What eats bark beetles?

SIGNS OF BARK BEETLE INFESTATION

Check your trees for signs of bark beetle infestation. Bark beetle activity starts on the bark surface and extends to the inner bark of twigs, branches or trunks. The following are signs that a tree may be infested by bark beetles:

In the Bark

  • Trees react by releasing pitch as their natural defense against bark beetle attack. This response from the tree will leave small white or reddish-brown pitch tubes on the outside of the bark. (Pitch tubes may resemble ½-¾ inch blobs of sap-like substance.)
  • A white pitch tube means the beetle was successfully repelled by the tree. If the pitch tube is reddish brown, most likely the beetle was successful in attacking the tree.
  • The pitch is accompanied by a sawdust-like substance, called frass, created by bark beetles and their larvae as they borethrough the bark.
  • Frass has accumulated in tree crevices and may have fallen to the ground, resembling very fine, reddish-brown coffee ground material at the base of the tree.
  • Bark flaking or holes in the bark caused by woodpeckers foraging for bark beetles are also a good indicator that bark beetles are present.
  • Removing bark sections will reveal holes created by bark beetles, as well as dead or degraded inner bark.

Leaves or Needles

  • The needles on conifer trees, like pines, begin to turn a reddish-brown color. Often the change begins at the top of the trees and moves down.
  • Some trees may slowly fade in color from green to brown.
  • Some trees may die within a few weeks of infestation, but may not show yellow-green, fading or red foliage for several months. Other types of trees may survive years before dying; by the time a tree appears dead, it cannot be saved.

Note: Unless trees are regularly monitored to detect bark beetle activity, any chemical spray application made after beetles have penetrated the bark is likely to be too late and ineffective. If detected early, chemical treatment must target the adults by spraying the bark so that beetles are killed when they land on trees and attempt to bore into the bark to lay eggs.

Caution: Chemically treating infested trees provides no benefit and could kill other beneficial insects. Please follow the instructions as directed on the label.

Download the Identifying Dead and Dying Conifers on Private Land in California pdf.

Illustration by Bill Mayer

There is an eerie feel to this grove of lodgepole pines that I can’t quite put my finger on as entomologist Diana Six tromps ahead of me, hatchet in hand, scanning the southwestern Montana woods for her target. But as she digs the blade into a towering trunk, it finally hits me: the smell. There’s no scent of pine needles, no sharp, minty note wafting through the brisk fall air.

Six hacks away hunks of bark until she reveals an inner layer riddled with wormy passageways. “Hey, looky!” she exclaims, poking at a small black form. “Are you dead? Yeah, you’re dead.” She extends her hand, holding a tiny oval, maybe a quarter of an inch long. Scientists often compare this insect to a grain of rice, but Six prefers mouse dropping: “Beetle in one hand, mouse turd in another. You can’t tell them apart.” She turns to the next few trees in search of more traces. Pill-size holes pock their ashen trunks—a sign, along with the missing pine scent, of a forest reeling from an invasion.

Beetles are chewing their way through American forests, sometimes felling as many as 100,000 trees a day.

These tiny winged beetles have long been culling sickly trees in North American forests. But in recent years, they’ve been working overtime. Prolonged droughts and shorter winters have spurred bark beetles to kill billions of trees in what’s likely the largest forest insect outbreak ever recorded, about 10 times the size of past eruptions. “A doubling would have been remarkable,” Six says. “Ten times screams that something is really going wrong.”

Mountain pine, spruce, piñon ips, and other kinds of bark beetles have chomped 46 million of the country’s 850 million acres of forested land, from the Yukon down the spine of the Rocky Mountains all the way to Mexico. Yellowstone’s grizzly bears have run out of pinecones to eat because of the beetles. Skiers and backpackers have watched their brushy green playgrounds fade as trees fall down, sometimes at a rate of 100,000 trunks a day. Real estate agents have seen home prices plummet from “viewshed contamination” in areas ransacked by the bugs. And the devastation isn’t likely to let up anytime soon. As climate change warms the North American woods, we can expect these bugs to continue to proliferate and thrive in higher elevations—meaning more beetles in the coming century, preying on bigger chunks of the country.

BEETLEMANIA

From 2000 to 2014, bark beetles destroyed large swaths of forests in the American West—and they’re not done yet.

In hopes of staving off complete catastrophe, the United States Forest Service, which oversees 80 percent of the country’s woodlands, has launched a beetle offensive, chopping down trees to prevent future infestations. The USFS believes this strategy reduces trees’ competition for resources, allowing the few that remain to better resist invading bugs. This theory just so happens to also benefit loggers, who are more than willing to help thin the forests. Politicians, too, have jumped on board, often on behalf of the timber industry: More than 50 bills introduced since 2001 in Congress proposed increasing timber harvests in part to help deal with beetle outbreaks.

But Six believes that the blitz on the bugs could backfire in a big way. For starters, she says, cutting trees “quite often removes more trees than the beetles would”—effectively outbeetling the beetles. But more importantly, intriguing evidence suggests that the bugs might be on the forest’s side. Six and other scientists are beginning to wonder: What if the insects that have wrought this devastation actually know more than we do about adapting to a changing climate?

A BUG’S LIFE

An adult mountain pine beetle lays her eggs under the bark. On her way, she disperses fungi that turn the trees’ tissue into food for her babies, eventually killing the tree.

Though they’re often described as pesky invaders, bark beetles have been a key part of conifer ecosystems for ages, ensuring that groves don’t get overcrowded. When a female mountain pine beetle locates a frail tree, she emits a chemical signal to her friends, who swarm to her by the hundreds. Together they chew through the bark until they reach the phloem, a cushy resinous layer between the outer bark and the sapwood that carries sugars through the tree. There, they lay their eggs in tunnels, and eventually a new generation of beetles hatches, grows up, and flies away. But before they do, the mature beetles also spread a special fungus in the center of the trunk. And that’s where things get really interesting.

Six focuses on the “evolutionary marriage” of beetle and fungi at her four-person lab at the University of Montana, where she is the chair of the department of ecosystems and conservation sciences. Structures in bark beetles’ mouths have evolved to carry certain types of fungi that convert the tree’s tissue into nutrients for the bug. The fungi have “figured out how to hail the beetle that will get them to the center of the tree,” Six says. “It’s like getting a taxi.” The fungi leave blue-gray streaks in the trees they kill; “blue-stain pine” has become a specialty product, used to make everything from cabins to coffins to iPod cases.

