May 14, 2008

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Questions and Answers

What is Bt maize?

‘GM maize’ usually refers to genetically modified Bt maize. So what is Bt maize?

Bt maize is maize that has been genetically modified to produce an insecticide – Bt toxin – that kills certain chewing insects. The gene transferred to the maize comes from the soil bacterium Bacillus thuringiensis, which is where the abbreviation Bt comes from. The bacterium has long been known to possess an insecticide effect. Unlike many chemical insecticides, Bt toxin is harmless to humans and is broken down quickly. This is why Bt preparations are frequently used as biological plant protectants in organic farming.

What is Bt toxin and what does it do?

Bt toxin is a protein. It is produced by Bacillus thuringiensis in a non-toxic form to start with. It is only in the gut of certain chewing insects that it is converted into a toxic form.

There are many different Bt toxins, but each one affects only certain insect groups. CryI, for instance, has a specific effect on certain moths, CryIII acts against certain beetles.

Is there a difference between the Bt toxin formed in Bt maize and the toxin used in plant protectants?

The toxins in Bt plants and in Bt preparations have a similar effect. In both, the toxin is present in a precursor form (protoxin) that is converted into active toxin only in the presence of certain enzymes in the gut. The difference is that the Bt genes introduced into the plant have been shortened and adapted to the plant. In the plant the protein is present in a dissolved form, rather than as a crystal.

Which maize pests is Bt maize effective against?

The main Bt maize varieties used around the world are those with resistance to the corn borer. The same is true of the Bt maize variety approved for cultivation in Europe, MON810, which produces the Cry1Ab Bt toxin that targets the European corn borer.

This small grey-brown moth is a major maize pest. The Julius Kühn Institute (formerly the Federal Biological Research Centre for Agriculture and Forestry - BBA) estimates that the European corn borer causes damage worth between eleven and twelve million euros in Germany each year. The larvae of the corn borer eat through the maize stems during the growing period. Even mild attacks can lead to the stems of the weakened plants snapping. The corn borer is found in all southern and south-eastern European maize-growing areas. In 1960 its presence in Germany was still restricted to isolated areas in the south of the country. Since then, the corn borer has been spreading northwards and has now reached the Baltic coast.

What are the benefits of Bt maize?

In regions with high levels of corn borer infestation, Bt maize has clear advantages over other control methods: its effect is targeted, effective and reliable. Farmers can avoid using plant protectants to combat the corn borer, saving labour time and machine operation costs. They also avoid the harvest losses caused by plants snapping – up to thirty per cent when a crop is heavily infested with corn borers.

If only a few pests are present, it is not worth controlling them and the farmers then choose to put up with small harvest losses.

However, Bt maize can also have another benefit: lower contamination with fungal toxins. Pathogenic fungi can enter plants through the sites where they have been chewed by the corn borer larvae. Some of these fungi, e.g. fusaria, produce highly effective toxins (mycotoxins) which contaminate the food and feed produced from maize. Several investigations have confirmed that Bt maize plants usually contain fewer mycotoxins than conventional maize plants.