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A New Weapon In the Bark Beetle Fight: Pheromones

Flickr user WBUR, CC BY-NC-ND 2.0
Mountain pine beetle.

Right now in California’s Sierra Nevada, an estimated 66 million trees have died, due to a deadly combination of drought and bark beetles, which take advantage of dry, thirsty trees. But could we prevent beetles from ever attacking trees in the first place? Researchers have been asking this question for decades, and a new tool fends off bark beetles using the very thing that makes them so deadly.

Take a look at a mountain pine beetle and you probably won’t see much. A tiny black and brown pellet, around the size of a mouse dropping. It looks simple. Kind of dumb, really. And yet, as forester Brian Block with Sequoia National Forest explains, these seemingly unassuming insects are taking out millions of towering, stately trees. He and I are looking at a tall, brown sugar pine near Lake Isabella. I ask him, “How does something so small kill something so big?” “Sheer numbers,” he says. “Thousands and thousands of little pinpricks.”

The tree can probably defend itself from one beetle on its own. But these beetles don’t travel alone. They ambush. And together that army burrows under a tree’s bark and cuts off the nutrient highway that runs from roots to canopy and back. So how do they do it? They’re not not super strong, they’re not super smart, and they certainly can’t talk or text each other—but, as Block explains, what they do have is a great sense of smell.

Credit Kerry Klein/KVPR
Forester Brian Block of Sequoia National Forest reveals bark beetle burrows, or “galleries,” beneath the bark of a dead pine tree.

“When the beetles come in and they start successfully hitting this tree, they emit pheromones that say ‘come get this tree,’” he says. “And so thousands of beetles will come and attack this tree. And that's usually how the trees end up losing.”

That’s right: mountain pine beetles navigate the world using pheromones. A bunch of beetle species do. Researchers at the forest service saw the insects’ carnage and asked: If pheromones can be used to help beetles, why can’t they be used against them? And that’s the principle behind a promising new tool called SPLAT Verb. “SPLAT Verb is a relatively new product that’s been designed for use for protecting trees from bark beetle attacks using their own pheromone,” says Chris Fettig, a research entomologist with the U.S. Forest Service and one of SPLAT Verb’s developers.

He describes it as a thick, gooey substance that squeezes out of a caulking tube. “It’s kind of a brownish grayish color,” he says. “In terms of consistency, it's a bit more viscous than, perhaps, toothpaste.”

The pheromone we talked about earlier is called an aggregation pheromone. It’s like an “open for business” sign for beetles around a tree. But beetles produce another important pheromone, too: one that says, “back off buddy, this tree’s already been eaten.”

“Once that tree begins to fill up, the beetles switch to production of an anti-aggregation pheromone, which essentially focuses as a ‘no vacancy’ sign,” Fettig says. “’This tree's full, please disperse and attack the next adjacent tree.’”

Credit U.S. Forest Service
A worker applies a "dollop" of SPLAT Verb to a tree.

That anti-aggregation pheromone is called verbenone, and it’s the “verb” in SPLAT Verb. “SPLAT” is an acronym for the gooey substance that surrounds the verbenone and releases it slowly over time. It stands for Specialized Pheromone and Lure Application Technology, and it’s actually been used to deploy many different pheromone-based chemicals.

This isn’t the first time verbenone has been used, either. The pheromone was first discovered in the 1960s, and research since then has explored which beetles are susceptible to it and how best to apply it. In the late 90s, researchers developed slow-release verbenone pouches. Then came a solid described to me as a “verbenone sandwich.” SPLAT Verb, the latest iteration, has been commercially available since 2014. You can find it online or in a handful of specialized farm supply stores throughout the state.

Studies show SPLAT Verb to be more effective than its predecessors—at least in some circumstances. “At the individual tree level, based on the results published by these researchers, I will say it seems promising,” says Nadir Erbilgin, a forest entomologist at the University of Alberta in Canada. He wasn’t involved in creating SPLAT Verb, but he did develop earlier verbenone treatments.

He talks about the individual tree level because that’s where the data show SPLAT Verb is most powerful. Developers found the substance was highly effective when applied to individual trees. But when they spaced it out in a grid pattern within a larger stand of trees, it didn’t work as well.

Erbilgin says, yeah, SPLAT Verb won’t save our forests, and it wouldn’t have, even if we’d used it before the latest outbreak. But he says we shouldn’t expect such power from a tool like this.

Credit Kerry Klein/KVPR
Firefighters chop down trees killed by drought and bark beetles around a campsite near Lake Isabella.

“Tree mortalities cannot be solved by entomologists or by this kind of management,” Erbilgin says. “I work in this subject, too;, I'm not being critical of my colleagues. These kind of activities might provide short-term solutions to deal with when the problem occurs, but we really need long-term solutions to improving forest health”—like forest thinning and maintaining species diversity.

“Forest health is really complex,” Erbilgin says. “It's not just the insects, not just the pathogens, not the diseases; it's a lot of things happening below ground that we're really scratching the surface right now.”

In other words, for a handful to a few acres of trees, SPLAT Verb could probably help protect them. But leave the forests to the foresters.

Kerry Klein is an award-winning reporter whose coverage of public health, air pollution, drinking water access and wildfires in the San Joaquin Valley has been featured on NPR, KQED, Science Friday and Kaiser Health News. Her work has earned numerous regional Edward R. Murrow and Golden Mike Awards and has been recognized by the Association of Health Care Journalists and Society of Environmental Journalists. Her podcast Escape From Mammoth Pool was named a podcast “listeners couldn’t get enough of in 2021” by the radio aggregator NPR One.
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