Bt maize versus the billion-dollar bug
The Western corn rootworm causes immense damage in the USA. It was introduced to Europe just 10 years ago and has been steadily extending its domain ever since. A new genetically modified maize aims to provide new control strategies. How best to prevent the development of resistance in this beetle, however, remains controversial.
|
It is said that the Western corn rootworm (Diabrotica virgifera) causes one billion dollars worth of damage annually in the maize-growing areas of the USA. The soil-dwelling larvae of the beetle eat the aerial roots, which quickly die back. Older larvae then penetrate the heart of the root and from there move further into the maize plants. Many infested plants collapse before harvest as a result of the root damage. In the USA the Western corn rootworm is controlled with the intensive use of insecticides. The annual bill amounts to around 200 million US dollars. Loss of earnings caused by the pest is estimated to be in the region of 800 million US dollars. Crop rotation can also be used to contain the infestation, since the larvae of the Western corn rootworm can only survive in maize roots. Cultivating maize varieties with roots which are able to regenerate quickly is another method of reducing the damage. From Belgrade to the rest of Europe The beetle arrived in Europe in the early 1990s. It was first discovered in 1992 at Belgrade airport. From there it spread throughout the vast Hungarian lowlands and later into other parts of Eastern Europe. The Western corn rootworm has since emerged in Italy and Switzerland. Despite intensive searches using traps, the pest has yet to be found in Germany. |
A new Bt strain to combat the Western corn rootworm
In late February 2003 a genetically modified maize strain (Mon863; YieldGard Rootworm Corn) was authorised in the USA as an alternative way of controlling the corn rootworm. As with the established strategy for controlling the European corn borer , these GM maize plants also produce a Bt toxi n, but one that acts specifically on the corn rootworm. The gene (cry3Bb1) comes from a subspecies of Bacillus thuringiensis (B. t. ssp. kurmamotoensis), but not the same one from which the genes originate which produce the Bt toxins used to combat the European corn borer and which have also been tested in Germany for some time.
This gene, which has been optimised for use in plants, was assessed by the US Environmental Protection Agency (EPA) back in 2001. Now the maize strain containing this gene has also been granted conditional, temporary approval. The conditions include resistance monitoring to promptly detect the possible development of resistant corn rootworms and adopt appropriate measures.
Discussion about the size of refuges
There is much debate about the size of the areas of conventional maize planted within the Bt maize cultivation zones. These "refuges" are a key component of the strategy aimed at preventing a rapid build-up of resistance in the Western corn rootworm.
The refuges are created to provide an area where susceptible pests can continue to multiply. If resistant insects emerge, they will mate with the susceptible insects. This will ensure that the resistance genes are “watered down” again in the next generation, so that the spread of resistance can be halted or at least slowed down.
This strategy only works if very few insects survive in the transgenic maize. If the transgenic plants produce too little Bt toxin, a large number of surviving insects could be partially resistant. If they mate with each other, different levels of resistance could arise. Therefore it makes sense to ensure that the Bt toxin content, as in previous Bt plants, is always relatively high.
However, it is significantly lower in the new Bt maize strain aimed at the Western corn rootworm. According to the agro-biotechnology company Monsanto, which developed the new Bt maize, the Bt toxin content is enough to control the pest effectively.
Although some experts recommend a non-Bt maize refuge comprising 50% of the total crop area, the EPA has specified 20%. The EPA estimates that it would take between seven and sixteen years for problems with resistance to arise, based on cultivation throughout the USA.
Monsanto has announced that it will be placing on the market seed from the new Bt maize strain in 2003. However, the company is assuming that it will be grown on no more than 6% of US maize-growing areas by 2005.
Approval is initially restricted to three years, after which the refuge concept will be re-examined.
More from GMO Safety
- Interview with Stefan Vidal: "In regions where we grow one maize crop after another there will be problems"
- Bt maize - A new concept for controlling the European corn borer
![[jump to top]](/images/icons/icon_gotop_9x9.gif){kind=icon}