BIO PEST CONTROL

Fertilizers of biological origin are called biofertilizers. These provide the essential elements to the soil and hence maintain or increase the soil fertility. Use of these biological fertilizers is safe both economically as well as ecologically.

There are two main categories of fertilizers of biological origin:

(1) Green manures

These are fast growing herbaceous crops which are ploughed down and mixed with the soil while still green for enrichment of soil. These provide both organic matter and nitrogen to the soil, in which Indian soils are generally poor. The green manure checks soil erosion by forming protective soil cover and also prevents leaching. Increase in yield by 30-50% has been observed by use of green manures.

Some important green manure crops, which are mostly members of family Leguminosae are as follows:

(i)    Trifolium alexandrianum (Barseem or Egyptain clover)

(ii)   Melilotus parviflora (Senji)

(iii)  Dolichos biflorus (Horse gram or Kulthi)

(iv)  Lens esculenta (Lentil or Masure)

(v)   Cyamopsis tetragonoloba (Guar or Cluster bean)

(vi)  Sesbania sesban (Dhaincha)

(vii) Crotolaria juncea (Sunhemp or Sunn)

(viii)                  Vigna sinensis (Lobiya or Cowpea)

(2)   Biofertilizers

They are organisms which bring about nutrient enrichment of the soil. Some of the important biofertilizers are :

Blue-green algae (Cyanobacteria) as biofertilizers

(i)    Symbiotic Nitrogen-fixing blue-green algae

Anabaena azollae, Anabaena cycadacae and Nostoc punctaeformae are present symbiotically in leaves of Azolla (water fern), Cycas roots and Anthoceros thallus respectively, which fix atmospheric nitrogen. Azolla-Anabaena symbiotic system is the main biofertilizer which is inoculated in rice fields in South-Eastern Asia, which is found to increase yield up to 50%

(ii)   Free living Nitrogen-fixing blue-green algae

Blue-green algae like Nostoc, Anabaena, Aulosira, Tolypothrix, Plectonema and cylindrospermum are the most common nitrogen fixing organisms, probably with the help of heterocysts. Photosynthesis provides the energy for nitrogen fixation. In the rice fields, Aulosira is the most active nitrogen fixing blue green alga. It is estimated that 7-8 lakh tons of nitrogen can be fixed by blue-green algae thus saving 15-17 lakh tons of urea every year.

Bacteria as biofertilizers

(i)    Free-living Nitrogen-fixing bacteria : Some bacteria like Azotobacter (aerobic), Clostridium (anaerobic), Rhodospirillum, Chromatium (photosynthetic bacteria) are free living nitrogen fixing bacteria of soil, which increase fertility of soil and hence yield of crop plants.

(ii)   Symboitic Nitrogen-fixing bacteria : Rhizobium leguminosarium bacteira live symbiotically in root nodules of legumes and some non-legumes, which fix atmospheric N₂ and hence increase the fertility of soil.

(iii)  Loose association of Nitrogen-fixing bacteria : A bacterium Azosprillum lipoferum forms loose association with roots of maize and some Brazilian grasses (Dobreiner, 1975), which increase the crop yield upto a large extent.

Fungi as biofertilizers

(i) Mycorrhiza : Symbiotic association between roots of higher plants and fungal hyphae, is called  mycorrihizal association or mycorrhiza. It is of 2 types :

(ii)   Endotrophic or Endophytic mycorrhiza : Here, fungal hyphae are present inside or between the cells of cortex, e.g., in Coffee, Pepper and Cardamon, etc.

In many grasses and some other crops, the fungal hyphae penetrate to the cortical cells, which swell to form vesicles or arbuscules, called vesicular-arbuscular mycorrhiza (VAM). It has significant role in Phosphate nutrition of plants.

(iii)  Ectotrophic or Ectophytic mycorrhiza : Here, fungal hyphae are present on the surface of roots. It increases water and nutrient absorption, growth, vigour and yield of plants, e.g., in Pinus, Oak, Eucalyptus, Peach, etc.

Biological pest control

Due to harmful effects of pesticides on organisms, some alternative methods of pest control are being used and biological pest control is one of the suitable methods, i.e., use of other organisms to kill the pests constitutes biological pest control and such organisms are called biopesticides.