A healthy tree can usually beat back invading beetles by deploying chemical defenses and flooding them out with sticky resin. But just as dehydration makes humans weaker, heat and drought impede a tree’s ability to fight back—less water means less resin. In some areas of the Rocky Mountain West, the mid-2000s was the driest, hottest stretch in 800 years. From 2000 to 2012, bark beetles killed enough trees to cover the entire state of Colorado. “Insects reflect their environment,” explains renowned entomologist Ken Raffa—they serve as a barometer of vast changes taking place in an ecosystem.

Under the microscope, Diana Six picks up a dead mountain pine beetle in her Missoula lab. Shawn Gust

Typically, beetle swells subside when they either run out of trees or when long, cold winters freeze them off (though some larvae typically survive, since they produce antifreeze that can keep them safe down to 30 below). But in warm weather the bugs thrive. In 2008, a team of biologists at the University of Colorado observed pine beetles flying and attacking trees in June, a month earlier than previously recorded. With warmer springs, the beetle flight season had doubled, meaning they could mature and lay eggs—and then their babies could mature and lay eggs—all within one summer.

That’s not the only big change. Even as the mountain pine beetles run out of lodgepole pines to devour in the United States, in 2011 the insects made their first jump into a new species of tree, the jack pine, in Alberta. “Those trees don’t have evolved defenses,” Six says, “and they’re not fighting back.” The ability to invade a new species means the insects could begin a trek east across Canada’s boreal forest, then head south into the jack, red, and white pines of Minnesota and the Great Lakes region, and on to the woods of the East Coast. Similarly, last year, the reddish-black spruce beetle infested five times as many acres in Colorado as it did in 2009. And in the last decade, scientists spotted the southern pine beetle north of the Mason-Dixon Line for the first time on record, in New Jersey and later on Long Island. As investigative journalist Andrew Nikiforuk put it in his 2011 book on the outbreaks, we now belong to the “empire of the beetle.”

In a weird way, all of this is exciting news for Six: She is not only one of the world’s foremost experts in beetle-fungi symbiosis, but proud to be “one of the few people in Montana that thinks bark beetles are cute.” (She’s even brewed her own beer from beetle fungi.) As a child, she filled her bedroom in Upland, California, with jars of insects and her fungus collection. But as a teenager, she got into drugs, quit high school, and started living on the streets. Nine years later, she attended night school, where teachers urged her to become the first in her family to go to college. And when she finally did, she couldn’t get enough: classes in microbiology and integrated pest management led to a master’s degree in veterinary entomology, then a Ph.D. in entomology and mycology and a postdoc in chemical ecology, focused on insect pheromones.

Entomologist Diana Six, who has devoted her career to bark beetles, believes that the bugs might hold clues to saving our forests in the face of climate change. Shawn Gust

Six, 58, has light-green eyes ringed with saffron, and long silvery-blond hair streaming down shoulders toned from fly-fishing and bodybuilding. As several fellow researchers stress to me, she is the rare scientist who’s also a powerful communicator. “I think about what it means to be a tree,” she told a rapt audience at a TEDx talk about global forest die-offs. “Trees can’t walk. Trees can’t run. Trees can’t hide,” she continued, her sonorous voice pausing carefully for emphasis. “And that means, when an enemy like the mountain pine beetle shows up, they have no choice but to stand their ground.”

To a tree hugger, that might seem a grim prognosis: Since trees can’t escape, they’ll all eventually be devoured by insects, until we have no forests left. Especially since, with our current climate projections, we might be headed toward a world in which beetle blooms do not subside easily and instead continue to spread through new terrain.

But Six has a different way of looking at the trees’ plight: as a battle for survival, with the army of beetles as a helper. She found compelling evidence of this after stumbling across the work of Forest Service researcher Constance Millar, with whom she had crossed paths at beetle conferences.

Millar was comparing tree core measurements of limber pines, a slight species found in the eastern Sierras of California that can live to be 1,000 years old. After mountain pine beetles ravaged one of her study sites in the late 1980s, certain trees survived. They were all around the same size and age as the surrounding trees that the beetles tore through, so Millar looked closer at tree ring records and began to suspect that, though they looked identical on the outside, the stand in fact had contained two genetically distinct groups of trees. One group had fared well during the 1800s, when the globe was still in the Little Ice Age and average temperatures were cooler. But this group weakened during the warmer 1900s, and grew more slowly as a result. Meanwhile, the second group seemed better suited for the warmer climate, and started to grow faster.

Pine beetles have increasingly attacked fragile whitebark pine trees, whose cones are an important food source for grizzly bears, Clark’s nutcrackers, red squirrels, and other animals in the Yellowstone area. Maddie Oatman

When beetle populations exploded in the 1980s, this second group mounted a much more successful battle against the bugs. After surviving the epidemic, this group of trees “ratcheted forward rapidly,” Millar explains. When an outbreak flared up in the mid-2000s, the bugs failed to infiltrate any of the survivor trees in the stand. The beetles had helped pare down the trees that had adapted to the Little Ice Age, leaving behind the ones better suited to hotter weather. Millar found similar patterns in whitebark pines and thinks it’s possible that this type of beetle-assisted natural selection is going on in different types of trees all over the country.

When Six read Millar’s studies, she was floored. Was it possible, she wondered, that we’ve been going about beetle management all wrong? “It just hit me,” she says. “There is something amazing happening here.”

Last year, Six and Eric Biber, a University of California-Berkeley law professor, published a provocative review paper in the journal Forests that challenged the Forest Service’s beetle-busting strategies. After scrutinizing every study about beetle control that they could get their hands on, they concluded that “even after millions of dollars and massive efforts, suppression…has never effectively been achieved, and, at best, the rate of mortality of trees was reduced only marginally.”

Six points to a stand of lodgepoles in the University of Montana’s Lubrecht Experimental Forest. In the early 2000s, school foresters preened the trees, spacing them out at even distances, and hung signs to note how this would prevent beetle outbreaks. This “prethinned” block was “the pride and joy of the experimental forest,” Six remembers. But that stand was the first to get hit by encroaching pine beetles, which took out every last tree. She approached the university forest managers. “I said, ‘Boy, you need to document that,’” Six says. “They didn’t. They just cut it down. Now there’s just a field of stumps.”

