Scientists to Bark Beetle: “No Vacancy”

Douglas-fir bark beetles can kill healthy trees, but a commercially available pheromone identified by scientists repels the insects. (Photo: Edward H. Holsten, USDA Forest Service, Bugwood.org)

You think it’s difficult to master a complex foreign language like Chinese or Greek?

Try learning how to speak “bark beetle.”

After about 30 years of study, researchers at OSU have done exactly that. Along with U.S. Forest Service colleagues, they’ve figured out what a particular pheromone is communicating to Douglas–fir bark beetles and now use that language to help protect high–value trees on thousands of acres across much of the West.

The pheromone, known as MCH, has been proven effective. The U.S. Environmental Protection Agency has approved its use, and two private companies are marketing it for application in such places as campgrounds, resorts and residential areas. It represents another success story in the development of chemical cues and signals to help resist insect attacks and epidemics.

“This system works like a charm,” says Darrell Ross, associate professor of forest science. “We’re definitely communicating with the bark beetles, and the result is we now have a way to protect some of our most valued trees. That’s pretty exciting.”

Douglas–fir bark beetles are opportunistic pests that need recently killed trees to breed and reproduce. Although always present, they rarely harm healthy forests.

They often come in after forest disturbance events such as wildfire, wind throw or other pest epidemics, and take advantage of the dead trees or finish killing weakened ones. Once an infestation reaches high levels, though, the pest is forced to attack live trees more than usual in order to support the population. It can cause major damage.

But in the 1970s, OSU and Forest Service scientists identified MCH. They then spent almost 30 years learning what it does and how it can be used to prevent beetle infestations. Essentially, this pheromone tells a Douglas–fir bark beetle, “This tree is already taken.”

“When bark beetles are looking for a place to lay their eggs, they don’t want to go to a tree that’s already heavily infested with their own species, because food might be limited,” Ross says. “So the insect has a communication pheromone that alerts other beetles to its presence.

This anti–aggregation pheromone is like a no–vacancy sign. It tells individual beetles to go somewhere else, this spot is already spoken for.”

Another pheromone has also been identified that attracts the beetles into traps, but it is less effective and not as widely used.

Many Pacific Northwest forests can survive years of defoliating attack by other insect pests, such as the spruce budworm or Douglas–fir tussock moth. But bark beetles, whose scientific name, Dendroctonus, actually means “tree killer” in Latin, often finish the job and can cause high levels of mortality in an infected forest.

The anti–aggregation pheromone used in this protection program is particularly effective, scientists say. It accomplishes the forest management goal nearly 100 percent of the time, and studies have shown that it’s environmentally safe, as well as inexpensive.

The technique also lends itself well to the concept of integrated pest management, where various approaches — such as silviculture, thinning, harvest of wind–thrown timber and use of pheromones — are all used together to improve forest health more than any one approach could by itself. Continued studies will work toward techniques that allow fewer pheromone dispensers per acre and further reduce cost of use.

“These are very powerful chemicals that the beetles use for specific communication purposes,” Ross says. “We continue to learn more about pheromones and now understand that insects and other animals use a whole complex of odors to communicate and make behavioral decisions. The hard part is learning how to speak that language.”

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