Could 'rewilding' allow crops to fight pests without chemicals?

Wild plants can protect themselves from pests, but scientists think this immune system has been bred out of domesticated crop plants. Now, they're trying to figure out how to bring it back. 

A team of researchers from Sweden and Mexico is arguing that farmers can give up harmful chemical sprays used to repel pests from their crops by giving nature back the tools it needs to chase off pests on its own.

These defensive tools – like odors and nectars – are already widely used in wild plants to deter pests or attract their predators, but have been bred out of domesticated food crops because they weren’t conducive to taste, appearance, growth rate, or efficient long-distance transportation, say researchers from the Swedish University of Agriculture Sciences and Mexican biotechnology institute Cinvestav-Irapuato.

“You don’t want to eat a spiny fruit or a very bitter fruit, so we’ve had to eliminate some of the defensive traits,” like bitterness, hairiness, toughness or toxicity, Martin Heil, a plant ecologist at Cinvestav, tells The Christian Science Monitor. This has left crop plants defenseless against pests like beetles and caterpillars, requiring modern-day farmers to use chemical pesticides to do the job that plants used to do on their own.

The chemicals used are toxic, though, and in many cases have been linked with numerous health issues in humans and animals, so the idea, says Dr. Heil, is to reduce their use.

After reviewing over a hundred studies on plant biology, the team argues in a paper published Sunday in the journal Trends in Plant Science that there’s a way to bring back the natural defenses of crops through “rewilding” of crop plants, planting techniques, and innovative agricultural technology.

"New regulations and changing consumer demands are gradually improving the prospects for more sustainable agriculture," Heil said in a research announcement. "This provides a ready market if we can give crops back their own immune system,” he added.

The wild rose is a good example of how the immune system of plants works, as many home gardeners already know. The rose’s pest is a tiny insect called an aphid. It loves to suck the sweet nectar on the underside of rose bush leaves and flower buds, gaining so much strength from the wholesome meal that it can quickly reproduce to form a rose-killing army.

But when the perennial flower is under attack by aphids, its natural defense system prompts the release of odors that signal to ladybugs, the mortal enemy of the aphid, that there’s a scrumptious feast of aphids nearby.

“The plant teams up with the enemies of its own enemies,” explained Heil. “That’s the idea of biocontrol,” or reducing pest populations by using their natural enemies.

The solution is more complicated than breeding the odors and nectars that repel pests or attract pest-eating predators back into food crops though. “Such defenses involve multiple genes,” the research teams writes in its announcement, “and it won't be easy to simply bring them back.”

Another option, the research team says, is called “intercropping,” or planting odor-emitting plants next to defenseless ones to deter pests or attract the right predators.

“My grandma always had some onions between the roses,” recalled Heil, since onions deter pests by masking the scent aphids love.

But Heil concedes that this option – which is used on some small farms where everything is done by hand – is too work-intensive and unreliable for industrial scale farms.

“If you try to recommend intercropping to a US farmer, it won’t work,” said Heil.

For this reason, his team in Mexico is developing mechanical dispensers that can produce the odors pests love or hate to be used on industrial farms.