For the timber industry and its friends, beetle invasions have been a handy excuse to open wild areas for logging.

Six and Biber’s paper came as a direct affront to some Forest Service researchers, one of whom told me that he believes changing forest structure through thinning is the only long-term solution to the beetle problem. Politicians tend to agree—and beetle suppression sometimes serves as a convenient excuse: “It is perhaps no accident that the beetle treatments most aggressively pushed for in the political landscape allow for logging activities that provide revenue and jobs for the commercial timber industry,” Six and Biber wrote in the Forests review.

Take the Restoring Healthy Forests for Healthy Communities Act, proposed in 2013 by then-Rep. Doc Hastings (R-Wash.) and championed by then-Rep. Steve Daines (R-Mont.). The bill sought to designate “Revenue Areas” in every national forest where, to help address insect infestations, loggers would be required to clear a certain number of trees every year. Loggers could gain access to roadless areas, wilderness study areas, and other conservation sites, and once designated, their acreage could never be reduced. The zones would also be excluded from the standard environmental-review process.

Six and other scientists vehemently opposed these massive timber harvests—as did environmental advocates like the Sierra Club and Defenders of Wildlife, the latter warning that the harvests would take logging to “unprecedented and unsustainable levels.” The bill passed the House but died in the Senate last year. But Daines, now a senator and one of 2014’s top 10 recipients of timber money, vows to renew the effort so as to “revitalize Montana’s timber industry” and “protect the environment for future generations.”

This summer, Six plans to start examining the genes of “supertrees”—those that survive beetle onslaughts—in stands of whitebarks in Montana’s Big Hole Valley. Her findings could help inform a new kind of forest management guided by a deeper understanding of tree genes—one that beetles have had for millennia.

If we pay close enough attention, someday we may be able to learn how to think like they do. University of California-Davis plant sciences professor David Neale champions a new discipline called “landscape genomics.” At his lab in Davis, Neale operates a machine that grinds up a tree’s needles and spits out its DNA code. This technology is already being used for fruit tree breeding and planting, but Neale says it could one day be used in wild forests. “As a person, you can take your DNA and have it analyzed, and they can tell you your relative risk to some disease,” Neale says. “I’m proposing to do the same thing with a tree: I can estimate the relative risk to a change in temperature, change in moisture, introduction to a pathogen.”

Signs of beetle invasion on a whitebark pine tree in Montana’s Big Hole Valley Maddie Oatman

Right now, foresters prune woodlands based on the size of trees’ trunks and density of their stands. If we knew more about trees’ genetic differences, Neale says, “maybe we would thin the ones that have the highest relative risks.” This application is still years off, but Neale has already assembled a group of Forest Service officials who want to learn more about landscape genomics.

Six, meanwhile, places her faith in the beetles. Whereas traditional foresters worry that failing to step in now could destroy America’s forests, Six points to nature’s resilience. Asked at TEDx how she wants to change the world, she responded, “I don’t want to change the world. We have changed the world to a point that it is barely recognizable. I think it’s time to stop thinking change and try to hold on to what beauty and function remains.”

Diana Six in her lab at the University of Montana Shawn Gust

This story was supported by a Middlebury College Fellowship in Environmental Journalism.

Bark beetles are one of the most destructive insects in western coniferous forests. It has been estimated that 90 percent of insect-caused tree mortality and more than 60 percent of the total insect-caused loss of wood growth in the United States is due to bark beetles. In the Southwest, bark beetle killed trees were scattered over more than 2 million acres between 2001 and 2003.

The Southwest has a large complex of bark beetles composed of many genera and species. Frequently, several species are found attacking the same host tree and, therefore, it may be difficult to discern what species initiated the attack. Although species of Dendroctonus are the most notorious tree killers in the western United States, Ips species also play a very important role in pine forests of the Southwest.

Figure 84. Life stages of bark beetles (adult, pupa, larva).

Bark beetles derive their name from their habit of living and mining between the bark and wood of trees and shrubs. Adults excavate egg galleries in bark phloem. All bark beetle life stages are spent in the phloem, inner bark and bark, except when adults leave the tree in which they developed to fly to new host material. Bark beetles feed on the phloem during adult and larval stages.

Most bark beetles are considered secondary mortality agents because they prefer weakened host material. However, during environmental conditions favorable for beetle development, populations may build up rapidly and successfully attack healthy trees. Most bark beetles have a symbiotic relationship with blue-stain fungi. The blue stain fungi can completely penetrate the sapwood within a year. The fungi occlude the outer conducting tissues in the xylem that halts upward water translocation. This action, plus that of the bark beetle feeding, causes the death of a host tree.

Figure 85. Adult Arizona five-spined ips (top) and western pine beetle (bottom). Note differences in the elytral declivity between the species.

Bark beetles produce chemical compounds called pheromones that are used to communicate with other beetles. Aggregation pheromones cause beetles to congregate in certain areas and mass-attack trees. Anti-aggregation pheromones cause beetles to disperse to neighboring trees or other areas. Pheromones of many bark beetles have been identified and synthetically produced. Both aggregation and anti-aggregation pheromones have been effective to mitigate impacts caused by some bark beetles in the western United States.

Figure 86. Exit holes of bark beetles emerging from ponderosa pine.

Crowns of successfully attacked trees turn from green to yellow to reddish brown. This color change, an indication of a dying tree, may occur from a month to more than 2 years after successful attack depending on the temperature, moisture conditions, and density of beetles in the tree. Close inspection of infested tree trunks will show either small globules of resin, small holes through the bark, or reddish boring dust in bark crevices and around the tree base. The removal of bark from infested trees will reveal two types of galleries, egg and larval. Egg galleries constructed by adult beetles are rather uniform in width. Larval galleries depart at right angles from egg galleries and increase in size as the young grow.

References: 24, 25, 41, 116

Figure 87. Small rust-colored pitch tubes are a sign that trees have been attacked by bark beetles. Figure 88. Boring dust in branch crotches, bark crevices or around the base of trees is a sign that trees have been successfully attacked by bark beetles. Figure 89. Blue stain fungi introduced during bark beetle attacks on conifers occurs in the sapwood.

