May 14, 2008

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Bt maize: Is it safe for humans and the environment?

Has Bt maize been thoroughly assessed for safety before being authorised?

Like all genetically modified crops, MON810 Bt maize has been approved only because it has been shown to be just as safe as conventional maize. The same applies to food and feed produced from such plants. In order for the necessary safety assessment to take place, the applicant has to carry out comprehensive tests and investigations and submit the data obtained for assessment.

Approval under European genetic engineering legislation is valid for all EU countries. The body responsible is the European Food Safety Authority (EFSA), which has the support of a panel of independent experts for scientific safety assessments. MON810 Bt maize was approved for cultivation in the EU as far back as 1998. Since then, EFSA’s responsible scientific expert panel has discussed MON810 maize several times. It established that there were no new scientific findings that would justify casting doubt on the safety assessment. MON810 Bt maize has been grown on several million hectares for years now, and is used primarily for animal feed. So far, no harmful effects have become apparent, either on the environment or on human or animal health.

Is it conceivable that Bt maize could get out of control in the environment?

Even Bt maize is maize first and foremost. It behaves like normal maize and contains the same substances – with one difference: it produces an active substance (Bt protein) which attacks certain chewing insects. Maize originates from Central America. Plant breeders have adapted it to European conditions so that it can be cultivated here. But maize plants cannot survive outside of cultivated fields. Neither can maize outcross to related plant species, since it does not have any related species in Europe.

What happens if undesirable effects of Bt maize become apparent only during cultivation after it has been approved?

Genetically modified plants continue to be monitored once they have been approved. Under current European legislation, genetically modified plants are authorised for cultivation only if an environmental monitoring plan has been set up. Post-market monitoring is designed to identify any unexpected negative environmental impacts of GM plants in good time.

What about environmental safety? If Bt maize is toxic to the European corn borer, couldn’t it also harm other insects?

Unlike conventional plant protection agents, Bt protein is highly specific in its action: its main effect is on the pest it is designed to target and its close relatives, but not on other insects. In Germany, numerous research projects have studied the biological safety of Bt maize. Trials lasting several years failed to find any indication of harmful effects on the ecosystem or biodiversity. The number and type of insects and spiders found in a maize field depends on a large number of factors, including the weather, soil composition, time of year and the maize variety. The Bt protein produced by the Bt maize plants is just one factor among many. By contrast, treatment with an insecticide had significant impacts on the presence of species in the maize field.

Last year, US ecologists evaluated 42 individual studies on possible effects of Bt plants on non-target organisms. They found that when Bt maize cultivation is compared with the cultivation of conventional maize and the usual use of insecticides to combat pests, species diversity is greater on the Bt maize fields.

The European corn borer, which Bt maize attacks, is a moth. Isn’t Bt maize harmful to other moths and butterflies?

MON810 Bt maize very specifically targets moths. For this reason, Bt toxin could be harmful to other butterflies and moths apart from the European corn borer. Different butterfly and moth species have different levels of sensitivity to Bt toxin. However, only the larvae of the corn borer live and eat in the maize plant. Most butterflies, like the cabbage white and peacock butterfly, do not feed on maize. Although they can come into contact with Bt maize pollen if they live near a Bt maize field, the quantities of MON810 maize pollen ingested by butterflies under natural conditions are below the minimum dose at which the first harmful effects are observed.

In feeding trials in the laboratory, the larvae of the diamond-back moth, cabbage white and peacock butterfly were sensitive to pollen from the Bt176 maize line. The pollen of MON810 maize contains much less Bt toxin. In laboratory trials, larvae of the diamond-back moth, which were particularly sensitive to Bt176, were not significantly harmed even when given a comparatively high dose of eighty pollen grains.

Various field trials have so far failed to find a negative effect of Bt maize on butterflies or moths.

A few years ago, a debate on the harmful effect of Bt maize on the popular monarch butterfly broke out in the US. Laboratory trials had found that the larvae of the monarch butterfly could be harmed if fed on Bt maize pollen. Subsequent studies then focused primarily on the question of how frequently monarch butterflies come into contact with Bt maize pollen under natural conditions and what effect this could have on the monarch butterfly population as a whole. It was found that only a small proportion of the caterpillars – between 0.012 and 2.5 per cent – is severely at risk as a result of Bt maize. The authors of the various studies see no risk to the monarch butterfly population.

Are bees harmed by Bt maize?

Bees can come into contact with Bt toxin via maize pollen. Pollen is rich in proteins and bees collect it to raise their larvae and young. Although maize is not particularly attractive to bees because it does not produce nectar, it does play a role as a source of pollen. If bees are kept near Bt maize fields it is likely that they will collect Bt pollen.According to a Swiss study, however, honeybee larvae are exposed to Bt toxin to a much lesser extent than had been assumed. The protein from pollen grains accounts for only around 2.5% of the larvae’s total protein requirements. The Swiss researchers also investigated possible effects on the brood food gland, which produces the protein-rich brood food for the bee larvae, and found no adverse effects.

In a recent meta study, US ecologists evaluated 25 unconnected laboratory studies. They found no effect on the survival rate of honeybees. Since the Bt toxin concentrations used for laboratory tests are usually much higher than the concentrations found in the field, an adverse effect on bees in the natural environment is unlikely. However, researchers have not yet established whether stress factors, like heat or disease, might increase the bees’ susceptibility to Bt toxin. In a biosafety project, bees infested with parasites (microsporidia) were more susceptible to Bt toxin than healthy colonies. But here too the toxin concentrations administered were much higher than would be ingested under natural conditions. The experiment has yet to be repeated.

What happens to the Bt toxin when it enters the soil? Can it accumulate in the soil over the years?

Bt maize plants also release Bt toxin into the soil, especially through the plant remains that are left on the field after harvesting. Biosafety research projects have been investigating the effect of Bt toxin on the soil for many years now. The scientists have discovered that the Bt concentrations in the soil are so small – the rhizosphere contains only a thousandth of the quantity required to produce a sensitive reaction in the European corn borer – that negative impacts on soil organisms such as earthworms are not likely.

Bt toxin breaks down quickly in the soil and does not accumulate there. A small proportion of the toxin binds to soil components, especially clay particles, and therefore takes longer to break down. But even on fields on which MON810 Bt maize was planted for three years in succession, no Bt toxin was detected six months after the harvest.

If Bt maize were grown on a large scale, could the pests become resistant to the toxin over time?

If resistant pests were to emerge, there would be a risk that the Bt concept would fail and classic Bt preparations would also cease to work. For this region, refugia are prescribed for large-scale Bt maize cultivation in the US. These are areas on which conventional maize continues grown. Since the resistance genes are inherited recessively, i.e. are expressed only in the second generation, any resistance traits that emerge can be diluted again in the refugia. So far this strategy has been successful. No resistant corn borers have yet been found.