Will GM damage the environment?
One of the main issues for GM critics is the environmental impact of growing modified crops. Once the crops are planted, we can't undo any harmful effects.
Can the modified characteristics spread from GM plants to wild ones? Will we create 'superweeds' that we can't get rid of? What about insects and other wildlife? Could we create plagues of insect pests that are resistant to our chemical sprays?
Can we answer these questions by looking at the 114 million hectares of GM crops already grown worldwide?
The great escape
One of the big concerns is that genes from GM crops will spread to non-GM crops and other wild plants. If the pollen from a GM plant were carried to a non-GM plant by a bee or other insect, then it is likely that the resulting seeds would contain modified genes.
But genes 'escape' when any plant breeds naturally – not just GM plants. What is it about GM plants that makes people so concerned about escaping genes?
Will we create superweeds?
Some GM crops are herbicide tolerant – that is, they survive when sprayed with certain kinds of weedkiller.
A Canadian farmer planted three fields of GM oil-seed rape, each field resistant to a different weedkiller. By 2000, a few years later, he found plants in his fields which were tolerant to all three weedkillers. The farmer had to use new chemicals to keep them at bay.
Experts in Canada and the UK have also found a few cases of resistance genes escaping from oil-seed rape to ordinary weeds.
Will we create superweeds?
Agricultural companies knew people were concerned about escaping genes. They tried adding extra genes to their new plants to make them produce seeds that wouldn't grow. These plants wouldn't be able to reproduce, so wouldn't pass their genes on.
This 'terminator' technology would have been a good solution to the problem of gene escape. However, anti-GM groups criticised this idea too, as it would have forced farmers to buy new seeds each year rather than collect them from their crops to replant. Because of this backlash, terminator technology was never fully developed.
Will we create superbugs?
Superweeds aren't the only possible problem. GM critics are concerned that we could create 'super insects' too.
Some maize crops have been genetically modified to make a substance poisonous to certain insects (but harmless to humans and other animals). These plants are called Bt plants, after Bacillus thuringiensis, the bacteria the modified gene comes from. Bt plants have reduced insect damage to crops worldwide.
But as more insects buzz around the Bt plants, the more likely they are to become resistant to the poison, through natural selection.
Once one bug develops resistance to the Bt poison, this characteristic will spread quickly through the local insect population, as resistant bugs are more likely to survive and reproduce.
Eventually the Bt crop won't survive the attacks of Bt-resistant insects any better than a non-GM variety. As resistance spreads, Bt technology will become useless.
So far, resistant insects have been spotted in America, China and Spain. To help prevent this, farmers are advised to plant 'refuge' areas of non-Bt crops around their fields where insects can thrive – but it only delays the inevitable.
What about wildlife?
Trying to get rid of the bugs that damage crops seems to make sense. But all sorts of insects live in and around crop fields, not just pests.
Insects such as butterflies are essential for pollinating wild flowers, and provide food for other animals. If GM crops kill off butterflies, it could have drastic consequences for the whole ecosystem.
If Bt crops produce a bug-killing chemical, will they kill these helpful bugs too?
An American study in 1999 looked into whether Bt maize would harm monarch butterfly populations. The scientists found monarch caterpillars fed with pollen from GM plants were more likely to die than those fed non-GM pollen.
However, the results of more recent experiments have contradicted this study. And scientists found that butterflies were far more threatened by pesticide spraying and habitat destruction than by any effect of GM.
The 'natural' world
GM critics warn that ecosystems are so interconnected that even the tiniest change in plant or animal populations can do unexpected and far-reaching damage. They argue that releasing GM crops into the environment will have effects we cannot control.
However, others argue that no agricultural ecosystem is natural anyway. Farming technologies including tractors and irrigation have changed the landscape and ecosystem – but they are widely used because they help farmers to farm effectively. Should we treat GM differently?