Plants calling for help

When pests attack your plants, believe it or not, the plants advertise it to predators who eat those pests. Of course, they don't scream out, "Hey, there's food here!" Instead, they release volatile chemicals—natural substances that easily evaporate and diffuse through the air—that cue beneficial organisms to come over and take out the pests.

Scientists are exploring how plants use volatile compounds to attract pest predators. Researchers refer to these chemicals as herbivore-induced plant volatiles (HIPVs) because they are made in response to a plant-eating pest.

Back in the 1980s, entomologists Marcel Dicke, Ph.D., of Wageningen University (then still a student) and Maurice W. Sabelis, Ph.D., of the University of Amsterdam, both in the Netherlands, were studying spider mites on bean plants when they had a crazy brainstorm. Did the infested bean plants have a mechanism for attracting predatory mites? As it turned out, they did.

Since then, scientists have found that nearly all plants produce HIPVs. For example, corn can release HIPVs under the soil to attract nematodes that parasitize corn rootworms, or above ground to attract parasitic wasps that kill stem-borer larvae. It can even sound the alarm when a stem-borer egg is laid on a corn leaf, before any damage occurs.

At the time, Dicke and Sabelis's discovery attracted little interest from plant breeders. Without realizing it, hybridizers sometimes bred the capability to produce HIPVs out of commercial crop varieties.

But times have changed. Individual plant varieties vary greatly in their ability to produce HIPVs, Dicke says. "There are some varieties that cry for help and others that whisper for help." Growers who use beneficial organisms in pest control want varieties that can "shout" with HIPVs. Breeders are now working with cucumbers to select for greater HIPV production.

Scientists also identified which volatiles corn produces when attacked by stem borers, and found that a legume in the genus Desmodium releases the same chemicals all the time. By intercropping corn with Desmodium and planting a grass attractive to stem borers around the perimeter of the field as a trap crop—a system known as "push-pull"—farmers in Kenya increased their yields more than threefold.

The technology that works in Kenya may not help American gardeners who deal with different pests and beneficial insects. More research is needed into how HIPVs might help organic growers. In the meantime, knowing about HIPVs helps us understand how our plants defend themselves against pests—without pesticides.

Originally published in Organic Gardening Magazine August/September 2013.