Bacillus thuringiensis (or BT) is a living microorganism belonging to the family of bacteria. It has the ability to kill the insects which are harmful for the crops, forests or agriculture. Due to this property, this bacterium is used in many insecticides and pesticides. However there are some proteins in bacillus thuringiensis which are considered harmful for the mammals. But in most of the cases it kills the larvae of caterpillars, mosquitoes and beetles by making them starved which leads to their death. It is found in the soil and provides a shield to the crops against harmful insects. It is the only bacterium present in the world which serves as an insecticide. There are some crops which have been genetically modified by inserting the genes of BT which enable them to kill the larvae of the insects when they attack them for example, corn and cotton.
Bacillus thuringiensis has the ability to produce such proteins which react with the gut lining of the insect larvae. Through this way, the digestive system of the insect becomes paralyzed and death occurs eventually due to starvation. In some cases, BT enters the bloodstream of the insects where it reproduces and causes death. It is interesting to know that even the proteins of the dead bacteria are useful and can act as insecticides.
To kill leaf and needle feeding caterpillars, most common strain used of BT is kurstaki strain but in the past decade new strains have been developed which are effective against fly larvae. In the recent years, some new strains of BT have been developed which act efficiently against leaf beetles. Every BT strain has specific insecticidal activity that is if one Bt strain is effective against mosquitoes, it will not work against caterpillars.
Mode of Action:-
Bacteria have the ability to produce spores when they do not find appropriate condition to reproduce. Bacillus thuringiensis also produces spores and waits for the suitable environment for its reproduction. The spores of BT make a crystal of protein around the spores which is a toxic component. When an insect eats the bacterium, the crystal around the spores gets dissolved in its gut. This way, the digestive system of the insect digests the crystal and toxic activity of the BT starts in the stomach. The toxin is called as delta-endotoxin. This endotoxin binds with the cells of the gut lining where they create pores in the membrane and disturb the ions present in the gut. As a result, the insect cannot feed itself and dies automatically.
Advantages of BT:-
Bacillus thuringiensis has the ability to kill only those insects which are harmful for the crops. It does not damage the beneficial insects because of its limited spectrum of activity. For example naturally occurring predators and parasites which are beneficial for the crops but harmful for the insects do not get affected by the BT. BT genes in corn have made the corn a natural insecticide and now insects which attack the corn plants, die when they eat corn. The most important beneficial aspect of BT is that it is not harmful for the pets, humans and animals.
Disadvantages of BT:-
Bacillus thuringiensis with its benefits contains some disadvantages also. It is the negative point of BT that it degrades in the presence of sunlight. Many strains developed to control the spread of leaf beetle have become ineffective within 24 hours. Scientists are doing research on this problem and are trying to insert the protein crystal into other bacterial species which have the ability to retain longer. The activity of BT may become restricted when several pests or nonsusceptible insects are involved in spreading the disease in plants for example grasshoppers. Though BT kills insects but this killing occurs in a slow process because first it attacks the insect gut, then makes it starved and after this the death of the insect may occur. So when the agriculturists use it, they get disappointed after one or two days when they see that it has not yet killed the insects. Products of BT do not live longer and they have short life span. Their effectiveness decreases after two or three years.
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