Plant rescue calls could provide a more environmentally friendly way to protect crops from pests, an opinion piece in the journal Trends in Plant Science argues. The technique is widely used by wild plants, and applied by some gardeners, but so far has not been scaled for the use of broad scale crops.

"Wild plants commonly emit natural odors when they are damaged that attract natural enemies of pest insects - even as humans we smell it when our neighbour is mowing the lawn,” author Martin Heil of CINVESTAV-Irapuato, Mexico, said in a statement. "Agriculture has bred such defenses out of crops, and since these odors have no negative effects on human consumers, we want to replace what the plant would already be doing."

Many plants and predators have formed mutually beneficial arrangements where the plants release a call for help that alerts animals that a tasty meal is available. Even bioluminescent dinoflagellates get in on the act. It's all a little like the climax of many films where the bigger predator resuces the desperate people from the more immediate threat.

The use of Volatile Organic Compounds (VOCs) to attract predators is almost universal among plants, the authors note, and many have taken this further producing extrafloral nectar (EFN) to sweeten the deal for species that feed on herbivorous insects. The paper notes that EFN not only attracts predators, but keeps them around longer while the sugar hit makes them more effective predators.

Nectar not only attracts some animals that are otherwise largely carnivores, but makes them more effecient hunters. Credit: Stenberg et al./Trends in Plant Science 2015

Strangely, however, this capacity is seldom shared by the plants we use as our major crops, although beans and cabbages are exceptions. Specific cases indicate that farmers bred out this incredibly useful trait deliberately because they couldn’t tell the difference between pest insects and those that were actually beneficial, and Heil and his co-authors propose this destructive practise was widespread.

“To the best of our knowledge, classical breeding has never aimed to improve anti-herbivore defense via VOCs or EFN,” the authors note, although an attempt has been made to genetically engineer wheat to produce a smell that repels aphids and attracts their predators.

Restoring something that has been lost over centuries is unlikely to happen quickly, even with modern Genetic Modification or targeted breeding techniques, so Heil and his colleagues are seeking speedier alternatives.

One option is to simply produce appropriate VOCs ourselves and release them around the crops we want to protect. Also known as infochemicals, VOCs are often specific to the particular herbivore the plant needs protection against, so careful choices must be made. However, such projects need to be undertaken with care. In some cases VOCs have been found to attract more plant-eaters, apparently alerted to the presence of vulnerable food, outweighing the predators they are meant to invite.

Nevertheless, the authors write, “It is our ambition to inspire plant breeders to consider [the use of protective predators] and inducible resistance to a greater extent.”