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Bt - A weapon against insects
We all know that insects are
the well known organisms on the earth. Some insects are
beneficial and other are harmful, because it transmitting
diseases in plants or eating our cash crops, trees and other
plants.
Man made insecticides having powerful toxic chemicals that
not only kill harmful insects but also beneficial ones. They
can effect animals, birds and peoples if not handled
properly. Some insects often develop resistance to these
insecticides. So keeping in view this, we have looked for
new and better ways to combat with these harmful insects.
Bacillus thuriengensis is a bacterium which is a natural
enemy for the certain specific insects. It is now widely
known as Bt. It produces toxic proteins which can kill
insects. It is the most important bacteria to kill wide
range of insects. More than 150 insects have been reported
to be killed by these bacteria.
HOW Bt TOXINS KILL THE INSECTS?
The mechanism of killing the insects by means of Bt is quite
interesting. The toxicity is actually resides in a large
protein (toxin) .This bacteria produces different types of
poisonous proteins which can kill only specific insects.
Upon eating the poisonous protein, within 15 to 30 minutes,
the poisons begin to create problems in the stomach and
destroy there stomach lining. The insect stops eating and
results in the death of insect.
USE OF Bt TOXINS FOR THE CONTROL OF INSECTS:
Bt toxin has been used in several ways to control the
insects. A relatively simple way is to grow the Bt bacteria,
dry out, and prepare the heat killed and dried bacteria in
such a way that they can be sprayed on the crops .These
preparations are initially highly effective, but the Bt
toxins is not stable after product is sprayed on plants .The
Bt toxins crystals are released from bacterium and quickly
disappear from plants.
Scientists introduced Bt gene in a different bacterium
(pseudomonas fluorescens, E.coli). These bacteria can
readily be grown in large fermentors,they are killed and
then formulated as spray .With this bacterium, the protein
crystals remain in the bacterial cells and as a result they
are stable even after they have been sprayed on the plants
.It works well but would be less effective.
TRANSGENIC PLANTS:
To control insects, scientists have transferred Bt gene
(code for toxin proteins) using particle gun (gene gun), in
the genome of the cash crops like cotton, tomato, tobacco,
potatoes and other plants. More than 50 genes have been
isolated, cloned and characterized.
Australian scientists have produced genetically engineered
species of cotton, known as killer cotton (kill insects). It
kills the predators specially Bollworms. The leaves of
transgenic plants produce toxins. When The Bollworm eats
upon the cotton leaves toxin is taken up by them .Toxin
activates in their gut and results in the death of Bollworm.
In Australia, pesticides used on Bt cotton crops has been
reduced by 50%, while American farmers report a 30% decrease
in insecticide used on Bt cotton (levidow, 1999).This is due
to the fact that Bt crops constantly emit their own
insecticide in the form of the Bt protein crystal. This
eliminates the need to spray insecticides designed to kill
the same pest that are susceptible to Bt. Today versions of
Bt cotton, Bt potatoes, and Bt corn are being grown in the
United states, Canada, Argentina, South Africa, France and
Spain.
Other transgenic plants with Bt gene:
Stem borer resistant rice, corn borer resistant maize,
potato beetle, tuber moth resistant potato, and tomato
resistant to pinworms.
Now Bt technology is being used in expressing two different
kinds of Bt genes in transgenic plants or microorganisms.
This technique help in preventing insects from developing
resistance to Bt toxin proteins.
NON TARGET EFFECTS:
These toxins does not harm ,spider, human and other non
specific species because toxin requires both high alkaline
PH and presence of specific proteases enzymes. It also
requires specific cell surface receptors proteins for
binding of these small toxin molecules. Such conditions are
not present in the body of non specific species, hence they
are safe.
To date, no known mammalian effects are found.
Bt are non toxic to most beneficial or predator insects.
Currently ,over 180 Bt products are registered in USA .There
are different strains or varieties of Bt available that have
been selected for the control of specific inects. These
products are applied as spraying on the crops or treatment
of water by aerial or ground equipment(vector control).
AVANTAGES OF Bt:
Bt have several advantages over chemical insecticides (man
made). Beneficial and nontarget insects are not harmed and
often kill only specific insects. These toxins are not
harmful for animals, birds and humans. If insects died from
Bt toxins, they are not dangerous to birds and animals. Bt
does not cause injury to plant on which it has been applied
.To date most cases of Bt resistant crops have been
reported. It is more difficult for insects to develop
resistance, because they can evolve as the insects evolve.Bt
insecticides are safer, they don’t hurt the people, animals,
plant and friendly insects. As we know that natural produce
is the best.
LIMITATIONS OF Bt :
Besides the advantages of using Bt insectides ,there are
also certain limitations .It must be eaten by insects to be
effective. Bt is effective against only immature stages of
insects. It is applied like chemical insecticides because it
lacks the ability of transmission and spreading. Bt is
susceptible to degradation by sunlight .Some Bt products
have shorter shelf life than other insecticides. Shelf life
is greater when storage conditions are cool, dry,and out of
direct sunlight.
As the world’s population increases and the demand for food
and resources grows, it becomes more and more important to
find safe and effective methods for controlling harmful
insects. Bt toxins are safe, carefully targeted and durable.
Because of their unique characteristics, especially target
specific, they show outstanding promise for the future.
MUHAMMAD FAISAL SIDDIQUI
BIOTECHNOLOGIST.
CENTRE OF BIOTECHNOLOGY UNIVERSITY OF PESHAWAR.
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