Crop weeds: controlling small weeds

Page last updated: Thursday, 4 June 2020 - 11:20am

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Biological control of weeds

Biological control is the management of weeds using the weed's natural enemies (biological control agents). The mode of action of a biological control agent is either to feed on the weeds (insects) or cause disease in the weeds (pathogens). This in turn slows growth, reduces seed-set and/or eventually leads to the death of the weed. There are three generally recognised types of biological control: ‘classical’, ‘inundative’ and ‘conservation’.

Patterson's curse is partially controlled by insects like the crown weevil.

Types of biological control

Classical control

Classical control is the most commonly used and widely known approach. It involves:

  • Exploration for natural enemies of the host weed in the plant's region of origin.
  • Rigorous host testing of potential control agents.
  • Importation and clearing through quarantine.
  • Approval followed by release onto the target weed in Australia.
Inundative control

The inundative approach involves mass production of biological control agents and then mass release of them in order to produce an epidemic against the weed. The inundative approach is best illustrated by the development of bioherbicides. Bioherbicides have an advantage over chemical herbicides in situations where the latter may be ineffective (for example, due to herbicide resistance) or inappropriate (for example, near sensitive wetlands or in organic agriculture). Although bioherbicides have been researched in Australia, they have not been brought to commercial use due to the constraints of production costs and limited markets.

Conservation biological control

Conservation biological control involves managing both the crop and the weed to favour the presence of naturally occurring biological control agents that attack the weed.


  • About 33% of biological control agents have resulted in substantial control of the target weeds. Biological control success can range from complete (no other weed control required), to substantial (other weed control methods are needed but the effort required has been reduced) to negligible (control is still dependent on other control measures).
  • When classical biological control is successful it is very cost-effective.
  • The recognised potential of bioherbicides as an effective form of weed control stimulates future investigation.

Issues to consider

  • Weed biology factors influence biological control efficacy. Not all weeds are suitable for biological control.
  • Survival of the control agent relies on survival of the weed.
  • The success of biological control is dependent on the existence of suitable agents and their degree of host specificity. It is difficult to find biological control agents for weeds that are closely related to crops (for example, wild oats).
  • Bioherbicide technology is limited in terms of the cost of production. The Australian market is generally too small to warrant bioherbicide development.

Contact information


Sally Peltzer