Pest evolves better resistance to insecticidal GM crops

A few days ago I reblogged a post about genetically modified (GM, also called genetically engineered, GE) crops. I said that in my opinion, my fellow blogger Nasir Butt at Agriculture Information Bank sounds a little bit naïve in that particular post. I said that for several reasons. One of those reasons is evolution. Pests evolve. Here’s a story about evolving pests in some of the world’s poorest countries.

The African maize stalkborer (Busseola fusca) is a serious pest in many countries of sub-Saharan Africa. Scroll through that link if you’d like to see a map and more detail. B. fusca is a moth whose caterpillars attack maize (corn, Zea mays), pearl millet (Pennisetum glaucum) and sorghum (Sorghum bicolor). Many people rely on those grains as their staple food. So people don’t like the stalkborer at all.

Along came GM crops, engineered to make an insecticide called Bt toxin. The GM crops that make it are called Bt crops. This goes down well with many farmers when it works. But after a while, the canny little pests evolve resistance to the toxin. I’ve written about how BT crops work until they stop working.

Here’s the ‘Bt crops working’ part. The ordinary, common kind of the African maize stalkborer carries at least one copy of a version (allele) of a particular gene which makes it Bt-susceptible. Most individuals are homozygous, meaning that they carry two copies of that allele, one from each parent. The allele is dominant. That means that, even if the stalkborer is heterozygous (having only one copy of that allele) it’s still Bt-susceptible. So whether the stalkborer is homozygous or heterozygous for that allele, it will die if it eats Bt toxin. So fields of BT maize end up free from the maize stalkborer.

This goes wrong for the farmer when, rarely, a stalkborer carries two copies of the Bt-resistance allele. That’s the recessive allele. A stalkborer which is homozygous for that recessive allele can eat Bt toxin without dying. Then it can breed. So homozygous recessives become common in fields of BT maize. In other words, the pest evolves. That’s the ‘BT crops stop working’ part.

One of the best ways to prevent the maize stalkborers on your farm from evolving Bt resistance is to plant ‘refuges’ for the Bt-susceptible stalkborers to breed more of their kind. These refuges are patches of the crop which don’t make Bt. Then, you can hope, each time a Bt-resistant stalkborer happens to appear, it’s likely to mate with a Bt-susceptible one. Their offspring will be heterozygous. Since it’s a recessive allele, heterozygotes that eat BT maize will die.

The refuge strategy works well in many places where Bt crops are being grown. But the team at Science20 tells us that another kind of Bt resistance appeared in South Africa about ten years ago. This time, initial studies provide evidence that heterozygotes are resistant. In other words, there seems to be a new Bt-resistance allele, this time a dominant allele. You can’t get rid of a dominant allele by providing refuges. Here’s the science.

One thing’s very clear: Bt-resistant maize stalkborers are spreading fast across Africa.

The authors at Science20 say, ‘single-toxin Bt maize is being progressively replaced by a stacked variety producing two different toxins but, in a worst case scenario, one cannot exclude that Busseola fusca could also quickly adapt to varieties expressing more than one toxin. In the long term, new Insect Resistance Management strategies, likely more complex, should be developed against Busseola fusca.’

I keep an open mind about GM. It may be useful sometimes. But I don’t jump to the conclusion that it’s any kind of magic bullet.


About argylesock

I wrote a PhD about veterinary parasitology so that's the starting point for this blog. But I'm now branching out into other areas of biology and into popular science writing. I'll write here about science that happens in landscapes, particularly farmland, and about science involving interspecific interactions. Datasets and statistics get my attention. Exactly where this blog will lead? That's a journey that I'm on and I hope you'll come with me.
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3 Responses to Pest evolves better resistance to insecticidal GM crops

  1. Pingback: Insecticides weaken plants | Dear Kitty. Some blog

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