How to Manage Pests

Chemical Control

Unless trees are monitored regularly so that bark beetle attack can be detected early, any chemical spray application made once the beetles have aggregated and penetrated the bark is likely to be too late and ineffective. Treatment must target the adults by spraying the bark so that beetles are killed when they land on trees and attempt to bore into the bark to lay eggs. Chemically treating trees that have been previously attacked will provide no benefit and could kill beneficial insects. Seriously infested trees, or trees that are dead or dying due to previous beetle attacks, cannot be saved with insecticide treatments and should be removed. Systemic insecticides, meaning those that are implanted or injected through the bark or applied to soil beneath trees, have not been shown to prevent attack or control populations of bark beetles. Although new systemic products are being investigated, they are not currently recommended for bark beetle control.

Circumstances for Effective Use of Insecticides

Highly valued, uninfested host trees may be protected by spraying their bark with a persistent, registered insecticide labeled as a preventive spray for bark beetles. Look for signs of recent infestation to help decide whether preventive spraying of nearby, lightly attacked or unattacked trees may be justified. Spraying a persistent insecticide on valuable, uninfested host trees near infested trees may be warranted to protect uninfested host trees from bark beetles. However, do not substitute preventive sprays for proper cultural care. The infestation status of a tree can be determined by inspecting the trunk or limbs for fresh pitch tubes or frass; peeling a small portion of the outer bark from the trunk or limbs and looking for signs of adult beetles or larvae; and inspecting the foliage for yellow or yellow-green needles or leaves. Frequently the infestation is diagnosed after the beetles have vacated the tree. For example, when reddish brown foliage is observed the tree is dead and the new generation of bark beetles has already emerged from the tree. Fading foliage throughout the tree crown indicates a dead tree and no insecticide treatment will be effective. Because each bark beetle species attacks only certain tree species, spray only healthy trees that are susceptible to the beetle species attacking nearby trees (for example, pine bark beetles do not attack oaks and oak bark beetles do not attack pines) (Table 1). Insecticide sprays are not recommended against shothole borer and cedar or cypress bark beetles.

How to Apply Insecticides

Insecticide products available to home users are not effective for bark beetle control. Most home gardeners also lack the high-pressure spray equipment and experience to effectively treat large trees. Protective spraying for bark beetles must be done by a licensed pesticide applicator. When hiring a professional applicator, discuss the specific pesticide to be used and effective timing of the application. Also see Pest Notes: Hiring a Pest Control Company. The applicator must use a product with bark beetles listed on the label, and mix and apply the formulation following label directions. Proper application involves thoroughly drenching the main trunk, exposed root collar near the base of the tree, and larger branches (for engraver beetles) with a pyrethroid, such as Astro or Dragnet, or any of the flowable (EC) formulations of the chemical carbaryl to prevent new bark beetle infestations. (Note: These products are not available to home users.) The material must be applied before the new adults penetrate the bark surface of the tree. Regardless of the insecticide used, the applicator should mix only what is needed and dispose of any excess insecticide by properly following label directions.

When to Apply Insecticides

Preventive treatments must be applied to the tree trunk or branches to kill adults before they penetrate the bark and lay eggs. Treatment following successful attacks and egg laying will not be effective. In most cases, the time to apply is in late winter to early spring in warm areas of the state and late spring in cooler and higher elevation areas. For most insecticide treatments associated with bark beetles listed on the insecticide label, generally only one application per year is necessary to provide season-long control. However, depending on local conditions, the life cycle of the beetle, and the insecticide used, in a few situations a second application may be needed several months later to protect individual trees. For example, in California a single spray applied for red turpentine beetle and engraver beetles around mid-February, before adults arrive on new trees, should provide enough control for the home gardener or arborist to implement cultural practices to improve the vigor and defense of pines. However, if strong spring rains or regular irrigation sprinkling of the stem remove the insecticidal barrier, a second application may be necessary.

Red Turpentine Beetle

This beetle is very common on Monterey pines planted in urban landscapes and highway corridors within about 100 miles of the California coast. It is also prevalent on most pines that grow in the Sierra Nevada, particularly on pines damaged by wildfire. Otherwise healthy pines often survive attacks by a few individuals of the red turpentine beetle. Prominent pitch tubes on the lower trunk of standing trees or stumps of recently cut trees nearly always indicate the beetle’s presence. A red turpentine beetle attack likely indicates that pines are stressed from an unfavorable growing environment, injuries, inappropriate cultural care, or that pines are declining from old age. Ensure that planted trees receive proper care and a good growing environment to enhance tree survival.

Western Pine Beetle

This native species attacks the trunk of ponderosa and Coulter pines and creates long winding galleries in the phloem. The trunk quickly becomes covered with small pitch tubes as the beetles can be attracted in large numbers (aggregate) in only a few days. Drought-stressed trees are highly susceptible to attacks by these bark beetles. Heavily attacked trees invariably die and should be removed as soon as attacks are observed. Unattacked trees that are particularly vulnerable, such as during drought or those adjacent to attacked trees, can be protected by watering, if possible, and by applying an insecticide to the outer bark surface before beetles have attacked the tree.

Elm Bark Beetles

Elm bark beetles are pests because they feed in the phloem of elms and spread the fungus that causes Dutch elm disease. The fungus kills the vascular system of elms, causing foliage to turn yellow and brown and then die. Be sure to distinguish diseased trees from those damaged by leaf-chewing caused by elm leaf beetles (Xanthogaleruca luteola). Chewed leaves turn brown, which, when viewed from a distance, resemble discolored leaves caused by Dutch elm disease. If planting elms, choose from among the many new elm cultivars that are resistant to both disease and leaf beetles, as discussed in Pest Notes: Elm Leaf Beetle.

WARNING ON THE USE OF PESTICIDES

Donaldson, S. G. and S. J. Seybold. 1998. Thinning and Sanitation: Tools for the Management of Bark Beetles in the Lake Tahoe Basin. Reno: University of Nevada Cooperative Extension Fact Sheet FS-98-42 (PDF).

