A team led by entomologist
Aaron Gassmann from the Iowa State University has concluded that the western
corn rootworm (Diabrotica virgifera
virgifera LeConte) has developed resistance against the toxins
produced by Bt corn. Bt corn is the term used for corn that has been
incorporated with the genes of the bacterium Bacillus thurinigiensis (Bt) such that it produces the toxins
that the bacterium produces. Currently there are three types of toxins being
used, and Gassmann’s team found out that the rootworm has developed resistance
to two of these.
The first Bt toxin to
be used was Cry3Bb1, used in the United States in 2003. A second toxin used was mCry3A. In 6 years, damage was already observed in plants producing
the first toxin, and by 2011 even the plants producing the second began to show
damage. Normally, the Bt toxins are able to kill 98% of the rootworms. This is comparably lower than the 99.99% killed in another pest, the European corn borer (Ostrinia nubilalis). The other 2% of
rootworms that survive are then able to propagate and pass on the toxin
resistance to their offspring. This is initially not a problem, but over time
more and more of the rootworm population become resistant to the toxin and the
effectivity of the toxin in preventing crop damage decreases. How fast can
resistance develop? In fields in Iowa where corn has been grown year after
year, resistance to the toxins showed up after an average of 3.6 years. [1]
Another entomologist,
Lance Meinke of the University of Nebraska-Lincoln has confirmed that rootworm
resistance has been observed in fields in Nebraska, saying that damage due to
resistant rootworm has been observed as early as 2011. After lab and field
testing, Meinke has concluded that the rootworms they have tested have
developed resistance specifically to the Cry3Bb1 toxin. Reaching the same
conclusion as the Iowa team, Meinke said that farming Bt corn numerous years in
a row promoted the development of resistance through natural selection. Meinke
hopes to communicate that rootworm populations can become harder to handle if
Bt technologies are continued to be overused and if resistance continues and
expands to other Bt toxins. [2]
What are the
suggested solutions to this problem? Both teams of researchers have suggested “pyramiding”
the different Bt toxins available. This means incorporating more than one Bt
toxin in the corn so that the rootworm population will take more time to
develop resistance. For example, Dow AgroSciences in Washington DC has begun to
sell seeds that have both the Cry3Bb1 toxin and the Cry34/35Ab1 toxin, one with no
observed resistance development at present. [1]
However, emphasis was
given by both teams on another solution: crop rotation, wherein instead of
farming Bt corn continuously, it is alternated with other plants, specifically
soybean according to Meinke. When the rootworm larvae hatch, even if they have
the genetic resistance to the toxins, they don’t find the nutrients they need
and they die.
Meinke suggests that
farmers move away from the “silver bullet” approach when using Bt hybrids. I
believe he has a good point in this, seeing as how the usage of enormous
amounts of money and time to develop Bt technology can be overcome and made
useless by the mindless application of it year after year. It is possibly very
frustrating for the researchers who developed Bt technology to see it become
useless in a few years due to resistance development that could have been
delayed or avoided. I think the usage of pyramided Bt toxins alone can only delay
development of resistance in the rootworm and thus is not a permanent solution.
It would simply produce a situation wherein researchers who develop new Bt toxins
are racing against natural selection’s ability to develop resistance; a race they
would probably lose. On the other hand the solution to this problem seems to be
not as simple as ordering the farmers to stop farming Bt corn continuously.
Those farmers are trying to maximize efficiency in their production, and
alternating crops may disrupt the established systems and thus cost money. However
if these farmers are truly to maximize efficiency, it may be better in the long
run to heed the call to alternate crops, along with pyramiding Bt toxins, despite the possible immediate costs it
may bring instead of continuing their current procedures and seeing a crisis
of super-resistant rootworms in the future. It is in this kind of situation
that scientists and lay people need to communicate well to effectively face
problems and provide long-lasting solutions.
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