Plants that are under attack emit distress signals to warn neighboring plants they are vulnerable, says WSU entomologist Dr. David James.

Plants that are under attack emit distress signals to warn neighboring plants they are vulnerable, says WSU entomologist Dr. David James.

Exposing plants to certain chemicals can arouse them to release their own aromas to warn each other of danger and beckon insect “bodyguards” to defend them against insect and mite pests, according to research by entomologists.

Dr. David James, entomology professor at Washington State University’s Irrigated Agriculture Research and Extension Center in Prosser, said the research is the first of its type in the United States. Good Fruit Grower reported on the project’s earlier stages last year.

“We’ve had a lot more trials,” said James, who addressed the topic at a February conference of the Washington Association of Wine Grape Growers in Kennewick.

“It’s the real thing; it does actually work,” James said. “Growers are experimenting with this in New Zealand and Australia.”

HIPPOs

Scientists have termed the chemical aromas herbivore-induced plant protection odors, more colorfully and memorably known as HIPPOs.

James said studies in Europe, Japan, and the United States have demonstrated that plants under attack respond by emitting distress signals in the form of a combination of volatile chemicals; the plants use the chemical “language” to warn neighboring plants that they also are vulnerable and should defend themselves.

While researchers have been aware of the protection odors for about 15 years, the whole concept of plants putting out distress signals is a new technology, James said.

“What was done here that nobody else had done was taking the research into the field. We tried playing around with it, and we got results.”

James said research indicates that plants also use this chemical language to attract beneficial predators, including parasitic wasps and mites, that somehow understand the chemicals they detect signal the availability of food and hosts.

Thus, the chemical dialogue that plants produce when attacked benefits the plants as well as the bodyguards they recruit for themselves, he said.

Methyl salicylate is one of the principal protection odors researchers have used to trigger alarm signals from plants to predators including lacewings, hover flies, big-eyed bugs, and mite-eating lady beetles. Methyl salicylate is an inexpensive and readily available compound known as oil of wintergreen and found in heating ointments, toothpaste, and other common consumer products.

Another such odor is hexenyl acetate, which attracts beneficial pirate bugs and mirid bugs.

The research team installed HIPPO dispensers in five hopyard blocks and five vineyard blocks last spring, using five untreated blocks of each as controls.

In the blocks with dispensers, they almost always got larger populations of beneficial insects.

Among the team’s key discoveries was that rather than simply luring useful predators to vulnerable plants, the chemicals actually triggered the plants’ own defense mechanisms, enhancing the effectiveness of the plant protection odors.

“Originally, we thought we were directly attracting insects,” James said. “But we were also communicating to the plants that there are pests around.”

Through trial and error, researchers calibrated the optimum timing and quantity of the odor releases.

“We went from 200 to 50 dispensers per acre, turning plants on to produce their own volatiles,” James said, adding that the initial high-volume releases, which the researchers could actually smell, generated little response.

“Halfway through the season, when we couldn’t smell it anymore, we were getting a response,” he noted. “When we used a lower number of dispensers, we were getting a good response earlier in the season.”

Pandora’s box

James said the aroma research, while promising, is still in its early stages.

“It’s a Pandora’s box,” he said. “Methyl salicylate may not turn out to be the best thing to use in the field.

“There’s a lot more work to be done, not just for grapes and hops but for all crops. We’re trying to enhance plant signals to…improve the population of natural enemies in the crop.

“It has a lot of potential to improve biological controls on any crop.”

While chemical “languages” can differ among different species, James said, some plants share common words—and meanings. For example, different kinds of plants emit methyl salicylate when attacked by spider mites, while plants attacked by caterpillars often emit methyl jasmonate.

James sees potential for using the plant protection odors to strengthen biological control components of integrated pest management programs. Beneficial insects and predators are often unreliable IPM techniques because population numbers of beneficials are not high enough early in the season to provide adequate control of the targeted pest.

James buys the HIPPO dispensers for $2 per unit from a company in Costa Rica, one of very few firms that manufactures them.

“That’s effective management for $100 per hectare,” he said, adding that a Colorado company is planning to market them soon.

James acknowledges many growers are dubious about his glowing reports regarding the potential for HIPPO technology.

The boy who called wolf?

“There’s a lot of skepticism, because it sounds too good to be true,” he said. “When I talk about this at conferences, I get a lot of good questions about the ramifications.”

One such concern, for example, is “if we trigger the distress call and there isn’t really a threat, will [the plants] ignore a real threat the next time?”

Noting the labor-intensive work of maintaining the sachet dispensers, setting and monitoring insect traps and collecting data, James said he is seeking more funding from the wine grape and hop industries for further studies.

“We want to learn more about the mechanics of what’s really happening,” he said. “If we know how it works, we can make it work better.

“The field is really exploding at the moment.”