5.5 MICROBES AS BIOCONTROL AGENTS:
The biological agents (organisms) which can be used to kill or check the proliferation of disease causing agents are called biopesticides.
Use of microorganisms or biological methods for controlling plant diseases and pests is called Biocontrol.
Use of chemical pesticides and insecticides is effective but these chemicals are toxic and extremely harmful to human beings, domestic animals and useful pollinators.
Moreover, they pollute our environment (soil, ground water and air).
Therefore, use of chemical pesticides should be minimum.
In modem agriculture microbes are used for controlling pests.
These microbes are either pathogens or predators or parasites on the pests.
Natural predation is useful and harmless.
Contrary to the conventional use of chemicals which kill both useful and harmful organisms indiscriminately, modem organic farmer tries to understand the food chains and the web of interactions between the organisms that constitute the flora and fauna of field and uses suitable biopesticide.
He knows their life-cycles, pattern of feeding and the habitats that they prefer.
Organic farmer knows that the biodiversity is important, more the variety a landscape has, the more sustainable it is.
This helps him develop and use appropriate biocontrol methods.
An example of microbial bio-control agent is Bacillus thuringiensis.
Dried spores of B.thuringiensis are mixed with water and sprayed onto vulnerable plants such as cotton, brassicas etc. when insect larvae eat the leaves, they get killed as toxins are released in their gut by the bacteria. (Now the gene which is responsible to produce the toxic substance is introduced into the cotton plant using r-DNA technology and this transgenic plant is called Bt.Cotton. You have learned this in chapter 3)
Four groups of biocontrol agents are known. They are bacteria, fungi, protozoans and viruses. Some common examples are given below.
a) Microbial pesticides and their host range:
|
Pathogen |
Host Range |
|---|---|
|
Bacteria: Bacillus thuringiensis |
Caterpillars (larvae of moths and butterflies), larvae of Aedes, black flies, some adult beetles, wax moths. |
|
Fungi: Beauveria bassiana |
Aphids, mealy bugs, mites, white flies, etc |
|
Protozoans: Nosema locustae |
Grasshoppers, caterpillars, some corn-borers and crickets |
|
Viruses: Nucleopolyhedrovirus (NPV) [125 types known] |
Gypsy moths and NPV caterpillars |
b) Microbial herbicides/weedicides:
Microbes are used as herbicides also.
Many dicot herbs grow in the field of cereals as weeds and these weeds can be killed by certain microbes.
For examples -
(1) Pathogenic fungi as mycoherbicides:
i) Phytophthora palmivora
ii) Alternaria crassa
iii) Fusarium sp.
(2) Bacterial pathogens as herbicides:
i) Pseudomonas sp.
ii) Xanthomonas sp.
iii) Agrobacterium sp.
