WHY PLANTS BECOME INVASIVE?
Plants that have become weeds in Australia are rarely invasive and troublesome in their home country. This is often because populations in the home country are regulated by a variety of natural enemies such as insects and pathogens (disease-causing organisms like fungi and bacteria) that attack the seeds, leaves, stems and roots of the plant.
If plants are introduced to a new country without these natural enemies, their populations may grow unchecked to the point where they become so widespread that they are regarded as weeds.
Generally, a weed becomes a problem in the introduced range because its population density fluctuates around an equilibrium that is above a threshold at which the weed begins to affect the economic or ecological sustainability of the ecosystem.
Typical infestation of Paterson’s curse (Echium plantagineum) in south-east Australia before the introduction of biocontrol agents.
HOW DOES BIOCONTROL WORK?
Biological control is the practice of managing a weed by the deliberate use of one or more natural enemies (biocontrol agents) that suppress it. Following their introduction and establishment, populations of biocontrol agents build up to very high levels due to the abundance of the weed. Eventually their attack on the weed causes a decline in the weed biomass, reproduction and/or population density.
This, in turn, leads to a decline in the numbers of biocontrol agents until equilibrium is reached between the amount of damage caused by the agents and regeneration by the weed.
Schematic representation of a common, desired goal of a weed biological control program, whereby the weed population is suppressed by the agents below the damage threshold that the ecosystem can tolerate leading to an increase of the associated plant community (Adapted from Briese 2000).
IS BIOCONTROL RISKY?
It is critical that the biocontrol agents do not become pests themselves. Considerable host-specificity testing is undertaken prior to the release of biocontrol agents to ensure they will not pose a threat to non-target species such as native and agricultural plants.
One of the agents, the root-boring weevil Mogulones geographicus, responsible for a major decline of Paterson’s curse populations.
WHAT ARE THE BENEFITS OF BIOCONTROL?
Other control methods, such as the use of poisons and mechanical removal, require continued reapplication. Biocontrol agents of weeds, once established, are self-sustaining and don’t have to be reapplied. A series of cost-benefit analyses in 2006 revealed that for every dollar spent on biocontrol of weeds, agricultural industries and society benefited by A$23. This was due to increases in production, multi-billion dollar savings in control costs and benefits to human health.
The initial costs of biocontrol programs are generally high. That’s because we have to find suitable candidate agents overseas, test them for safety in quarantine, and comply with regulations around release.
But once biocontrol agents are released and affect the weeds across its range, follow up control costs are greatly reduced.