Some crops which have been genetically-modified (GM) to produce pesticides may have an "environmental advantage" over sprays, because they do not harm useful insects, according to UK researchers.

The BBC's Christine McGourty reports: "The plant seems to do exactly what it's supposed to do"

The team at the Institute of Arable Crops Research (IACR), Hertfordshire, showed that GM oilseed rape which produces the Bt toxin does not harm a wasp which kills a pest moth.

Nor did the crop prevent the parasitic wasp finding the moth's larvae.

The scientists conclude that the GM rape, which allows the wasps to continue killing the moths, may therefore be better for the environment than insecticide sprays, which kill both the moths and the wasps.

Dr Guy Poppy: "We need to assess what is important"

Their work appears to contrast with the discovery in May that pollen from one GM corn crop can kill the larvae of monarch butterflies. That discovery was made by a team at Cornell University, who said it showed an urgent need for more research on the environmental risks of biotechnology in farming.

The IACR researchers, led by Dr Guy Poppy, believe their work shows that the behaviour of non-target insects can be as important as toxicity in determining the impact of GM plants.

Same toxins

The new evidence is published in the science journal Nature.

The laboratory-based experiment involved GM oilseed rape, the diamondback moth (Plutella xylostella) that damages the crop, and the parasitic wasp (Cotesia plutellae), which kills the moths' caterpillars by laying its eggs in them.

The rape was modified to produce the same Bt (Bacillus thuringiensis) toxin used in the Cornell study. The diamondback moth has gained notoriety for being the first insect pest to evolve resistance to Bt sprays in the field.

Diamondback moth larvae (Photo Extension Entomology, Dept of Entomology, Texas A&M University)

One experiment allowed adult wasps to lay their eggs in the larvae of highly Bt-resistant moths, which had been fed on the Bt rape. But the presence of the toxins in the moth larvae made no difference to the survival rates of the wasps, as larvae or adults.

The authors say: "The presence of Bt toxin had no direct effect on wasp survival."

The parasitic wasps only find their host moths thanks to a chemical "alarm bell" released by the plant when its leaves are damaged. So in another experiment the team used a wind tunnel to see if the wasps would make a distinction between Bt rape and wild rape.

No preference

Overall, when the damage to the two sorts of rape was equivalent, the wasps showed no significant preference for either sort of leaf.

Monarch butterfly: a sharp contrast

Based on their experiments, the IACR team believe that: "The continued ability of the wasps to locate and lay their eggs in Bt-resistant larvae on transgenic crops might even help to constrain the spread of genes for Bt resistance in the moths.

"Our results highlight the need to consider behavioural as well as toxicological aspects when looking at possible side effects of transgenic crops on non-target organisms."

The BBC's Helen Briggs examines what has become a hot issue in biotechnology

Dr Guy Poppy told the BBC: "What we're saying is that it is important to judge the impact of a transgenic plant against a conventional insecticide. What our research shows is that the transgenic plant has much less impact on these non-target organisms than the alternative of using a broad-spectrum insecticide."

The UK Government's official wildlife advisory body English Nature, which has raised concerns about GM technology, welcomed the research. "I think it is a very interested piece of research because it gives us pointers to the sorts of things that we need to take outside from these laboratory trials into the filed trials," says Chief executive Dr Keith Duff.