Bt cotton is cotton genetically engineered to produce toxins from the bacterium Bacillus thuringiensis (Bt), to kill selected insect pests such as the cotton bollworm. However, over time, the pests can develop resistance to the Bt toxins. A popular strategy recommended by the biotech industry has been to create non-Bt cotton refuges around Bt cotton fields to delay insect resistance. The use of natural refuges of non-Bt plants other than cotton, has also been recommended, in cases where the target pests feed on a wide variety of plants.

A recent study in China has found that while natural refuges delayed resistance, they were not as effective as an equivalent area of non-Bt cotton refuges (Item 1). The researchers’ main  recommendation is to switch to a Bt cotton which can produce two or more toxins to further slow resistance down. 

The latter strategy is, however, seriously flawed, according to Dr. Doug Gurian-Sherman of the Center for Food Safety, in a commentary on the study (Item 2). Firstly, a dominant resistance gene found in the bollworm has increased in frequency from 37% in 2010 to 84% in 2013 in bollworm populations in cotton fields in China. The frequency of resistance is not likely to reduce but is in fact more likely to exceed 50% by 2016 unless effective measures are taken to prevent it. Other weaknesses of the recommended strategy include the possibility of cross-resistance arising i.e. resistance to one Bt toxin conferring resistance to other Bt toxins, and higher costs charged by companies for the additional genes added.

He concludes that “the use of GE pesticidal genes to prop up an unsustainable industrial agriculture system is futile”. He recommends agroecological practices like crop rotation as one of the best ways to naturally reduce pest numbers and the likelihood of resistance.

Large-scale test of the natural refuge strategy for delaying insect resistance to transgenic Bt crops

Jin, L., Zhang, H., Lu, Y., Yang, Y., Wu, K., Tabashnik, B. E., & Wu, Y. (2014). 

Nature Biotechnology.
http://www.nature.com/nbt/journal/vaop/ncurrent/full/nbt.3100.html

http://gmwatch.org/index.php/news/archive/2014/15828-gmo-industry-techno-fixes-for-bt-resistance-in-pests-are-counterproductive 

A new study looks at the ability of “natural refuges” versus non-Bt cotton refuges to delay Bt toxin resistance in pests. Natural refuges are non-Bt crops of any description – i.e. they are not confined to non-Bt cotton.

The authors come down in favour of non-Bt cotton refuges and introducing more different Bt traits in an attempt to delay resistance.

Below Dr Doug Gurian-Sherman of the Center for Food Safety points out the limitations of these tactics and says agroecological approaches are the best sustainable solution to pest resistance to Bt crops. 

Doug Gurian-Sherman: The authors emphasize the ability of "natural refuges" to forestall the development of resistance in insect pests that are controlled by Bt genes. Refuges, which allow the insect pest to produce non-resistant young to mate with any resistant insects that emerge from Bt crops, are needed to "dilute" those resistance genes and keep resistance from increasing, or at least to substantially slow it down. Natural refuges are nearby non-Bt crops, other than cotton, that the insect pest feeds on. These can serve the same purpose as deliberately planted non-Bt refuges. Natural refuges can work for pests like bollworm, the subject of this paper, that feed on a wide variety of crops. They do not work for insects that feed on only one or a few crops, like the corn rootworm in the US.

But this paper also presents troubling information, some of which is not found in the abstract:

1) One of the resistance genes found in bollworm is dominant (it needs only one copy to produce resistance), and it has increased in frequency from 37 percent in 2010 to 84 percent in 2013 in bollworm populations in the cotton fields of China. The refuge model is dependent on recessive resistance genes (takes two, one from each parent, to produce resistance) to function well;

2) overall resistance went from 0.93 percent in 2010 to 5.5 now and is up to 11 percent in one province), which is getting to be a lot;

3) resistance will probably now take off (to over 50 percent by 2016, according to the authors’ models) unless effective measures are taken to prevent it;

4) there seems to be no reduction in fitness in the pest due to the dominant resistance gene when it is observed in non-Bt cotton crops. Some resistance genes have a side effect of weakening the pest when it is not being given the big advantage of Bt killing the non-resistant cousins. That is not the case here. In other words, even where there are no Bt genes in the crop selecting for resistance, the frequency of resistance would not be expected to decrease, and it can increase especially rapidly when Bt genes are used (the authors’ test for fitness is fairly crude, but a good first approximation); 

5) the natural refuge is only about half as effective as Bt crop refuge.

The better solution: agroecology

One of the best ways to slow resistance is to grow crops ecologically. Crop rotations and other similar measures naturally reduce pest numbers and thereby reduce the likelihood of resistance. Of course, they may also greatly reduce the value of Bt genes. The harm from many pest insects is greatly reduced or even eliminated by using these methods. Another way, favored by the GE industry, is to try to find other engineered genes, e.g. other Bt toxins, to kill the now-resistant pests. This is mentioned and favored by the paper authors. But resistance to one Bt toxin sometimes confers resistance to others. This is called cross resistance.

So far, cross resistance between Cry1 genes (the kind currently used to control bollworm on cotton in China) and Cry2 proteins and Vip (two other available options) does not seem to have been observed, but that does not mean that resistance genes that can confer cross resistance are not out there at low frequency, waiting to be selected and increased. We simply do not know whether they exist. But there are examples of cross resistance between other Bt genes. And it takes time to develop these crops, and meanwhile resistance is increasing.

The idea of stacking several Bt genes into a crop as a way to prevent more resistance faces several big challenges. First, in areas where there is already substantial resistance to one gene, as in China, at least two new genes are needed, not just one, to have the best chance to slow resistance down substantially. That is more work, but the companies love it as a business strategy, because the newer genes, assuming they were patented later, will be on patent longer than the older (Cry1) genes, and the companies can charge more for two or three genes than for one, although without any increase in effectiveness than the single gene originally had (this may catch up with the companies eventually).

And there are not an unlimited number of Bt genes available to enable this strategy. For example, in the US, where rootworm is resistant to Cry3Bb1, this leaves only Cry34/35, for now at least, because at least some of the Cry3Bb1 resistance is also cross-resistant to the third rootworm Cry, modified Cry3A. This is one reason Monsanto is pursuing the new RNAi technology for rootworm control, but this brings many unanswered risk issues, according to independent scientists queried by US EPA.

In the end, the use of GE pesticidal genes to prop up an unsustainable industrial agriculture system is futile. We end up running faster and faster just to maintain our pest-control status quo.  We need to use agroecology to fundamentally change the way we grow food and fiber. The longer we continue to rely on technology to fix the problems of agriculture without fundamentally changing the way we grow our crops, the longer we will delay doing what is really needed, and the more harm that will be done in the interim. But agroecology is contrary to the GE seed and pesticide company bottom lines because much less of their products would be needed. So expect more of the same from those selling GE and pesticides.