MICROBES AS BIOCONTROL AGENTS

Biological Pest Control or Biopesticide

1. Biopesticides are the organisms which are applied to destroy the pests. They are used to destroy the weeds as well as the insect pests. Two basic types are bioherbicides and bioinsecticides.

2. Transgenic plants are genetically engineered plants to develop resistance against pests. e.g., transgenic tobacco and transgenic cotton.

3. Smoother crops are those which do not allow the weeds to grow nearby e.g., barley, rye, Sorghum, millet, sunflower, alfalfa, soyabean, marigold etc. Smoother crops eliminate weeds through chemicals. Crop rotation with these crops will naturally reduce the incidence of weeds.

4. Catch/ trap crops : Around the major crop in the field some early growing crop is sown in strips which is termed as catch or trap crop. The pests get attracted towards the early grown trap crop and then can be easily killed by cutting and destroying the trap crop. A good example of trap crop is bhindi (okra) which is sown around the cotton field to attract the jassid and spotted bollworm. Sesame is also good trap crop to attract the red hairy caterpillar from the cotton field.

5. Bioherbicides : It involves the biological control of weeds by some living organisms. e.g., use of insects feeding on a specific weed or use of micro-organisms which will cause diseases in weeds. Some of the common examples are given below :

(a) In India and Australia, the overgrowth of Opuntia (prickly pear cactus) was checked by the introduction of the cochineal insect (Cactoblastis cactorum).

(b) The first bioherbicide was mycoherbicide called Devine, derived from a fungus Phytophthora palmivora which controls the growth of milk weed vines in Citrus orchards.

(c) Another mycoherbicide called Collego has been derived from conidia of fungus Colletotrichum gloeosporioides. It controls the growth of northern Jointvetch (Aeschynomene virginica fam. Leguminosae) growing in rice fields.

(d) Extensive growth of Hypericum perforatum or kalmath weed was checked in USA by the introduction of Chrysolina beetles.

(e) Water hyacinth has been successfully controlled in Florida using the indigenous fungus Cercospora rodmanii.

6. Bioinsecticides : These are non persistent, non toxic and biodegradable. They include

(a) Pathogens, parasite and predators :

A well known example of biological control of an insect pest is the destruction of large populations of aphids (a pest on crucifers) by an insect called lady bug or praying mantis which feeds on the aphids.

The hoover fly larvae (Syrphid larvae) are very effective in keeping the aphids (plant bugs) under check as they feed on the aphids only. Dragon flies are useful to get rid of aphids and mosquitoes.

The mosquito larvae are easily controlled by rearing the larvicidal fish Gambusia (mosquito fish).

The sugarcane scale insects are controlled by the coccinellid predators (Cailochorus negriti and Pharoscymnus homi), the fluted scale insect (lcerya purchasi), a common pest on Citrus trees by the lady bird beetles (Rodolia cardinalis) and Nephantis serinopa, is a dangerous pest on coconut palms, by Perisierola nephanticdis and Trichospilus pupivora.

Baculoviruses are pathogens that attack insects and other arthopods. NPV (Nuclearpolyhedrovirus) based insecticide has been found to eliminate bollworms which cause extensive damage to cotton. These are species specific and narrow spectrum.

Trichoderma species are effective biocontrol agents of several plant pathogens. Trichoderma species are free-living fungi that are very common in the root ecosystems.

(b) Sterilisation strategy : Screw worm (Cochliomyia hominivorax) was eradicated by releasing sterile males.

(c) Insect hormone or Pheromones :

The pheromones are those chemical messengers which help in communication, sending alarm signals, marking trails or for attracting males.

Pheromones are secreted by females. Traps containing pheromones are placed in infested
fields. Males attracted by the trap become unavailable for reproduction.

In confusion technique, the pheromone containing papers are spread all over the field, so
males can no longer locate the females.

Introduction of moulting hormone ecdysone or juvenile hormones at inappropriate times results in the early death of insect pests.

(d) Natural insecticide :

These are obtained from living organisms (Plants) e.g., rotenones (obtained from the roots of Derris elliptica), nicotine (obtained from tobacco), pyrethrum and cinerin (pyrethroids, obtained from Chrysanthemum cinerarifolium), azadirachtin from margosa (Azadirachta indica) leaves, thurioside from mutant strains of a bacterium called Bacillus thuringiensis (Bt).

Thurioside is a proteinaceous toxin and is effective against several insects such as moths, flies, mosquitoes and beetles which accumulate as crystals inside the bacteria during sporulation.

Integrated Pest Management (1PM)

Sustainable pest management is otherwise known as Integrated pest management i.e., integration of tactics for control of single pest on one or more crops.

The overall objective of IPM is to create and to maintain situations in which insects are prevented from causing significant damage to crops.