Bio-pesticides Types: Bio-Herbicides and Bio-Insecticides!
Bio-pesticides are those biological agents that are used for control of weeds, insects and pathogens.
The micro-organisms used as bio-pesticides are viruses, bacteria, protozoa, fungi and mites. Some of the bio-pesticides are being used on a commercial scale.
Most important example is the soil bacterium, Bacillus thuringiensis (Bt). Spores of this bacterium possess the insecticidal Cry protein.
Therefore, spores of this bacterium kill larvae of certain insects. The commercial preparations of B. thuringiensis contain a mixture of spores, Cry protein and an inert carrier.
This bacterium was the first bio-pesticide to be used on a commercial scale in the world, and is the first bio-pesticide being produced on a commercial scale in India.
Bio-pesticides are of two types: bio-herbicides and bio-insecticides.
(i) Bio-herbicides:
Herbicides are chemicals that are used for inhibiting the growth of plants in unwanted places. Herbicides used for controlling weeds in the cultivated areas are called weedicides. A number of risks are involved in the use of chemical herbicides. This can be avoided if herbicide resistance can be introduced in the crop plants. It is possible through genetic engineering or recombinant DNA technology. Transgenic Tomato and Tobacco plants have been developed which show tolerance to specific herbicides.
Certain crop plants do not allow the weeds to grow nearby. They are called smoother crops, e.g., Barley, Rye, Sorghum, Millet, Sweet clover, Alfalfa, Soybean, Sunflower. Smoother crops eliminate weeds through chemicals. Crop rotation with these crops will naturally reduce the incidence of weeds.
Another way of weed control is the introduction of specific insects which feed on the weeds. Extensive growth of Opuntia in India and Australia was checked through the introduction of its natural herbivore, cochineal insect (Cactoblastis cactorum). Similarly, growth of Hypericum perforatum or Klamath weed was checked by U.S.A. through the introduction of Chrysolina beetles.
An organism which controls or destroys unwanted plant growth without harming the useful plant is called bioherbicide. The first bioherbicide happened to be mycoherbicide. It was put to use in 1981. The herbicide is Phytophthora palmivora. The fungus does not allow the Milkweed Vine to grow in Citrus orchards. Growth of Eichhornia crassipes (Water Hyacinth) is being controlled by Cercospora rodmanii in USA and Alternaria eichhorniae in India.
Puccinia chondrilla has controlled the growth of skeleton weed, Chondrilla juncea in Australia. Fungal spores are now available to be sprayed over weeds for their elimination. Two of them are ‘Devine’ and ‘Collego’. The spores are ideal for marketing because they can tolerate adverse conditions and can remain viable for long periods.
(ii) Bio-insecticides:
Bio-insecticides are those biological agents that are used to control harmful insects. They include the following.
(a) Predators:
Destructive insects or plant pests can be brought under control through introduction of their natural predators. The predators should be specific and unable to harm the useful insects. Introduction of ladybugs (Lady Bird Beetles) and Praying Mantis has been successful in combating scale insects or aphids which feed on plant sap.
(b) Parasites and Pathogens:
This is alternate biological control of plant pests through the search of their natural parasites and pathogens. They include viruses, bacteria, fungi and insect parasitoids. Parasitoids are organisms that live as parasites for some time (as early or larval stage) and free living at other times, e.g., Trichogramma. Nucleopolyhedrovirus (NPV) are species specific.
For example, Baculovirus heliothis (a virus) can control Cotton bollworm (Heliothis Zea). Similarly, Bacillus thuringenesis (a bacterium) is effective against the cabbage looper (Trichoplausiani) and Entomophthora ignobilis (a fungus) the green peach aphid of Potato (Myzus persicae). In U.S.S.R. the fungus Beauveria bassiana has been successfully employed in controlling Potato beetle and Codling moth.
(c) Natural Insecticides:
They are insecticides and related pesticides which are obtained from microbes and plants. A number of natural insecticides are available. The common ones include (i) Azadirachtin from Margosa or Neem (Azadirachta indica). It occurs in Margosa extract. Spray of the same keeps away the Japanese beetles and other leaf eating pests because of the antifeedant property of azadirachtin. (ii) Rotenones. They are powerful insecticides which are harmless to warm blooded animals. Chinese are believed to be first to discover their insecticidal properties. Rotenones are obtained from the roots of Derris elliptica and Lonchocarpus nicou. (iii) Squill. The red variety of Sea Onion (Red Squill, Ureginea maritima) produces a radicide which does not have any harmful effect on other animals, (iv) Nicotine. It is obtained from Nicotiana species. The purified chemical is highly poisonous. Nicotine sulphate is one of the most toxic insecticides, (v) Pyrethrum.
It is an insecticide which is obtained from the inflorescence of Chrysanthemum cinerarifolium (Dalmation Pyrethrum), C. coccineum and C. marshallii. The active compounds are pyrethrin and cinerin. Pyrethrin is also used in fly sprays, aerosols, mosquito coils, etc. (vi) Thurioside. It is a toxin produced by bacterium Bacillus thuringenesis. The toxin is highly effective against different groups of insects like moths, flies, mosquitoes and beetles. It does not cause any adverse environmental pollution or disturbance.
Thurioside occurs as crystals in the bacterium. It kills the susceptible insects through inhibiting ion transport in the midgut, formation of pores in gut epithelium, swelling and bursting of cells, (vii) Transgenic Plants. They are crop plants which are modified through genetic engineering to develop natural resistance to insects by inserting cry genes of Bacillus thuringenesis into them, e.g., Bt Cotton. Similarly, transgenic Tomato has been developed which is resistant to homworm larvae.