Armies of tiny bark beetles are ravaging drought-weakened pine trees throughout California in a fast spreading epidemic that biologists fear could soon turn catastrophic.

Local, state and federal officials are virtually helpless against the pestilence, which has turned hundreds of thousands of acres of forest brown and left huge fire-prone stands of dead wood.

The trees are being devoured by millions of native beetles, each about the size of a grain of rice. The insects, thriving in the warm weather and lack of freezing temperatures, are overwhelming the defenses of water-starved trees, attacking in waves and multiplying at a frenzied pace, depositing eggs under the bark that hatch into ravenous larval grubs.

“Things are looking really, really bad,” said Tom Smith, a forest pest management specialist for the California Department of Forestry and Fire Protection. “Basically we’ve got native bark beetles that are attacking the pines. They are only successful in attacking the trees when the trees are stressed. Right now all the trees are stressed because of drought.”

The infected trees are on private and public lands, in national parks, wilderness areas and managed forests. There seems to be no solution short of removing the dead and dying trees and hoping against hope for rain and cold. The worst of it is in the southern part of the state, but pest management experts say the plague is moving north.

Mount Diablo is one of the hardest hit areas in Northern California, where a forest of Coulter pine, also known as big-cone pine, has been devastated. Bill Miller, an environmental scientist for the California Department of Parks and Recreation, said close to 50 percent of a nearly 1-square-mile stand of Coulters on the north side of the mountain is dead or infested with beetles.

Conditions favor beetles

“It’s a combination of these trees being stressed and a little weaker from drought, and the number of beetles that are there right now,” said Miller, who first noticed a line of dead trees on Meridian Ridge last spring and has watched helplessly as the marauding beetles have spread. “Conditions right now favor the development of beetles. We have a lot more susceptible trees and the beetles are making use of an opportunity.”

Large numbers of dead pines have been reported in the forested hills above Clear Lake and in some areas of the Sierra around Lake Tahoe, but it is south of the Bay Area where the trees are really going into death throes.

State forestry officials estimate that from 20 to 40 percent of the trees are dead or dying between Calaveras County and the Kings County area near Fresno, with entire hillside forests completely brown.

In Cambria, the picturesque coastal community between Big Sur and San Luis Obispo County, 80 to 85 percent of the region’s native Monterey pine forest is dead or dying. Tree mortality is also bad in the Tehachapi Mountains, in the far southern part of the state.

“In the southern Sierra Nevada it’s devastating,” Smith said. “It’s epidemic and its probably going to get even worse.”

Tree deaths are not unusual during sustained hot, dry periods. It happened during previous droughts over the past four decades. In 2003, then-Gov. Gray Davis proclaimed a state of emergency for Los Angeles, Riverside, San Bernardino and San Diego counties because of a bark beetle infestation that at one point threatened more than 6 million acres of California forest.

But experts say this die-off has the potential to be even worse. The beetle population is normally kept in check by the winter cold, but three years of above-average temperatures and lack of snowfall have given the growing bug hordes free rein to search and destroy.

“It’s scary in areas — the amount of dead and dying trees I am seeing,” said Smith, who has been studying bark beetles in California forests for 15 years. “This is the worst I’ve seen.”

Tree mortality doubles

Sheri Smith, the regional entomologist for the U.S. Forest Service in California, said bark beetle and drought-caused tree mortality more than doubled across forests in California last year and is expected to increase even more this year. The Forest Service mapped tree mortality across 820,000 acres of forested land in 2014 compared with 350,000 acres in 2013, according to the service’s annual Aerial Detection Survey Program results. The results of this year’s surveys, which are just getting under way, are not expected until later in the year.

Smith and others said the drought is impacting many tree species, including oaks and incense cedars, but pine and fir trees are the most obvious victims. Some 460,000 acres with dead or dying fir trees were observed last year by the Forest Service, she said.

“Many communities are in that lower elevation pine belt, so the trees around them are what they notice dying,” she said. “That’s where people live, where they drive and what they see.”

The bark beetle infestation is not limited to California. The ravenous insects have decimated 45 million acres of forest in the Western United States in recent years, including 15 million acres of Forest Service land. Scientific studies have shown that trees are dying faster than ever in the old-growth forests of California and the mountains of the West. Scientists have linked the phenomenon to rising temperatures, earlier than normal snowmelt, drought, forest fires and insect infestations.

There are 19 different types of pines in California, including sugar, ponderosa and lodgepole pine. Red and white fir are the most common species of fir in California, Smith said. Forest pathologists say bark beetle infestations are the primary difference between pine and fir trees and other species that are also suffering ill effects from the drought.

