their native range. This means that they have an ability to be a landscape scale solution, allowing for out-competed native plants to recolonise.

Disadvantages of Biocontrol?

It is important to remember that biocontrol will not totally eradicate a weed, and so may not be the ‘silver bullet’ of weed control. It also takes time for control agents to establish and achieve successful control. Impact and success of biocontrol agents on target weeds is unpredictable, it depends on elements such as the site, number of agents, and traits of the individual weeds. Weather, chemicals, reduced weed density or mismanagement can also eliminate the biocontrol agents.

But it should reduce its vigour and abundance to a level that is either tolerable or that can be managed by more conventional means. In the long term, a more natural balance between the weed and its predators is achieved. Best results have occurred when more than one agent attacks a specific weed and/or when it is part of an integrated management program.

Availability of Biocontrol Agents.

It is important to establish nursery sites in local areas under local conditions. These sites need to be protected while numbers build up. For this reason they need to be in areas where exposure to herbicides and cultivation are generally at a minimum. The specific needs will vary with the type of agents being used. Once established and acclimatised to a local area they may be transported locally to increase the rate of spread.

Various states in Australia are working on different biological control agents and there are already some successful releases of agents in Tasmania. Some have increased enough in populations to be self-sustaining, others still need a helping hand with nursery sites and manual spread.

Tamar NRM is developing a community-led biological control action plan for long-term operation with multi-species that would result in relatively low-cost management of a number of established weed species. A field day in December 2019 kick-started the development process for the first two species – gorse and Paterson’s curse. A third species was added when the February 2020 Ragwort Raids were used to assess the status of the Ragwort biocontrol agents in the Tamar area.  Further community involvement for these and other weed species will take place over the next twelve months.

A key part of the biocontrol action plan will be to source existing approved agents for release in the local area, to establish nursery sites of agents where we can and to educate the community on how to distribute the agents that are available. Once established in our area, the agents’ natural increase in range can be facilitated by community and Landcare groups much as dung beetles have been in the past.

Before You Start

Before you attempt to implement biocontrol, make sure:

  • you have identified target weeds correctly (all are host specific so if the weed is miss-identified there will be no response at all)
  • you have obtained the required permits for transport and release (and landholder/manager permission if not on your land)
  • you can obtain approved agents from a reliable source
  • you have a good understanding of the likelihood of success and have considered the need to integrate biological control with other methods of control
  • that adequate measures have been implemented to prevent dispersal when transporting materials, such as sealed containers
  • you are ready to release the agents straight away, as many do not last long away from their hosts.