Insecticides, GM Crops and Resistance
How insecticide work
Insecticides kill moths (and other insects) by binding irreversibly to a specific protein (encoded by a specific gene) referred to as the target. The binding of the insecticide typically renders the target inactive. Since the activity of the target is required for survival, the binding of the insecticide kills the insect.
How insects become resistant to insecticides
Moths have approximately 200 different genes that encode enzymes with the potential to detoxify insecticides. Typically the insecticide concentrations used overwhelm these detoxification enzymes. However, resistance occurs when a mutation arises that allows the insect to produce more of an enzyme with the capacity to degrade the insecticide being used. A second common route to resistance involves target site modification. Here a mutation in the gene that encodes the target leads to the production of a modified protein that fails to bind the insecticide.
Chemical insecticides are key weapons in the fight against the moth. Unfortunately, the moth has become resistant to many of the widely used chemicals, reducing the number of viable options for insecticide-based control.
Resistance to Bt has been detected in a number of
species, includingH.
armigera.
Are GM crops the answer?
For one crop, cotton, genetically modified
crops are being used to improve control and reduce
insecticide usage. One or more genes isolated from the
bacterium Bacillus thuringiensis, have been
introduced into cotton. These genes provide the
instructions for making insect toxins. Undoubtedly the Bt
cottons have been a huge success with widespread adoption
in a number of countries including Australia, China, India
and the U.S.A.
For cotton Bt crops provide the centrepiece of an excellent
control strategy. However, Bt cannot be viewed as a
permanent solution. Resistance to Bt has been detected in a
number of species, including H. armigera. That
resistance has not spread in this species may indicate that
the resistance mutations reduce the capacity of the moth to
survive and reproduce. Even if this is the case, ultimately
evolution may throw up another resistance mutation with a
great capacity to spread.
Public opposition to the genetic modification of food crops
means that in the short term Bt is not a viable alternative
for controlling the moth on the majority of the crop
species it attacks. Even for cotton the next generation of
control agents must be researched and developed. The genome
sequence would inform this research.
