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The first experiments with plants genetically modified to repel aphids are under way in the UK.

Wheat has been engineered with a gene from a peppermint plant so that it emits a particular pheromone.

The smell is the alarm signal given off by aphids to warn of an attack by predators.

The researchers hope that this will act as a "no parking sign" to keep the pests at bay without needing insecticide.

This is the first trial of a plant deliberately modified to use pheromones to ward off pests.

The work is taking place at Rothamsted Research, the plant science centre in Hertfordshire.

Eight plots of land - each six metres by 6m - have been planted with the GM wheat in the past few days.

Aphids are one of the major threats to the cultivation of wheat in the UK and other countries.

Professor Maurice Moloney, director of Rothamsted Research, has described the insects as a "ten billion dollar problem".

The idea of creating a variety of wheat that could use smell to repel insects was first mooted in 1985.

According to Professor John Pickett, Rothamsted's scientific leader on chemical ecology, tests have proved the technique in the lab.

"The aim is to use natural processes instead of pesticides and address the public's concerns about pesticide use," he said.

The trials are being conducted behind security fencing to keep out animals and to guard against environmental protestors.

One set of wheat in the experiment is fitted with the peppermint gene only; another set has also been engineered with a synthetic gene.

The goal is to see if the plants give off a highly pure version of the pheromone, known as A-Farnesene.

Experiments have shown that the aphids' sense of smell is highly sensitive; the insects will not react if they suspect the pheromone has been released by a plant.

A further aim is to see if the pheromone attracts the aphids' main predator, the tiny parasitic wasp.

Lab tests show that the smell has the twin effect of repelling aphids and luring the wasps.

One possible risk, raised during a briefing, is that the pests are driven from the GM wheat on to neighbouring crops.

The researchers say that existing wheat fields are protected by pesticides and wild plants have their own defence mechanisms.

The scientists are hoping that GM plants which cut insecticide use will help gain support for this controversial technology.

The science suffered a setback a decade ago amid negative publicity and public nervousness about GM food.

Britain's major supermarket chains do not stock GM food and approval for GM crops has to be given by the European Union.

Trials like this one, which are purely for research, can be authorised by national governments.

The widespread view among plant scientists is that the challenge of feeding a global population of seven billion requires genetic modification.

But they face a struggle to win over retailers and politicians not just in Britain but in the EU as well.