Bioherbicides/Biological control of weeds

Bioherbicides are organisms or their extracts which destroy weeds without harming useful plants. The first bioherbicide is devine, which is a mycoherbicide, based on fungus Phytophthora palmivora. It is being used since 1981 to control Morrenia odorata (milkweed vines) in Citrus orchards. Similarly Collego is another mycoherbi­cide from conidia Colletotrichum gloeosporioides fungus.

Bioinsecticides : They are living organisms or their products which are able to kill or repel specific insects.

(1) Use of parasites, predators and pathogens : e.g., control of aphids by use of praying mantis or lady bug, i.e., Mantis.

Bacillus thuringiensis : One of the most widely used bioinsecticides is a naturally occurring soil bacterium called Bacillus thuringiensis or Bt. that produces a protein, poisonous to insects. Insects affecting maize, cotton, cabbage, sunflowers, etc. are controlled by mutant strains of Bacillus thuringeinsis (Bt) bacteria (sporeine). Often within 15 minutes of being eaten, the poisons begin to create ulcers in the insect’s stomach lining. The insect stops eating and eventually dies. Researchers have identified between 500 and 600 strains, or types of Bacillus thuringiensis. Bt is very selective it affects only a specific species of insect pest and does not harm humans, birds, fish or beneficial insects.  

Prickly pear cactus (Opuntia) in Australia and India was effectively controlled by larvae of Cactoblastis cactorum (Cochineal insect).

By use of lady bugs or praying mantis, aphids (plant bugs or homopterous insects) can be kept under control. Fluted scale insect (Icerya purchasi), a pest on citrus can be effectively controlled by lady bird beetles. Similarly mosquito larvae can be easily controlled by fish Gambusia and sugar cane scale insects are controlled by coccinellid predators.

(2)   Use of natural insecticides : The insecticides of plant or micro-organisms origin are called natural insecticides. These have little toxicity for animals, e.g., Rotenone (from roots of Derris sps. and Lonchocarpus sps.); different alkaloids like Nicotine (from tobacco), Pyrethrum and Cinerin (from Chrysanthemum); Azadirachtin (from Margosa or Neem) are useful natural insecticides.

Neem or margosa (Azadirachta indica) is most useful natural insecticide. It is resistant to about 200 species of insects, nematodes, mites, etc.

(3)   Sterilization technique : This is a modern method of biological pest control. In this technique, male insects are sterilized by irradiation, these are released at the time of mating and hence their multiplication is checked, e.g., screw worm (Cochliomyia hominivorax) and red weavil (a pest on coconut) have been checked by this method.

(4)   Use of insect hormones (Sex attractants) : The insect hormones called pherohormones (Pheromones) are useful in controlling insects. These pherohormones attract opposite sex insects during breeding season. The natural and synthetic pherohormones are now used to attract the insects towards death traps. The Orient-fruit fly has been eradicated by this method. Similarly, gipsy moth, a pest of conifers can be trapped

(i) Use of insect hormones like juvenile hormone and molting hormone or ecdysone is also made as bioinsecticides, Juvenile hormone should be present in early stages of growth to prevent early maturation, but if the same is given artificially at later stage of growth, the insect is transformed into giant larva (immature adult) which dies quickly. Similarly, periodic shedding of insect cuticle (molting occurs during the process of growth and ecdysone) hormone is associated with molting. Use of this hormone at inappropriate time also results in early death of insect.

(ii)   ‘Confusion technique’ is a variation of this approach and it involves use of pheromones or sex attractants. In this technique, hydrophobic paper having pheromones or sex attractants is placed over the crop area, due to which characteristic smell is spread over the whole field and thus males are unable to locate the females.

Integrated Pest Management (IPM)

IPM involves use of different pest control methods, which are ecologically sound (i.e., not cause hazard to environment), e.g., biological control methods, better agricultural practices like crop rotation, sanitation, etc. starvation method, i.e., growing of target crop away from major crop, ultra low volume spraying method, i.e., use of very low and most effective concentration of chemicals, which does not cause pollution etc.

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