Genetic engineering: Insect resistance

Bacterial arsenal to combat chewing insects

Plants are producing their own insecticide and using it to ward off their attackers. Genetic engineering has fundamentally changed the face of plant protection. Maize and cotton with genetically engineered insect resistance have been extensively cultivated for about ten years now, particularly in the USA. And these sorts of maize varieties are already approved in the European Union as well.

Made possible by genetic engineering: The active ingredient used to combat the voracious European corn borer caterpillars actually comes from a bacterium. It is now produced in the maize plants.

Bt maize: cultivation in the USA (in million ha) Some transgenic maize varieties have herbicide resistance as well as Bt resistance to the European corn borer (“stacked genes”). In 2009 86% of maize cultivation in the USA consisted of GM maize.

Source: NASS (National Agricultural Statistics Service)-Report 2010

Other countries in which Bt maize is grown commercially: Canada, Argentina, South Africa, Spain, Czech Republik, Rumania and Portugal

Germany: trial cultivation on around 500 hectares per year since 1998. In 2006 Bt maize MON810 was grown commercially for the first time. Since 2009 the cultivation is ban.

EU authorisations: In the case of Bt maize, two “events” have been approved for cultivation in the European Union: MON 810 and T 25. Further applications are still awaiting a decision. All varieties grown in the EU are based on MON810.

It has been known since the start of the last century that a commonly occurring soil bacterium Bacillus thuringiensis (Bt) - has a lethal toxic effect on certain insects. This is due to a protein produced by the bacteria, known as Bt protein.

With the Bt protein, as with all proteins, the “blueprint” is encoded in a specific DNA sequence (gene). The gene is expressed and the respective protein produced. This process is basically the same in both bacterial cells and plants. If the Bt protein gene isolated from bacteria is inserted into a plant’s genetic material, the plant cells will obey the “command” of the foreign gene: the plant then produces its own Bt protein. Chewing insects sensitive to the toxin strain in question absorb the active ingredient, which is specifically toxic to them, along with the plant parts. If the concept works, insecticides will no longer be needed to control these pests.

Maize: the most important Bt plant

Reports of Bt genes being integrated into plants started emerging as far back as the 1980s, initially with tobacco and tomatoes. Then in 1995 the US authorised the first Bt plant: Bt maize.

The Bt concept was particularly attractive for maize, since it made it possible for the first time to combat the European corn borer caterpillars inside the plant. Bt maize has now been grown on a large scale for over a decade, particularly in the USA. In 2009, insect-resistant Bt maize was grown on 16 per cent of the total maize cultivation area, and Bt maize with a combination of insect and herbicide resistance was grown on a further 47 per cent. Bt maize MON810 has been approved for cultivation in the EU, and is commercially grown mainly in Spain, the Czech Republic, Romania and Portugal. However, since 2009 cultivation has been banned

In recent years there has been increasing focus on another maize pest, the Corn root worm, that is spreading all the time – including in Europe. Bt maize which is resistant to the Western corn rootworm has been authorised in the USA since 2003 and has been grown on a large scale since then. In Europe too, Bt maize MON810 have been approved for cultivation mainly in Spain, Czech Republik, Rumania and Portugal.

Bt not just for maize

However, maize is not the only plant into which Bt genes have been transferred. Genes for different toxin variants are transferred depending on the pests that a plant needs to be able to fend off:

  • European corn borer: Bt toxin cry 1Ab (cry=crystal protein)
  • Western corn rootworm: cry 3Bb1
  • Cotton bollworm: cry 1Ac
  • Colorado potato beetle: cry 3A

Distribution in the plant

Bt plants and varieties differ not just in terms of the particular strain of protein they produce but also in terms of the amount of Bt ptotein produced and its distribution in the plant.

The first commercially grown Bt maize plants contained high levels of Bt protein in all plant parts - pollen, stems and maize cobs. They contained greater amounts of toxin than was strictly necessary to achieve the desired effect.

More recent Bt maize varieties not only produce smaller quantities of Bt protein, they produce it only where it is needed. Today, Bt genes are provided with tissue-specific promoters, which activate protein production only in specific plant parts. The Bt maize developed in recent years to combat the Western corn rootworm, for instance, produces Bt protein primarily in the roots.

Bt maize: Effect on non-target organisms

Bt varieties – particularly maize and cotton – are now firmly established in some countries. But they still attract controversy.

In the USA a furore was created by a study which found evidence to suggest that pollen from Bt maize was harming the much loved monarch butterfly. The excitement has since abated. The study findings were based on laboratory trials which did not reflect natural conditions and were put into perspective by subsequent studies. Nevertheless, fears that the protein could harm not just the target organism it is designed to combat, but also other living organisms in the maize field, are behind numerous safety research projects looking at the effect of Bt protein on Non-target Organisms. No clear impacts that would indicate harmful effects have yet been found.

Development of resistance in the European corn borer and the Western corn rootworm

The fact that chewing insects can develop resistance to insecticides over time is nothing new. This has not occurred yet with classic Bt preparations, with the exception of a few isolated incidences. There are fears that large-scale cultivation of Bt plants will accelerate the development of resistance. This would devalue the effect of classic preparations, and organic farming would lose a vital method of plant protection.

In the USA the cultivation of Bt plants is accompanied by statutory resistance management. Proportionate refuge areas must be planted with conventional varieties without the Bt protein.

On this issue too it has been shown that many fears were unfounded. The development of resistance to Bt protein in the European corn borer appears to be much slower than was expected a few years ago.

A number of biosafety projects have looked at mechanisms for the development of resistance in the European corn borer and Western corn rootworm.