Biological Control Agents

Australian pastoralists and graziers have long been the victims of introduced species impacting on productive land.

When chemical, mechanical or other methods of control become ineffective, too costly or have an adverse environmental effect, then a biological control option becomes a suitable long-term solution.

The CSIRO has been heavily involved in research for the control of weeds using biological control agents on invasive weed species in Australia since the 1920s.

What is biological control?

According the CSIRO: “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.”

An example of this is the introduction of the flea beetle, Longitarsus species, which was released in 14 local government areas in NSW to control the weed blue heliotrope, a declared noxious weed.

Before it was released, a considerable amount of testing was conducted to ensure the beetle, sourced from Argentina, did not itself become invasive, effecting non-target species such as native flora and fauna or agricultural plants.

Since their introduction the flea beetles have been credited with the death of blue heliotrope plants within its native range.

The CSIRO has many active biological control projects under way for both temperate and tropical Australian weeds that cause problems in natural, pastoral and agricultural ecosystems.

How biological control works

A noxious weed is determined as being undesirable, troublesome and difficult to control, and thereby legislated for their specific control and management. Often this is a result of the weed affecting the economic or ecological sustainability of an ecosystem such as range land, pasture or natural ecosystem. 

Courtesy of the CSIRO, this graphic below depicts the population of a weed species over time and its ecological and economic impact measured by the damage threshold.

As the biological control agent is introduced its population quickly increases, overcoming that of the target weed. With its effect on the target species reducing the weed density and therefore its host, the biological control agent population also fluctuates as the weed density gradually finds a new equilibrium below that of the damage threshold of a noxious weed.

The role of bilological control

Weed management is an integrated process that combines the use of chemical and mechanical control methods, livestock and vegetation management as well as proper agricultural and forestry biosecurity.

Biological control is just another weapon in your arsenal to combat woody weeds.

For example, blackberry spp. infests about 8.8 million hectares of temperate Australia, with a significant proportion of it inhabiting highly inaccessible areas or regions where risk of damage to native vegetation makes control by herbicides or cultural means difficult or impossible.

Blackberry will persist indefinitely unless managed and it is in these areas where biological control can play a role.

A fungus known as leaf rust has demonstrated good suppression of blackberry development in research by the CSIRO. The leaf-rust fungus has shown to cause most severe disease on emerging blackberry leaves during active shoot growth; older leaves on shoots have been shown to be more resistant to the rust. Also areas with sub-optimal growing conditions such as low rainfall and high temperatures have not demonstrated the same level of control as those in more suitable conditions.

This has shown that in key areas, particularly in Victoria and Tasmania, biological control agents can play a key role in suppressing the further spread of blackberry but not entirely eradicate it.