A genetically engineered wheat designed to scare away aphids has, in the end, just not proved scary enough. Researchers had hoped that the wheat modified to emit a warning pheromone would ward off aphids while also attracting their natural enemies, thereby allowing farmers to spray less insecticide. Despite promising signs in the laboratory, the field trial—which made headlines in 2012 after opponents of genetic modification (GM) threatened to obstruct it—failed to show any effect. “It was disappointing news,” says Paul Temple, a wheat farmer with the U.K. Agriculture and Horticulture Development Board, which was not involved in the research.

Aphids can cause major headaches for wheat growers. The tiny insects feed on sap, and an infestation can reduce harvests significantly. Some species inject a toxin, which causes the leaves to wither and fold, providing a safe spot for laying their eggs. Other kinds of aphids transmit devastating viruses that further weaken the plant. Luckily, other creatures prey on aphids. Parasitic wasps, for example, lay their eggs inside them, which eventually kills the insects. When attacked by such wasps, aphids release a chemical alarm to encourage nearby aphids to flee. In a nefarious twist, however, other wasps follow this alarm to find victims.

Huw Jones, a molecular biologist, and other scientists at Rothamsted Research, a mainly publicly funded agricultural research station in Harpenden, U.K., wanted to exploit this alarm to help defend wheat plants.

The researchers knew that some kinds of plants make this alarm pheromone, although apparently not to fight aphids. They designed a new gene, based on one in peppermint, and adapted it to work in wheat plants. That’s “a major accomplishment,” says ecologist Marcel Dicke of Wageningen University in the Netherlands, whose group works to design ways to integrate crop protection with biological control, plant engineering, and breeding. Wheat has six copies of each of its chromosomes, he notes, making effective genetic modification quite a challenge.

Lab experiments showed promise: The wheat released the pheromone, three kinds of aphids steered clear, and parasitic wasps were attracted. “It’s really the first time that someone makes a GM plant with a gene for coding for an insect pheromone,” says Thierry Hance, an ecologist at the Catholic University of Louvain, Louvain-la-Neuve, in Belgium.

A field trial was next, but when a group called Take the Flour Back got a whiff of that study, they worried about the spread of the engineered genes. In 2012, the group threatened to uproot the test fields, but after a public appeal by scientists and a media debate, the trials went ahead. Fencing and extra security cost £2.2 million—about three times the price of the 5-year research project itself.

The researchers planted engineered wheat and non-GM varieties in 16 plots, each 6 meters square. But this time, the wheat was a bust: There were no statistically significant differences in aphid numbers between the GM and non-GM plots, as the team describes today in Scientific Reports. Failure in field trials is not uncommon, as plant breeders weed out poorly performing varieties en route to selecting likely commercial winners, says Penny Maplestone, chief executive of the British Society of Plant Breeders, in Ely.

Part of the problem might have been that the summers of 2012 and 2013, when the trials took place, were wet and cold, which depressed the aphid populations. That may have made it harder to detect an effect of the alarm pheromone. Jones suspects something else is wrong: The wheat plants released a steady supply of pheromone rather than a short burst, as aphids do when they are attacked. Dicke agrees. “If you make a transgenic plant that produces that alarm continuously, it’s not going to work,” he says. “You have a plant crying wolf all the time, and the bugs won’t listen to it any longer.”

In fact, in greenhouse experiments the researchers did see aphids eventually ignore the pheromone when they were constantly exposed to it. The reason that Jones and his colleagues decided to engineer a constant supply was to ensure that the wheat plants would make enough. In addition, they hoped that aphids flying into the field would suddenly encounter the alarm, as if it were a sharp new scent.

The researchers are considering creating wheat that sends out pulses of pheromones or releases them only when under attack. Jones says that wheat plants turn on certain genes when nibbled by insects or sucked by aphids, so he and others might be able to find the switches for these genes and hook them up to the engineered genes that make the pheromones. It will likely take several years before the team is ready for a new field trial, he says.

Despite the disappointing results, the overall approach is worth pursuing, other plant biologists say. “We are in urgent need of new ways to control insect pests on crops, with very limited options available from pesticide sprays and conventional breeding. Alternative approaches ranging from new agricultural practices to genetic modification must be explored,” said Ottoline Leyser, a plant biologist at the University of Cambridge in the United Kingdom, in a statement. "This field trial is an excellent example of the sort of work that is needed."