Dreistadt, S. H., D. L. Dahlsten, and A. B. Lawson. 2004. Pest Notes: Elm Leaf Beetle. Oakland: Univ. Calif. Nat. Res. Publ. 7403.

Dreistadt, S. H. and E. J. Perry. 2004. Pest Notes: Clearwing Moths. Oakland: Univ. Calif. Nat. Res. Publ. 7477.

Flint, M. L., ed. 2004. Pest Notes: Carpenter Bees. Oakland: Univ. Calif. Nat. Res. Publ. 7417.

Lewis, V. 2000. Pest Notes: Wood-Boring Beetles in Homes. Oakland: Univ. Calif. Nat. Res. Publ. 7418.

Lewis, V. 2001. Pest Notes: Termites. Oakland: Univ. Calif. Nat. Res. Publ. 7415.

Marer, P. J., and M. Grimes. 1995. Forest and Right-of-Way Pest Control. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3336.

Mussen, E. C. 2000. Pest Notes: Wood Wasps and Horntails. Oakland: Univ. Calif. Nat. Res. Publ. 7407.

Paine, T. D., J. G. Millar, and S. H. Dreistadt. 2000. Pest Notes: Eucalyptus Longhorned Borers. Oakland: Univ. Calif. Nat. Res. Publ. 7425.

Rust, M., and J. Klotz. 2000. Pest Notes: Carpenter Ants. Oakland: Univ. Calif. Nat. Res. Publ. 7416.

Sanborn, S. R. 1996. Controlling Bark Beetles in Wood Residue and Firewood. Sacramento: California Department of Forestry and Fire Protection, Tree Notes 3.

PUBLICATION INFORMATION

Pest Notes: Bark Beetles

UC ANR Publication 7421

Produced by UC Statewide IPM Program, University of California, Davis, CA 95616

Produced by University of California Statewide IPM Program

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Bark Beetles in Trees and Firewood

Description of bark beetles

Bark beetles feed and develops within the cambium layer just under the bark of trees. They are widespread, common and frequently abundant. Bark beetles are found in trees that are under stress or are in the process of dying. They are also common in firewood cut from recently-dead or cut trees. There are hundreds of species of bark beetles. Most are host plant specific and will attack only one species of group of related trees.

Bark beetle emergence holes. Photo by DR Lewis.

Life cycle of Bark beetles

Bark beetles are small (1/16 to 1/8-inch in length) cylindrical, brown to black beetles. Adults typically appear in the spring and females deposit eggs in galleries just under the bark. The eggs hatch into small white legless larvae with brown heads. The larvae tunnel under the bark as they eat and grow, producing winding tunnels between the bark and the sapwood of the tree. New adults emerge through small round exit holes in the bark. There may be 2 or 3 generations of beetles produced each season and dying trees, logs or firewood may contain hundreds of individuals.

Damage caused by bark beetles

Bark beetle attack of trees, logs or firewood is recognized by powdery, sawdust-like frass created as the beetles chew and tunnel under the bark. Small, buckshot-sized emergence holes indicate past bark beetle activity as most of the holes are made when the beetles emerge from infested wood. Small winding tunnels or galleries (less than one-eighth inch wide) under the loose bark show where bark beetles were feeding.

Bark beetles are usually a secondary problem in the landscape. Most bark beetle species attack trees that are severely stressed or dying. They do not cause trees to decline or die but rather are taking advantage of a tree weakened by previous events or circumstance (even though the tree may not outwardly appear compromised). Newly transplanted trees are particularly susceptible to attack. Healthy, well-established trees are rarely successfully attacked by bark beetles.

Management of bark beetles

Bark beetle galleries. Photo by Pennsylvania
Department of Conservation
and Natural Resources via Bugwood.org.

Control of bark beetles is usually not practical after they are established inside a tree (as evidenced by numerous emergence holes or loose bark). Prune and discarded bark beetle-infested limbs and trees. Prevent bark beetle problems by promoting tree vigor through proper site selection, planting, mulching, watering, pruning, and avoidance of injury. Insecticide sprays are not effective when applied to trees that are already heavily infested.

Bark beetles attack green logs and firewood for one or two seasons following cutting but do not attack cured wood or lumber. Bark beetles that emerge inside the house can be annoying and a source of anxiety. However, they are never a threat to people, furniture or the house structure. Spraying infested firewood is of no practical benefit and is not advised.

Do you live in Iowa and have an insect you would like identified?

The Iowa State University Plant & Insect Diagnostic Clinic will identify your insect, provide information on what it eats, life cycle, and if it is a pest the best ways to manage them. Please see our website for current forms, fees, and instructions on preserving and mailing insects.

Contact information for each states diagnostic laboratory for U.S. residents. If you live outside of Iowa please do not submit a sample without contacting the Plant & Insect Diagnostic Clinic.

(Available from the Texas AgriLife Extension Service) L-1826, Carpenter Bees E-412, Carpenter Ants E-394, Structure-Infesting Wood-Boring Beetles Table 1 lists generic or approved common names for insecticides. These ingredients may be found in a large number of products, whereas trade names are used by specific manufacturers or distributors of these insecticides. For convenience, examples of some of the most commonly available trade names for these generic ingredients are given on page 7.

Appendix

Summary of information obtained from product labels for treatment of wood-boring insects of trees and shrubs currently registered by the Environmental Protection Agency. Read and carefully follow directions provided on the actual product label.

Trunk sprays

Carbaryl, a carbamate insecticide

Sevin® Brand 4F Carbaryl Insecticide (43.0 percent carbaryl): Caution. For agricultural or commercial use only. In forested areas such as non-urban forests, tree plantations, Christmas tree farms, parks and rural shelter belts, and for rangeland trees, for treating cypress tip moth, locust borer, Nantucket pine tip moth, olive ash borer and pitch pine tip moth, apply 1 quart of product per acre; as a preventive treatment only, for elm bark and Ips engraver beetles apply as a trunk spray a 2 percent solution (5 fluid ounces per gallon) per acre no more than two times per year. On pecans for twig girdler, apply 2 to 5 quarts of product per acre. On peaches, plums, prunes and nectarines, apply 2 to 3 quarts of product per acre for lesser peachtree borer or peach twig borer. Observe bee caution. Repeat applications as necessary up to a total of three times per year per crop but not more often than once every 7 days. For lesser peachtree borer, thoroughly spray the limbs and tree trunks at weekly intervals during moth flight. Also see the product labels for rates of other formulations: Sevin® Brand 80S, 80WSP and XLR Plus Carbaryl Insecticides.

