Biological control of weeds

What is biological control?Biological control covers two key concepts:

•the deliberate use of a weed's "natural enemies" to suppress its population; and

•the use of these live organisms to maintain this lower population density

The biological control approach makes use of the invasive naturally occurring enemies.

Biological control of weeds• It simply aims to reunite weeds with their natural enemies and achieve sustainable

weed control.

• A weed's natural enemies may be arthropods (insects, mites and their relatives), bacteria or fungi

• These "control agents" feed upon or cause disease in the weed, thereby limiting its growth, reproduction and spread

• The aim of biological control is to shift the balance of competition between the weed and the crop in favour of the crop and against the weed.

Types of biological control:Classical (inoculative) biocontrol:

•It involves the release of a relatively small number of control agents.

•These agents feed on the weed, reproduce and gradually suppress the weed as their population grows.

•Generally arthropods are used as control agents.

Inundative biocontrol:

•In this type of biological control, large quantities of a control agent, generally a pathogen are applied to weeds

•It uses large quantities of pathogens to create artificial disease epidemics

•It does not persist for long, requires to be reapplied

Conservation control:

•It is an indirect method, which manipulates the habitat around the weeds with the aim of encourging those organisms that attack the weed.

•This is a long term strategy that requires a detailed knowledge of the ecology of the crop weed habitat, the target weeds and the control agents.

•Livestock can also be considered as biological control agents which can give a broad spectrum control of weeds in various situations.

How are biological control agents identified and introduced?

Step 1: Identifying target weeds to be considered a good candidate for biological control.

Step 2: Identifying control agents and assessing level of specialization

Step 3: Controlled release: All biological control agents must be approved under the Plant Protection Act and are reviewed by an advisory panel of Agriculture

Step 4: Full release and identifying optimal release sites: If the agents survive the harsh climate and damage or suppress the weeds in the controlled tests, they can be released on other weed infestations

Step 5: Monitoring release sites: Release sites should be monitored periodically to assess the size of the biological control agent population and the effect of the agent on the weed.

Step 6: Redistribution (classical) If a biological control agent is released on a site with favourable conditions, its population can grow quickly

Step 7: Maintaining control agent populations (classical)

Process of weed control by biological agents.

• A weed becomes a problem as its population density is above a threshold at which it affects the economic or ecological sustainability of the ecosystem

• Biological agents are released at this point

• Populations of biological control agents build up to very high levels due to the abundance of host plant.

• Eventually their attack on the plant causes a decline in the weed population.

• It leads to a decline in the population of biological control agents until an equilibrium is reached between the population of weeds & bio agents; such that this new equilibrium is below the damage threshold that the ecosystem can tolerate

Some examples of effective biological control agents.

Bactra verutana has been evaluated for efficacy as a biological control agent for C. rotundus

The mycoherbicide Dactylaria higginsii is a biological control fungus against purple nutsedge

Bipolaris sp has been effective in control of Johnson grass

Aceria malherbae used to control field bindweed

Hyphena, Telionema & Prospodium

tuberculatum- lantana rust [clockwise from

left] has been effective biocontrol agent for

Lantana sp.

• Some species of weeds do not have effective biological control agents.

• Some of them are

Cyanodon sp

Panicum sp

Argemone sp

• While species like Avena is being biologically controlled ants