(Mind nod to the Thumb and ResearchBlogging)

A post at Further Thoughts discusses a paper in Nature Biotechnology that reviews the research on the evolution of resistance to a Bt toxin that has been engineered into Cotton.

Cotton, as you may or may not know, requires a HUGE amount of pesticides to grow a good crop. The introduction of Bt cotton in China in 2001, engineered to produce a bacterial gene toxic to certain insects (but not mammals), eliminated a quantity of pesticides roughly equal to all the pesticides that California uses in a year.

The insects were not particularly pleased. Evolution charges on, however, and it would seem that resistance is only a matter of time, and eventually we’d be back where we started. Well, some resistance has evolved to the Cry1Ac protein (the specific Bt toxin inserted- there are thousands of different Bt toxins), but only in a few places. What did the paper find?

In an article published in Nature Biotechnology, Bruce Tabashnik and colleagues looked at the actual pattern of evolution of resistance to Bt toxin Cry1Ac in cotton over a 10-year period. They used studies conducted in Australia, China, Spain and the United States focusing on six pest species: Helicoverpa armigera, H. zea, Heliothis virescens, Ostrinia nubilalis, Pectinophora gossypiella and Sesamia nonagrioides. They found that in only one of these species - H. zea - had the frequency of resistance genes increased substantially.

One explanation for the evolution of resistance in H. zea is the observation that resistance to the Bt toxin Cry1Ac appears to be the dominant trait. This greatly reduces the effectiveness of refuges, since the resistance genes aren’t diluted out as effectively as they would be if they were recessive.

Refuges are areas of non-Bt crops planted to ensure that there are non-resistant insects to breed with any resistant insects that come up. If the resistance is a recessive trait, then refuges would, in theory, drastically slow the evolution of resistance because it would make it very unlikely for two resistance alleles to come together in one individual bug.

Gustafson et al. [2006] meticulously estimated that the effective refuge abundance during each of three generations when H. zea fed on cotton was 39% in Arkansas and Mississippi and 82% in North Carolina. With these refuge sizes, H. zea is projected to evolve resistance after 9 years in Arkansas and Mississippi. By contrast, in North Carolina, resistance evolution should take >20 years, with the expected resistance allele frequency still <0.005 after 10 years.

Where the refuges were larger, resistance, even in a species where the resistance is dominant, still takes a long time. (On a human scale)

What this paper shows is that not only do refuges work for most insects, but we can predict how well they work. We can predict what the gene frequencies are going to be after 10 or 20 years, and plant refuges accordingly. What this means for the introduction of genetically engineered crops is that if we do it right, we can stretch out the effectiveness of individual genes for decades.

But that’s not what we’re going to do. By “Pyramiding,” or stacking several resistance traits in each crop, the chance of resistance to each trait simultaneously coming together in the same insect becomes astronomic. If resistance to two different Bt toxins behaves like the one Bt toxin does alone, then after ten years, resistance would be below 0.000025 of the population - you might not see double-resistance before you retire. If you stacked four such genes in one cotton plant, you might not see it your whole lifetime!

However, if we grow dual-gene and single-gene varieties next to each other, resistance could evolve to the single-gene crop and allow the pests to ’step up’ to the dual-gene crop. Therefore, if we want to make this work right, we need to phase out single-gene crops as dual-gene crops being to be used. Dual-gene crops should be phased out in favor of triple-gene crops, etc.

It is interesting to note that insect resistance to Bt crops is coming about very slowly overall. Resistance to pesticide sprays that hit nontarget insects, malform frogs, and wreak general havoc on the environment, evolve much faster. Bizarrely, anti-GE folks have been saying for years that resistance in GE crops would come faster, that pesticide sprays are actually better than insect-resistant GE crops!

Go figure.

Someday, we’ll have a society that thinks about these things rationally and would rather wear GE cotton shirts that used far fewer pesticides (is it a dream to say none at all?) but doesn’t require more land to grow it. Imagine if we could have informative labels on cotton shirts, to help people decide which shirts to buy?

Organic: Tilled five times as much as conventional, requires minimum-wage migrant workers. 12-33% more land required for the same amount of cotton. (rough estimate from power point presentation linked above) Fewer insecticides, but not zero.

Non-GE conventional: Requires huge amounts of pesticides - cotton accounts for 25% of pesticide use on 2.5% of the land. But, reduced tillage and erosion.

GE conventional: Reduced use of pesticides while maintaining yield and erosion benefits.

GE organic: Could it be possible that this is the ticket to a higher-yielding organic cotton?

GE New Agriculture category? Is the opposition of the Organic movement to new agricultural techniques, and the problems associated with conventional agriculture enough to start a new category of agriculture that addresses the flaws in both?

Enough digression from the Bt topic. As you can see, I dive into this stuff!