GardenTech® Sevin® Ready To Use Bug Killer (0.126 percent carbaryl): Caution. For trees including shade trees and those in shelter belts, plantations, parks and recreational areas, and for ornamentals including roses and wooded shrubs; for treating European pine shoot moth, locust borer, Nantucket pine tip moth, ash borer, pitch pine tip moth and subtropical pine tip moth. Where pests appear, direct the spray toward the upper and lower leaf surfaces and small trunks, stems and twigs to the point of runoff. Repeat as necessary up to a total of four times for trees and six times for ornamentals and shrubs but not more often than once every 7 days. On fruit trees including peaches and plum, for treating lesser peachtree borer and peach twig borer. Do not apply within 3 days of harvest.

Chlorpyrifos, an organophosphate insecticide

Dursban® 50W (50.0 percent chlorpyrifos): Danger. For use by certified applicators or people under their direct supervision; this product may be used only on ornamentals grown in nurseries and to treat evergreens, shade and flowering trees, and nonbearing fruit trees infested with:

  • Weevils such as blackvine, cranberry, yellow poplar and pine reproduction weevils, use 1 pound per 100 gallons;
  • Borers, including clearwing moths such as ash, dogwood, lesser peachtree, lilac, oak, peachtree, rhododendron borers; metallic wood borers such as bronze birch, flatheaded appletree and twolined chestnut borers; longhorned borer beetles such as cottonwood, locust, red oak borers, pales weevil adults and Zimmerman pine moth: Use 2 pounds per 100 gallons, spraying trunks and lower limbs of trees and shrubs when adults begin to emerge for borers, and for peachtree borers spraying flowering trees and shrubs on the genus Prunus as a trunk spray before newly hatched larvae enter trees and thoroughly wet all bark areas from ground level to scaffold limbs;
  • Pales and northern pine weevils, use 6 pounds per 100 gallons applied as a cut stump spray or drench;
  • Other beetles such as ambrosia, Anobiidae, black turpentine, European elm bark, mountain pine, native elm bark and southern pine beetles: Use 16 pounds per 100 gallons, to achieve a preventive treatment by spraying the main trunk of trees in the early spring or when the threat of attack exists from nearby infested trees or to achieve remedial treatments spraying the main trunk of infested trees or logs when damage occurs but before beetles begin to emerge;
  • Weevils such as northern pine, pitch eating weevils: Use 32 pounds per 100 gallons for pine seedlings, treating immediately after transplanting to thoroughly wet the foliage and stems to the point of runoff, not using more than 6 gallons of spray dilution per acre.

Bifenthrin, a pyrethroid insecticide

OnyxPro™ Insecticide (23.4 percent bifenthrin): Warning. For commercial nonfood use in interiorscapes and on outdoor ornamentals, Christmas trees, nurseries, golf courses and other listed sites. Use as trunk sprays to ornamental trees including Christmas trees to control bark beetles and boring beetles, not applying more than 12.8 fluid ounces (0.2 pounds of active ingredient) of this product to trees per acre using spray volumes as directed on the product label and spraying until the bark is thoroughly wetted. Use specifically for:

  • Dendroctonus bark beetles such as mountain pine beetle, southern pine beetle, and black turpentine beetle and engraver (Ips Species) beetles at 16.32 fluid ounces per 100 gallons, applying to the trunk of the tree with a hydraulic sprayer in the early spring or before adult flight and tree infestation, applying the spray directly to the main trunk from the base of the tree to at least halfway into the live crown;
  • Other bark beetles such as ambrosia beetles, elm bark beetles and metallic wood borers at 16.32 fluid ounces per 100 gallons, spraying mixture to the trunk, scaffolding and limbs of the tree with a hydraulic sprayer in the early spring or before beetle flight and tree infestation;
  • Clearwing moth borers such as ash borer, banded ash clearwing, dogwood borer, lesser peachtree borer, lilac borer, oak borer, peachtree borer, rhododendron borer and coleopteran borers such as bronze birch borer and flatheaded appletree borer use 6.4 to 12.8 fluid ounces per 100 gallons, spraying the branches and trunks prior to adult emergence.

Talstar® P Professional Insecticide (7.9 percent bifenthrin): Caution. For use outdoors on residential, institutional, public, commercial and industrial buildings, greenhouses, lawns, ornamentals, parks, recreational areas and other listed sites. For ornamentals and trees including but not limited to trees and shrubs, greenhouses and interiorscapes, to control:

  • Beetles including twig borers and weevils, use 10.8 to 21.7 fluid ounces per 100 gallons, treating trunks, stems and twigs in addition to plant foliage;
  • Pine shoot beetle adults use 21.7 to 43.5 fluid ounces per 100 gallons.

Ortho® Bug-B-Gone® MAX® Lawn & Garden Insect Killer Concentrate (0.3 percent bifenthrin): Caution. For use on shrubs and small, nonbearing trees for northern pine weevil, pine shoot weevil and Zimmerman pine moth, and on fruit trees for peachtree borer and lesser peachtree borer.

Permethrin, a pyrethroid insecticide

Astro® Insecticide (36.8 percent permethrin): Caution. For use on ornamental trees and shrubs around buildings, parks, recreational areas and other listed sites, such as ornamental greenhouse, interiorscapes and plantscapes, lawns, trees and shrubs. On ornamental trees, for:

  • Clearwing moth borers such as banded ash clearwing, dogwood borer, lesser peachtree borer, lilac borer, oak borer, peachtree borer and rhododendron borer: Use 1 to 2 quarts per gallon;
  • Bark beetles such as Dendroctonus sp., Ips species, elm bark beetles, mountain pine beetle, pine engravers, turpentine beetles and white pine beetle: Use 2 to 5 quarts per gallon;
  • Coleopteran borers such as bronze birch borer and flatheaded appletree borer: Use 2 to 5 quarts per 100 gallons;
  • Maximum residual control of all the insects listed above, use 5.35 quarts per 100 gallons. Apply to the lower branches and trunks prior to adult emergence, which varies according to pest species, host tree, environmental conditions and geographical location. Thorough coverage of bark is required for control;
  • Nantucket pine tip moth, coneworms and seedbugs, use 4 to 8 fluid ounces per 100 gallons. Use 5 to 10 gallons of finished spray per tree for coneworms and seedbugs. Begin application when the adults appear. Repeat applications may be made on 5- to 7-day intervals as needed;
  • On noncommercially grown peach trees, for lesser peachtree borer and peach twig borer use 1 /4 to 3 /4 teaspoon per gallon for 436 square feet. Do not harvest fruit within 14 days after application. Do not apply more than 3 3 /4 teaspoon per 436 square feet per year.

