Resistance To GM Crops
    By Gunnar De Winter | July 30th 2011 06:11 AM | 7 comments | Print | E-mail | Track Comments
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    Large amounts of research and money have been invested in the development of transgenic, or GM (genetically modified) crops. These crops are genetically engineered to withstand drought, excessive rain or other weather conditions, or to improve their yield or increase their rate of development, or to express certain toxins that would limit the amount of insects feeding on them.

    While transgenic crops might potentially be a means to increase food production, they have been, and remain to be, quite controversial. There are many reasons why people oppose them, but one of them is that insect resistance is unlikely to last. Now, a study seems to provide some evidence for the occurrence of field-evolved resistance to toxins produced by transgenic maize in the western corn rootworm (or Diabrotica vrigifera virgifera, don’t let the common name fool you, it is, in fact, an insect) (see figure 1).


    Figure 1: The western corn rootworm.

    (Source: wikimedia commons, user: Bemoeial2)


    Bt maize has been engineered to produce toxins derived from the bacterium Bacillus thuringiensis (hence Bt) (see figure 2). Bt crops are planted on millions of hectares each year and strongly reduce the need for conventional insecticides, as well as the occurrence of pests. Resistance to the toxins, however, could significantly impact these benefits. More specifically, fields which have been used to grow Cry3Bb1 maize (a type of Bt maize) for several consecutive growing seasons, showed more severe feeding injury from rootworms, indicating higher rootworm survival.


    Figure 2: The makings of Bt maize

    (Source: The Science Creative Quarterly)


    Rootworms from several fields were collected, and their larvae, reared in the laboratory, were introduced to Bt maize. Turns out that the larvae whose parents came from fields with a longer history of growing Cry3Bb1 maize exhibited a higher survival. In fact, the survival correlated nicely with the number of consecutive growing seasons of this type of maize (see figure 3).


    Figure 3: Correlation between the number of years that Cry3Bb1 maize has been grown on a field and rootworm survival (C: control field, P: problem field).

    (Source: Gassmann et al., 2011)


    The authors conclude:

    To date, the widespread planting of Bt crops has resulted in pest resistance for only a small subset of all pest populations managed by this technology. However, these recent cases suggest a need to develop more integrated management solutions for pests targeted by Bt crops. A common pattern observed among problem fields in this study was the consecutive planting of the same type of Bt maize over several seasons. Even with resistance management plans in place, sole reliance on Bt crops for management of agriculture pests will likely hasten the evolution of resistance in some cases, thereby diminishing the benefits that these crops provide.

    Does this mean the transgenic crops are without value? Of course not. Resistance to weather patterns, or improving yield will not be challenged in this way  (but perhaps in other ways?). And even transgenic crops engineered to produce insect toxins needn’t be labeled as ‘failed’. The key is not to give the insects a chance to develop resistance, for example, as the authors propose, by alternating different transgenic breeds, or alternating transgenic and 'regular' crops.

    As always, further research is required to understand exactly how this resistance evolves, and what the most appropriate means are for preventing this.



    Gassmann, A.J.; Petzold-Maxwell, J.L.; Keweshan, R.S.and Dunbar, M.W. (2011). Field-Evolved Resistance to Bt Maize by Western Corn Rootworm. PLoS ONE 6(7): e22629. doi:10.1371/journal.pone.0022629.


    Yeah, that last part is spot on. It isn't so much about GMOs as it is simple evolution, something GMOs do nothing to change. If you apply strong selection pressure to a fast reproducing population expect to see some sort of genetic shift. It doesn't matter if we're talking about herbicides and weeds, antibiotics and germs, soot and moths, or pesticides and pests, be those pesticides synthetic, so-called 'organic', or built into the plant (GMO or otherwise). This isn't good, but it isn't entirely unexpected, and is a case for the introduction of a wider range of pest control methods (and making sure that proper refuge areas are maintained in GM crop fields).

    But ugh, I hate to think what will happen if the anti-GMO groups catch wind of this. They will no doubt paint this as 'yet another failure' of GM crops, and are unlikely to present this issue with the nuance it deserves. Expect a wave of stories claiming 'GMOs create superpests,' with no mention that the pests are only resistant to the protein of the one type of pest resistant GMOs or that the threat is not enhanced pests merely losing the benefit that GMOs already provide. I don't think it is overly pessimistic to expect that because it is the exact same thing they did with glyphosate tolerant weeds, or 'superweeds' as they are often and erroneously called. It will be lost on them that the fact that these crops are GMO is not the point so much as that relying on a single mode of pest control, regardless of what that mode is, is generally a bad idea for numerous reasons, and preventing resistance is one of them.

    I agree that transgenic plants are a valuable pest control tool, and that they should not be a target for neo-luddites. However, the resistance issue should not be taken lightly either. There is considerable geographic separation among the problem fields (in particular, P4 seems to be completely isolated). So, we are not dealing with a single pocket of resistance.

    In light of what appears to be an increasing number of cases of pest resistance to Bt crops, I would question a continuous relaxation of refuge requirements by the EPA. With the introduction of pyramided plants, refuge requirements for rootworms have been reduced to 5% blends of transgenic pyramided seed with non-transgenic seed. The decision was made against the recommendations of an independent Scientific Advisory Panel convened by the EPA.

    Arguably, there is little scientific justification for reducing the refuge size (Nature Biotechnology 29, 577–578 [2011] doi:10.1038/nbt.1911). Furthermore, when one toxin in the pyramid has already failed (as is the case for the populations tested in this article), the pyramid is compromised because there is only one toxin remaining to kill the pests. So, even if a 5% refuge is sufficient for truly pyramided crops (and we do not know that), it is highly unlikely to be efficient when the pyramid is no longer a pyramid.

    Refuge size for blended rootworm-protected Bt corn is, of course, 10%. Five percent is the refuge size for caterpillars. Sorry for the typo.

    Gerhard Adam
    One of my biggest concerns involves the legal aspect of this.  Who owns the patents on these plants? or the genes? or whatever it is that is motivating companies to engage in this activity (because I know it isn't altruism).

    Given the recent ruling on human gene patents, I'm not particularly enthralled with the idea of granting that kind of control of our food supply to economic interests alone.
    Mundus vult decipi
    Steve Davis
    "I'm not particularly enthralled with the idea of granting that kind of control of our food supply to economic interests alone."
    I think you'll find that's already happened Gerhard.
    Gerhard Adam
    At present, I know I can at least grow and supply most of my own food without concern that I'll be sued for patent infringements or licensing violations.
    Mundus vult decipi
    Steve Davis
    Thank goodness thinking people started seed banks when this bandwagon got rolling!