Of the 220 species of bark beetle, about a dozen feed on California pines, including the Western pine beetle, the pine engraver or Pinyon ips and the red turpentine beetle. Smith said the beetles attack stressed trees, which have trouble producing enough pitch to drive away the insects. The beetles then release pheromones that attract other beetles, which quickly overwhelm the tree’s defenses.

The different beetle species specialize in certain types of pines and generally stay in specific regions. The trees on Mount Diablo are being attacked by the California fivespined ips. Western pine beetles attack ponderosas, but have also been known to dive into Coulter pines. The mountain pine beetle is wreaking havoc on the lodgepole and ponderosa forests in the Rocky Mountains and in British Columbia.

The insects bore through the pine’s bark where they lay eggs. The larvae then feed on the tree’s living tissue, which cuts off the tree’s ability to transport nutrients. An infestation can involve several thousand beetles, which can then spread to neighboring trees.

Acres of tinder created

The result is that there are now large pockets of California forest, in some cases stretching hundreds of acres, full of mostly dead tinder.

Experts say frequent fires once controlled the beetle populations in California until flame suppression became a forestry management hallmark. Although fire officials believe the dead wood from beetle outbreaks increases the likelihood of out-of-control fires, a recent University of Colorado study found that bug-infested forests are no more likely to burn than other forests.

But forestry officials and biologists say the issues are bigger than just fire. Dying bug-infested forests disrupt the food web, wildlife and local economies. Falling trees have crushed cars and closed campgrounds. The soil in denuded forests can erode away, taking with it forage material for birds, mammals and insects.

“The amount of dead and dying trees is certainly a very significant fire hazard,” said Ken Pimlott, the director of Cal Fire. “It’s a multifaceted challenge. Large stands of dead trees create another problem also because, when you get into the Sierras, many of the homes have been built in and around these trees. We don't want to create alarm, but this is a forest health issue that has become a public safety issue.”

Pimlott said the situation in California screams for more diligent forest management, including thinning of overcrowded stands of timber, removal of dead trees and clearing of dry brush. Problem is, all of that costs money, which many property owners can’t spare and legislators are generally reluctant to part with.

Climate scientists aren’t sitting idly by while others do all the hand wringing. As the forests shrink, they say, less carbon dioxide is absorbed. That means more greenhouse gases will enter the atmosphere resulting in the acceleration of global warming, researchers said.

Changes likely permanent

Experts say some of the changes to the ecosystem are likely to be permanent unless the weather pattern in the state gets moist soon.

“Even if we see a wet pattern this year, the damage is already done to these forests and it is going to take a few years to recover,” Pimlott said. “We are watching to see what scale it grows to. There is really nothing we can do when it gets to this point to change the outcome.”

Meanwhile, the drought persists and, as if things weren’t already bad enough, Mother Nature has produced even less rain and snow in California this year than last. It is a situation ripe for hungry beetles, which, in turn, create more fuel in a wildfire-prone state.

Peter Fimrite is a San Francisco Chronicle staff writer. E-mail: pfimrite@sfchronicle.com. Twitter: @pfimrite.

California bark beetles

Individuals are not much larger than a piece of cooked rice.

They survive in trees that are stressed or diseased and cannot secrete enough defensive resin, or sap, to drown the beetles.

The beetles emit pheromones that attract other beetles.

They lay eggs in the moist inner bark of the tree and the larvae feed on the living tissue, cutting off the tree’s ability to transport nutrients.

Find more information about bark beetle management at http://www.fs.usda.gov/main/barkbeetle/barkbeetlemanagement

Common California species

Mountain pine beetle: Native to the forests of western North America, it attacks ponderosa, lodgepole, sugar and other types of pine trees, but does not infect Jeffrey pine.

Western pine beetle, right: Native to Western North American forests, it primarily attacks ponderosa and Coulter pines in California.

Douglas fir beetle: Native to the forests of western North America, it only attacks Douglas fir trees, often in areas where there has been wind damage.

Pinyon ips: This pine engraver beetle has a spiny back end typical of all the “ips” species. It attacks pines and green slash pine and often kills from the top down.

Fir engraver beetle: This native California beetle attacks fir trees, but not Douglas fir. Its larvae spend the winter under the bark and emerge from June through August.

Jeffrey pine beetle: Native throughout the range of Jeffrey pines, it has a distinctive reddish hue. These voracious beetles will kill a tree and then move on to neighboring green trees.

Red turpentine beetle: The largest of the pine bark beetles in California, it primarily infests sick or damaged trees and freshly cut stumps. Attacks by this reddish-brown beetle don’t always kill trees, but may predispose them to attack by other beetles.