Jun 23, 2003

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Safety research

When earthworms eat Bt maize

Following various other organisms, it is now the turn of earthworms to be fed Bt maize. Swiss researchers wanted to know whether harmful effects can occur as a result of leaves from Bt maize plants falling to the ground and rotting. Under field conditions it was shown that Bt maize does not lead to increased mortality among earthworms. But, as so often, the findings throw up more questions.

Earthworms do not die as a result of Bt maize.

Bt maize, which protects itself against the greedy larvae of the European corn borer, is grown on a large scale in the US and other countries. It is not just in Germany that biological safety research is investigating possible harmful effects on non-target organisms.

So far, researchers have been interested primarily in insects which come into direct or indirect contact with maize – and therefore also with the Bt toxin. But the Bt toxin also enters the soil through rotting plant material. A Swiss working group has been investigating both in the laboratory and in the field the impact that this could have on earthworms, which are important for soil quality.

Earthworms do not die

Earthworms were fed normal and Bt maize over the course of two hundred days. Each worm was placed with the relevant leaf material in a special glass tube (laboratory experiment) or net cages installed in arable soil.

• Bt maize feed did not lead to any change in the mortality rate of the earthworms either in the laboratory or under field conditions.
• The worms fed on Bt maize were, however, considerably lighter at the end of one trial series (in one type of experiment).

Interestingly, this effect was found only with mature earthworms kept in glass tubes filled with soil in the laboratory. Young earthworms that were still developing and were examined in the field were not affected by the Bt maize.

Differences between the laboratory and field trial were also evident in the degradation rate of the Bt toxin. In the laboratory, the toxin content in the maize leaves on which the earthworms were fed fell rapidly in the first few weeks of the trial. It then remained at a constant low level until the end of the trial.

In the field, the toxin level in the rotting leaves was still high enough to achieve the desired effect, i.e. kill the European corn borer larvae, on the eightieth day of the trial. After 240 days only small amounts of the Bt toxin were detected.

Lighter earthworms?

One issue that has not been fully explained is whether the Bt toxin caused the observed weight loss in the earthworms. Earlier comparable experiments found no Bt-related effects on earthworms.

It is possible that the weight loss has nothing to do with the Bt toxin itself. There are in fact other differences between the leaves of Bt maize and non-Bt maize apart from the toxin content. For instance, the lignin content of the Bt maize used in these trials (Bt11) is up to 97% higher and could well have a negative effect on the feed quality.