Some plants that emit an airborne distress signal in response when attacked by insects or herbivorous animals may actually attract more enemies, according to new research by a team of Swiss scientists.

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In laboratory tests, the researchers found that the odour released by maize plants under attack by insects attract not only parasitic wasps, which prey on the insects, but also more pests. In the case of maize plants, alarm signals can attract caterpillars of the Egyptian cotton leafworm moth, which feeds on maize leaves.

The moth used in the experiment has only been recorded in the UK on a handful of occasions, though it is a pest of fruit, vegetable and field crops in Africa and Mediterranean Europe.

When damaged, many plants release hydrocarbons called volatile organic compounds, similar to the compounds that cause the characteristic smell of freshly cut grass. These volatile organic compounds are known to be attractive to parasitoid wasps that lay their eggs inside (and kill) other insects.

Plants appear to use this strategy to fight back against insect attacks by calling for their enemies' enemies. Herbivorous insects therefore tend to avoid the volatile organic compounds emitted by certain plants.

Professor Ted Turling, one of the study's co-authors from the University of Neuchâtel, said, "Adult moths and butterflies avoid food plants that are under attack by conspecifics [other insects of the same species]. This seems adaptive, because it reduces both competition and the risk of predation... But we found that caterpillars are actually attracted to the odour of damaged maize plants, even when this odour is mimicked in the laboratory with a mix of synthetic compounds,".

To determine what kind of odours the caterpillars preferred, the researchers let the caterpillars chose among several odours by placing them in an olfactometer, a device consisting of four tubes connected to a central chamber, with each tube introducing an airflow carrying a different odour. The caterpillars were more than twice as likely to crawl towards the 'maize' smell than towards undamaged plants, especially if the damage was recent and the caterpillars had already fed on maize.

The researchers believe that the caterpillars' preference for plants' distress odours may enable them to rediscover a plant on which they fed. They said that, when caterpillars drop from a plant, they are highly vulnerable to predators and pathogens in the soil, as well as to starvation.

When exposed to high concentrations of the distress odours, the caterpillars fed less and moved away. Prof. Turling that suggested, "By moving away from freshly damaged sites, they can minimize risk of predation and avoid competition."  Adult moths, which are more mobile, take no such risks and will avoid maize that is already under attack, preferring to explore their environment for other places to feed.

In the UK, researchers at Rothamsted Institute are trialling a genetically engineered strain of wheat, modified to mimic an odour that repels aphids and attracts predators. Critics of the trials have warned that aphids could develop resistance to the defence, as has been the case with other GM crops, though the Rothamsted Scientists remain determined to persevere.

The scientists were granted permission to extend their research and plant an autumn-sown trial in June.