Bacterial Pathogens in Bio-controls of Crop Pest
1. These bacteria are present in the digestive tract of insects but seldom cause pathogenic infection because they lack the invasive power to penetrate through midgut wall.
2. They are relatively harmless when remain in midgut but pathogenic when introduced into blood.
3. Their pathogenicity when introduced in blood is due to temperature extremes, other pathogens and due to poor food quality.
4. Little efforts have been to use them as microbial control agents for example, Serrstia entomophile against grass grubs.
Spore Forming Bacteria:
1. These bacteria are rod shaped, gram positive, motile with many flagella.
2. They form endospores in insect and promising organisms for microbial control.
3. More than 100 species have been identified as insect pathogens of the genus Bacillus are identified as insect pathogens and they are of commercial importance.
4. Bacillus species having bi-pesticidal potentially are classified under three groups as under:
i) Obligate pathogens: Bacillus papillae – milky disease causing organisms.
ii) Crystalliferous spore former: Bacillus thuringiensis
iii) Non crystalliferous facultative pathogen: Bacillus cereus and B. sphaencus.
1) Milky diseases Bacillus papillae and B. lentimorbus
Milky disease organisms are able to infect only certain closely related beetles of the family Scarabacidae. About 62 species of insects have been found to pick up infection by injection or soil inoculation. These bacteria multiply and speculated readily in the haemolymph of the grubs. The turbidity due to accumulating spores lead to milkiness of their haemolymph and hence the “milky disease”.
The discovery and practical application of the milky disease bacteria for control of grubs of Japanese beetle in USA provided the first real encouragement for use of bacteria for insect control. Preparations of B. popillae are resisted and commercially marketed as ‘Doom’. The spores are produced in vivo by inoculation of grubs. B. popillae cause true septicemia in susceptible hosts. In nature, spores are ingested and germinated in the insect get followed by penetration these. Upon death of insect, the disintegrating cadavers form a continuing inoculation in the soil. The necessity of producing by the time consuming and uneconomical method of inoculating grubs limits the usefulness of these organisms at the present time.
2) Bacillus Thuringiensis (BT):-
1. It is crystalline spore former: It produces endospores and proteinaceous parasporal crystal in sporulation time.
2. The cells produce crystal which contains an endotoxin capable of pralysing the gut of larvae. The toxic is known as “Delta endotoxin”.
3. BT is widely distributed in nature and it can be isolated from various sources, viz., dead insects, litters of sericulture farm, soils, water, dusts etc.
4 BT infests 525 species of insects from stress from crowding etc. make the insect more susceptible to the pathogen.
5. The UV light is harmful to spores and crystals of BT
6. Generally susceptible insects are killed by toxic crystals or a lethal septisemia.
7. Recently genetic engineering has helped to incorporating genes coding for Bt delta endotoxin into transgenic cowpea, cotton, tobacco and tomato plants so that internal production of toxic crystals by such plants can ward-off the attack of pests when visit for feeding.
8. Local strain Bacillus thuringiensis ver. Kenyae was isolated from meal moth, Ephastie cautella by BARC. Bombay which is effective against potato tuber moth.
9. Bacillus thuringiensis var. israelensis was isolated from south India and marketed under trade names as ’Doom’ Japidemic etc.
10. Bt is reported to be safe to honey bees and non-toxic to man. The reason for lack of toxicity is mammals; the primary digestion of protein is at low PH. The stomach enzyme pepsin, which has optimum pH value of 2, degrades the endotoxin into a non toxic compound. But exotoxin produced by some of strains of Bt has been found to be toxic to mice.
11. The product manufactured from Bacillus thuringiensis var. kurstaki is marketed trade names viz. Delfin, Dipel, Halt, Thuricide etc.
12. Twenty seven Bt biotypes/strains are known according to structure/sequence of several genes and gene products.
13. When ingested crystalline inclusion initially dissolve in the midgut releasing one or more proteins. These crystal proteins are activated by digestive gut enzymes with correct pH into toxic polypeptides. Smaller activated toxins bind to cell membrane liming the gut generating pores that disturb osmatic balance and lead to cellular swelling and lyses. Intoxicated insect larvae quickly stop feeding and eventually die. BT also produce other toxic metabolites such as beta exotoxin, alpha exotoxin and gamma exotoxin. These exotoxins are more persistent and inhibit reproduction in Spodoptera but have high mammalian toxicity and banned in USA by EPA.