Abstract:
The number of pathogenic microorganisms that are capable of killing insect
pests is extremely large. Some bacteria are more attractive to researchers
developing microbial biological control agents because their genomes are
simple enabling easier study and manipulation. Several bacterial species
are already used on a large scale as control agents against some insects.
Commercial preparation of these bacteria in theform of insecticidal powders
are available for use in the field. Although these preparations are somewhat
more expensive and also less e/jicient than chemical insecticides, for several
reasons including recent awareness of environmental safety, development of
biological control agents has recieved increased attention of biologists.
Several varieties of Bacillus thuringiensis have been well studied for
their to:xic effects on insects. These bacteria produce proteinaceous parasporal
crystals during their sporulation. When these crystals are ingested
by insect larvae protein protoxins are solubilised in the alkaline environment of
the insect midgut releasing polypeptides toxic to the epithelial cells resulting
in quick death of the larvae.
Toxicity of Bacillus thuringiensis var. israelensis is specific to mosquito
and blackfly larvae. However, large scale use of this bacterium for the
control of mosquitoes is limited by its low persistenee in the field after
application. Four genes coding for 4 different toxic polypeptides have been
isolated from this bacterium and all these have been cloned in E. coli and
studied. Research is being done to study the posible manipulation of these
genes in order to obtain potentially much more efficient bacterial strains for
the control of insects.
