The drought is now bringing an onslaught of tree-killing beetles
As if California’s drought—likely the worst in 1,200 years—wasn’t hard enough on trees, the oxygen-pumping plants are facing another challenge: ravenous beetles. Millions of hungry insects the size of rice grains are eating forests across the state from the inside out.
And … they’re here.
“We’re seeing bark beetles up and down Highway 1 on Monterey pines, along the coast of San Mateo county and even some dieback in the Bonny Doon area,” says Cal Fire division chief and forester Rich Sampson. “The drought has stressed a lot of pines and opened them up for infestations.”
These bark beetles have left millions of dead trees in their wake, according to the U.S. Forest Service. “We’re entering our fourth year of drought and so far we have over two million trees that have died from bark beetles across 820,000 acres in 2014,” says Forest Service entomologist Cynthia Snyder. “It’s pretty big right now, and we expect it to only increase.”
Santa Cruz County survived a beetle infestation after the 2008 Martin Fire, which opened up lots of stressed and dead trees to mountain pine beetles, which had a feast on the ecological reserve on Mountain Road near Bonny Doon.
“That pretty much took out 90 percent of the ponderosa pine that were growing up there,” says Sampson. “We’re just now seeing the seedlings coming back.”
Beetles are like fire; they’re both naturally occurring drivers behind fast-paced die-offs. Wildfire ravages stands of dry and stressed trees in waves. Trees die, which frees up resources for new trees, which grow up—and when conditions again favor fire, the cycle begins anew. Similarly, beetles rip through dry and stressed trees in waves. The catch is that as drought conditions continue on, it’s harder for the forest to recover.
The beetles are hitting hardest in Southern California, where drought conditions are the most extreme. Their reach is sprawling and has already infiltrated forests in San Luis Obispo, Mount Diablo and the southern Sierra Nevada. Since 2002, the Forest Service reports that more than 400,000 acres of trees in the Modoc National Forest show signs of death by bark beetle.
“It’s really bad in some areas,” says Thomas Smith, Cal Fire’s central and southern Sierra forest pest management expert. “In the southern hemisphere, we’re talking about 12 million dead pine trees right now. But things are starting to pick up all over the state. Mount Diablo has it bad. We’re starting to see a lot more activity in Lake County, too.”
Bark beetles and trees usually coexist in a less one-sided way; weak trees fall victim while strong trees fend off beetle assailants. Healthy trees expel boring beetles from their woody flesh by pumping out resin, which traps the invading insects. But that sticky residue doesn’t come cheap. When trees grow stressed from water deprivation—as so many have from the record drought—they lack the resources they need to make resin and are rendered defenseless.
“Bark beetles are native insects,” says Snyder. “They’re always in the woods. They have a natural role of creating gaps in the forest by killing one or two trees at a time. But with the drought, the trees are weakened and become susceptible to attacks.”
Once the beetles target a sufficiently weak tree, be it pine, oak, cedar or fir, they emit pheromones that attract other beetles. A single tree may grow to be infested by thousands of beetles. Adults dig through the tree’s outer tissue to create pathways called galleries (each species has its own distinct gallery) that extend into the phloem: the inner tissue where plants transport nutrients. Once inside, the beetles lay their eggs, which hatch hungry larvae. Adult beetles also smuggle fungi into the trees, which blocks nutrient flow and hastens the whole fatal process. A few bark beetles aren’t so bad, but when they send their signal, explains Snyder, tree death soon follows. “It’s the mass attack that overwhelms the tree’s defenses,” she says.
The Forest Service reports that older and slow-growing trees are more susceptible to beetle attacks than younger trees. The combination of nutrient starvation, water deprivation and being carved from the inside can kill a tree that has lived for hundreds of years in a relatively short time.
The Forest Service assessed over 820,000 forest acres in 2014 and found that more than twice as many of the surveyed trees died from drought and bark beetles than in the previous year.
But die-offs like this one aren’t new. In 2003, Gov. Gray Davis declared a state of emergency in several Southern California counties when the same beetles threatened millions of forested acres. Scientists fear this event could be the worst yet, though, as drought conditions weaken more trees every year and a warming climate favors beetles, which are usually slowed by cold wet weather.
The most worthwhile way to combat the beetle onslaught, suggests the Forest Service, is large-scale thinning. Creating distance between trees frees up resources, which strengthens remaining stands and mitigates the spread of pheromone plumes. Thinning also prevents forest fires, a concern looming large in the minds of forest homeowners, as large amounts of dry needles could fuel flames.
“The real concern is that we’re going to have so much dry fuel that the fire conditions are going to be really bad,” Smith says. “Everything is incredibly dry to start with, and we’re adding all of this really dry fuel to the mix. It’s going to be a bad fire year.”
Smith cautioned that while thinning is the main focus, it must be timed correctly. “Thinning makes space between trees, the stands are less likely to burn in the first place, fires are more likely to die down when they get in there, and the stands are less likely to be attacked by bark beetles,” he says. “But during the drought and during the bark beetle epidemic, it’s not a good time to thin. All those freshly cut stumps tend to attract even more bark beetles.” Smith says many property owners are left to just wait it out. “We’re pretty much in a no-win situation right now.”
PHOTO: California bark beetles are killing native trees weakened by drought conditions.