Transgenic crops were originally conceived as having the potential to improve agriculture and help feed a growing world population. Some of the original goals were to improve crop yield, impart drought tolerance, and to render crops resistant to common pesticides and insect pests.
Pest resistance is directly imparted to transgenic crops by rejiggering the crops' genetic structure, to produce toxins that kill pests that feed on crops. A danger of this approach is that it might select for insects that are resistant to the toxin, negating the value of the genetic tinkering.
A twin approach is taken as a precaution against this danger.
Transgenic corn currently approved for controlling corn rootworm does not meet this excess toxicity requirement. The proportion of pests that emerge from such corn fields with toxin resistance is unknown.
Bruce Hibbard (University of Missouri, Columbia) and coworkers have shed light on this question. They have shown that, within three or six generations, the western corn rootworm can become partially resistant to low-dose pesticides produced by transgenic corn that is not grown alongside normal corn.
Why choose western corn rootworm?
The western corn rootworm, Diabrotica, is a hardy insect. They can easily develop resistance to chemical pesticides, and they survive crop rotations often used to control pest populations.
Additionally, the insects can retain pesticide resistance for multiple generations. They are a challenge to control, afflicting roughly 1/3 of American corn farms, and cost American farmers an estimated $1 billion in lost revenue annually.
The western corn rootwrm is therefore a natural object of study for demonstrating the effectiveness of transgenic corn, against a costly pest, under rigorous conditions.
The pests and transgenic corn.
The transgenic corn, engineered to produce the protein toxin Cry3Bb1 (originally discovered in Bacillus thuringiensis bacteria) by the Monsanto Company, as well as the rootworm pests, were raised either in a greenhouse or in a field trial at the Bradford Resarch and Extension Center.
The insects were exposed to normal corn, or exposed to transgenic corn as either neonates, later in life, or throughout development. Additionally, in order to vary toxin exposure, different colonies of the insects were raised on the crops under different conditions.
Resistance to transgenic corn.
After three generations in a greenhouse (a controlled environment), the insects exposed to transgenic corn throughout life were resistant to the corn toxin. The number of surviving larvae on transgenic corn was the same as the number of larvae on normal corn.
Additionally, after six generations of living on transgenic corn, followed by six generations of living on normal corn, resistance was maintained. This suggests that Monsanto's corn quickly selects for pesticide-resistant insects, a resistance that lasts for succeeding generations.
Field tests.
Controlled conditions such as these rarely lead to findings that correspond to those found in the field. The scientists therefore needed to perform field tests in order to confirm the significance of their findings.
After six generations, these field tests with insects raised on transgenic corn throughout life produced offspring that were roughly twelve times as likely to survive than control insects. A lesser level of resistance was observed among insects exposed to the transgenic corn earlier or later in life.
Insect development -- as measured by dry weight, number of larvae produced, and number of resulting adults -- was normal, except that development was slower. The resistant insects were therefore mostly normal, except for their resistance to transgenic corn.
In the field, the trasngenic corn still killed off plenty of insects, in contrast to the greenhouse experiments. However, as noted above, a large number of insects did survive and quickly develop resistance.
It is important to note that the scientists killed the insects before they became adults in these field experiments. This was done to prevent resistant adults from spreading elsewhere.
How did the insects develop resistance?
One reason why the insects developed resistance to transgenic corn was that the amount of toxin produced by the corn was low. As mentioned previously, a very high dose is necessary to kill off all insects except those that are completely resistant to the toxin.
A contributing reason is that the insects altered their feeding behavior; developing larvae eat less of the corn in order to be exposed to less of the toxin. This allows even slight resistance to build gradually, unless the toxin levels in the corn are vey high.
What this means for transgenic corn.
Rapid development of pest resistance to transgenic corn is a serious concern. Pests that developed resistance to pesticides that were banned in the 1970s are still highly resistant today.
Additionally, genetic analysis suggests that the insects developed resistance not from random genetic mutations, but from selection pressure. The transgenic crops themselves imparted pesticide resistance.
These results re-emphasize the need to engineer transgenic corn with a high level of protein toxins, and to plant normal corn alongside it. Not doing so risks the real and serious danger of selecting for resistant pests, and defeating the entire purpose of the genetic rejiggering in the first place.
for more information:
Meihls, L. N.; Higdon, M. L.; Siegfried, B. D.; Miller, N. J.;
Sappington, T. W.; Ellersieck, M. R.; Spencer, T. A.; Hibbard, B. E.
Increased survival of western corn rootworm on transgenic corn
within three generations of on-plant greenhouse selection.
Proc. Natl. Acad. Sci. USA. 2008, 105, 19177-19182.