An adult of the striking red and black beetle Deuterocampta quadrijuga.

An adult of the beetle Deuterocampta quadrijuga, a biocontrol agent released on the weed blue heliotrope.

Biological control of weeds

Biological control is a long-term solution which is most effective as part of an integrated weed management approach.

  • 31 July 2008 | Updated 1 February 2013

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 prevalent that they are regarded as weeds.

What is biological control?

The biological control approach makes use of the invasive plant's naturally occurring enemies, to help reduce the invasive plant's impact on agriculture and the environment. It simply aims to reunite weeds with their natural enemies and achieve sustainable weed control.

Biological control is rarely a 'silver bullet', as not all weeds have biological control agents that would be considered safe for introduction in Australia.

These natural enemies of weeds are often referred to as biological control agents.

It is critical that the biological control agents do not become pests themselves. Considerable host-specificity testing is done prior to the release of biological control agents to ensure they will not pose a threat to non-target species such as native and agricultural plants.

Not all weeds are suitable for biological control. Developing a biological control project requires a substantial investment, sometimes costing millions of dollars, from stakeholders.

A biological control agent is generally only used when the cost of conventional control methods such as herbicides, mechanical control or fire is so great, both in dollar terms and impact on the environment, that there is little option than to pursue the biological control avenue.

CSIRO has been working on the biological control of weeds since the 1920s, starting with the biological control of prickly pear.

CSIRO now has many active biological control projects underway for both temperate and tropical Australian weeds which cause problems in natural, pastoral and agricultural ecosystems.

The process

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.

Graph depicting the course of events in a classical biocontrol program.

Best case scenario of the course of events in classical biological control programs targeting weeds invading habitats such as rangeland, pasture, or natural ecosystem.

 

Following their introduction and establishment, populations of biological control agents build up to very high levels due to the abundance of their host plant. Eventually their attack on the plant causes a decline in the weed population.

This, in turn, leads to a decline in the numbers of biological control agents until an equilibrium is reached between the amount of damage caused by an agent and regeneration by the weed. In a successful biological control program this new equilibrium is below the damage threshold that the ecosystem can tolerate.

Learn more about the Steps in a weed biological control program.