Bonide® Borer-Miner Killer Concentrate (2.50 percent permethrin): Caution. Protects fruits, nuts, trees, shrubs, roses, flowers and vegetables from borers, leafminers and other listed insects.

  • For pine beetles use 1 fluid ounce in 1 gallon water applied as a thorough spray, wetting the leaves and branches to dripping point. Spray in late afternoon or evening, when temperature ranges from 50 to 75 degrees F and when there is little or no wind. Spray at the first sign of insects. Repeat as necessary; use at intervals or 4 to 8 days. Do not exceed 16 applications per season.
  • For peach tree borers (lesser peachtree, peach twig) apply 2 fluid ounces in 1 gallon water when insects appear. Wet the plants to dripping point. Do not apply within 7 days of harvest. Do not make more than eight applications per year.

Bonide® Total Pest Control Concentrate Outdoor Formula (13.30 percent permethrin): Warning. For ornamental trees around the home, use sufficient spray to obtain full coverage of all foliage. Avoid heavy wetting. Use to control pine moths and pine beetles. On peaches to control lesser peachtree borer and peach twig borer: Use 23 /4 teaspoons per 5 quarts water. Apply when insects appear. Do not apply within 7 days of harvest. Do not make more than eight applications per season.

Spinosad, a spinosyns insecticide

Ferti. lome® Borer, Bagworm, Tent Caterpillar & Leafminer Spray (0.5 percent spinosad): Caution. For use on fruiting, tuberous and leafy vegetables, stone fruits, apple and citrus trees, ornamentals and lawns: Use 2.0 fluid ounces per gallon for peach twig borer (a caterpillar of a clearwing moth species) on fruit trees only.

Soil drenches

Dinotefuran, a neonicotinoid insecticide
Spectracide® Tree and Shrub Insect Control + Fertilizer Concentrate (0.43 percent dinotefuran): Caution. Available to homeowners through retail outlets for roundheaded borers apply 3 fluid ounces per inch of tree circumference or 9 fluid ounces per foot of shrub height. Note: For professional use, see label for Safari ®.

Imidacloprid, a neonicotinoid insecticide

Marathon® 60 WP (60 percent imidacloprid): Caution. For systemic control on ornamentals in greenhouses, nurseries and interior plantscapes as a soil injection for flatheaded borers including bronze birch and alder borers to trees at a rate of 20 grams per 8 to 16 inches of cumulative trunk diameter or to shrubs at a rate of 20 grams per 8 to 16 feet of cumulative shrub height applied as a soil injection or soil drench as directed on the product label. Also Marathon® II (21.4 percent imidacloprid), using different rate.

Bayer Advanced™ 12 Month Tree & Shrub Insect Control (1.47 percent imidacloprid): Caution. For flatheaded borers including bronze birch and alder borers apply to the soil as a drench to trees at 1 ounce per inch of distance around the trunk and to shrubs at 3 ounces per foot of height.

Discus™ Nursery Insecticide (0.70 percent cyfluthrin plus 2.94 percent imidacloprid): Caution. For ornamentals, nonbearing fruit and nut trees, and in field and container nurseries. Use as a drench and soil injection application for shrubs, evergreens and trees including nonbearing fruit and nut trees.

For flathead borers including bronze birch and alder borers, use 3.4 to 5.6 fluid ounces per 1,000 square feet. Use at the high rate for borer control and with high populations. Application to heavily infested trees may not prevent the eventual loss of the trees because of existing pest damage and tree stress. Note: Cypermethrin is a pyrethroid contact insecticide and will not move into the tree systemically.

Tree injection products

Acecap® 97 Systemic Insecticide Tree Implants (97 percent acephate): Caution. For ornamental trees with a trunk diameter of 3 inches or larger for treatment of borers. For small trees and shrubs with trunk diameters of 1.5 to 3 inches see the label for Mini Acecap 97.

Acejet Systemic insecticide for Micro-Infusion™ (97.4 percent acephate): Caution. Micro-Injectable and Micro-Infusable Insecticide for use with the Arborjet Injection Systems to manage of specific insect pests of trees and landscape ornamentals, including conifers, Christmas tree and deciduous tree farms, seed orchards and plantations, and forest trees, shrubs, evergreens and conifers, and trees in Christmas tree plantations and palms in forest areas including nonurban forests, tree plantations, seed orchards, parks, rural shelter belts, rangeland and woodlands including conifers. For treatment of carpenterworm, clearwing borers, cottonwood twig borer, Nantucket pine tip moth, pine coneworm, pine tip moth, buprestid borers including bronze birch borer, flatheaded borers, longhorned borers including red oak borer, and root weevil adults including black vine weevil. Do not treat trees that are moisture stressed or suffering from herbicide damage. Do not inject trees within 2 weeks of any other spray or soil chemical treatment.

Dicrotophos

Inject-A-Cide B® (82 percent dicrotophos): Danger. Contains Bidrin®, for internal treatment by microinjection for systemic suppression of certain insects on ornamental trees. Not for use on trees for sale or other commercial use, for commercial seed production, for the production of timber or wood products, or for research purposes. For dogwood twig borer and sycamore borer (American plum borer) on oaks and sycamore, and for lesser peachtree borer on flowering stone fruit (noncrop).

Oxydemeton methyl

Mauget Inject-A-Cide® (50 percent oxydemeton methyl): Danger. Internal treatment by microinjection for systemic suppression of certain insects on ornamental trees. Treatment limited by tree species to treatment of bark beetles on cedar, cypress, juniper and pine; engraver beetles; red and black turpentine beetles; and Nantucket pine tip moth on Douglas fir and pine.

