LONDON - A genetically engineered wheat that gives off a special smell designed to repel aphids has flopped in field-scale tests, underscoring the difficulty of harnessing the controversial technology.

Scientists said the result was disappointing but they aim to amend the process to do better in future, believing that genetic modification (GM) offers a way to develop resilient crops that don’t need to be sprayed with pesticides.

Critics, however, fear such GM plants risk contaminating the environment and could jeopardise the food chain. The work at Britain’s Rothamsted Research institute in southern England was the first test of a crop engineered to release an anti-insect pheromone, or smell, and it provoked protests from anti-GM activists who threatened — but failed — to rip up the plants.

While the crop survived human attack, however, it fared less well against the aphids. Results from the five-year project published in the journal Scientific Reports on Thursday showed the GM wheat did not repel aphid pests in the field as hoped, despite initial success in laboratory tests. Aphids damage wheat by sucking sugar out of plants and spreading viruses, prompting extensive spraying with insecticides made by companies like Bayer and Syngenta. The Rothamsted team added genes to make the wheat produce the pheromone (E)-beta-farnesene, which is found naturally in other plants, including peppermint, and acts as an alarm call telling aphids to disperse.

It is not clear why the GM crop failed to work as expected but scientists said the aphids may have simply become attuned to the constant alarm signal, in the same way that people get used a car alarm that never stops ringing. One idea now being pursued is to make the plants produce “puffs” of pheromone when aphids arrive, said Rothamsted’s John Pickett, who still sees a future for pheromone-based repellents.

“We see this as heralding a process of controlling insects without necessarily using a spray-on toxicant insecticide,” he told reporters. “It’s the beginning of an alternative approach.” The Rothamsted trial, which was not backed by commercial interests, was a relative rarity in Britain, given political opposition to GM at the European level and the cost of doing such research.

Opposition to GM meant the project required 2.2 million pounds ($3.5 million) of spending on fences and other security measures — three times more than the scientific costs. The study was funded entirely by the UK stated-backed Biotechnology and Biological Sciences Research Council.