Imidacloprid

IMAjet Systemic Insecticide (5.0 percent imidacloprid): Warning. Microinjectable systemic insecticide for use with Arbojet Injection System in the management of specific insect pests of forests, trees, landscape ornamentals, palms and interior plantscapes (trees, shrubs and evergreens), palms in forest areas including nonurban forests, tree plantations, Christmas tree farms, parks, rural shelter belts, rangelands and woodlands including those with conifers. For treatment of buprestid borers including bronze birch borer, flatheaded borers, longhorned borers and pine tip moth larvae.

Mauget® Imicide Systemic Injecticide for tree injection use in ready to use capsules (10.0 percent imidacloprid): Caution. Injection into trees more than 2 inches in diameter that will not produce food within a year after treatment: Use as directed for black vine weevil larvae, bronze birch borer, cottonwood longhorned borer, flatheaded borers including alder and birch borer, pine tip moth larvae and other listed pests. Also see Mauget Imicide Hp for use in loadable injectors.

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The Natural Enemies of Bark Beetles

Fact sheet for the practitioner

The biology and the environmental needs of bark beetles are already well known. The knowledge about their antagonistic agents however is limited. The fact sheet describes in detail the natural enemies (antagonists) of the spruce bark beetles (Ips typographus und Ips amitinus) and other bark beetles in Central Europe. The enemies of bark beetles may be classified in three superordinate groups: pathogens, predators, and parasites.

Pathogens

Fig. 1 – Three-toed woodpecker
Photo: A. Aichhorn

  • Bacteria are taken up by the beetle with food. The body content turns into a bad smelling-decayed pulp.
  • Under humid, warm conditions, fungi have a significant influence on the mass reproduction of numerous bark beetle species.
  • Protozoa mainly attack the larvae stages of the hosts. However, there is little known about the effect of protozoa on bark beetles.
  • Nematode worms can usually be found in the breeding system of the bark beetles. There is, however, still too little known about their effects.
  • According to the newest state of art, viruses also occur but play a secondary role as pathogens of bark beetles.

Predators

  • Mites can be found in almost all bark beetle breeding systems. The suck on the eggs, larvae, and cocoons of the bark beetles. However, there is still little known about their effect.
  • Among the beetles, there are numerous species which are as larvae and/or in the adult stage important predators for bark beetles. The most conspicuous one is the checkered-beetle (see photo).
  • The snakeflies also play a role with the natural regulation of the population density of various bark beetles. However, it is only the larvae which are feeding on bark beetles.
  • The true flies play an important role as predators of bark beetles. The larvae of the long legged flies (Dolichopodidae) and lance flies (Lonchaeidae) are particularly important predators.
  • Birds can intervene in a regulatory manner in bark beetle populations, as long as these are not too large. The black woodpecker, the lesser spotted woodpecker and, in particular, the three-toed woodpecker are important enemies.

Parasites

  • Various parasitic wasps among the Hymenoptera, contribute as parasititoids of bark beetles significantly to the maintenance and the reestablishment of the natural balance.
Fig. 2 – European checkered-beetle (Thanasimus formicarius) eats a bark beetle.
Photo: Beat Fecker (WSL)
Fig. 3 – Parasitic wasp laying eggs onto a bark beetle.
Photo: Beat Wermelinger (WSL)

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  • This fact sheet, issued by the Swiss Federal Institute for Forest, Snow and Landscape Research WSL, is not available in English language.

  • Die natürlichen Gegenspieler der Borkenkäfer (PDF)
  • Les ennemis naturels des scolytides (PDF)
  • I nemici naturali degli scolitidi (PDF)

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More on waldwissen.net

  • About the Biology of the Eight-Toothed Spruce Bark Beetle Species
  • Insects in the forest ecosystem
  • Application of Insecticides against Bark Beetles
  • Prevention and Control of Bark Boring Insects
  • Prevention and Control of Wood Boring Insects

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Bark Beetles – Trees and Shrubs

Evidence of bark beetle activity

Bark beetles are very small insects, usually less than 8mm. They are brown to black in color and cylindrical in shape. The beetles generally attack shrubs that are stressed or weakened. Often several beetle species will attack and individual plant.

Bark Beetles on needled evergreens

Needle color will turn from bright green to light green, then to yellow and finally to reddish brown. The initial signs of attack are pitch tubes and/or boring dust. The pitch tubes are small masses of pitch (sap), which are usually whitish in color and often mixed with reddish boring dust or frass.

Borers that attack evergreen trees and shrubs are primarily long-horned beetles (round headed borers), such as the southern pine sawyer. These beetles are much larger than the bark beetles (an inch or more with long antennae). They are stress pests and generally attack stressed plants. Symptoms include branch dieback due to feeding damage, and exit holes. Exit holes of long horned beetles are oval and about 1/4 inch or larger in diameter.

Initial symptoms are fading and yellowing of needles

The pitch tubes are small masses of pitch (sap),
which are usually whitish in color and often
mixed with reddish boring dust or frass
Southern pine sawyer adult
Southern pine sawyer damage

Damage on mugo pine

Bark beetles on deciduous trees

The beetles generally attack stressed or weakened trees (oak, elm, birch, ash, hickory, hackberry, beech, sweetgum, maple, cherry, pear, etc.). The immature beetles construct galleries as they feed on the cambium, under the bark. Healthy trees can usually withstand bark beetle attacks by literally trapping the beetles in a sticky flow of pitch. Bark beetles emit a chemical signal, or aggregation pheromone, which attracts more bark beetles to the tree under attack. Beetles often carry pathogenic fungi such as Dutch elm disease and oak wilt, which are deposited in the vascular system of the tree. These fungi quickly multiply and clog the water conducting vessels of the tree which hastens its death.

Management

The best control for bark beetles and borers is prevention. Healthy plants are able to withstand most bark beetle attacks. Insecticidal sprays will not kill the beetles or larvae inside the shrubs and sprays will not reverse the damage that has already occurred. Plants should receive adequate water in times of summer drought. Dead or dying shrubs should be removed to eliminate breeding sites for additional beetle generations.

Additional Resource

(PDF) HG1 Conifer Bark Beetles

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