Photo: Bark beetle damage, northern Idaho. © 2016 Delena Norris-Tull
Bark beetles and forest ecosystems in North America
Bark beetles have caused billions of dollars of economic damage to US and Canadian forests in recent decades. Bark beetle populations expanded dramatically in the 1980s. Billions of coniferous trees have been killed by bark beetles since that time (Bentz, et al., 2010). These beetles have decimated over 40 million acres of forests over the past few decades (Oatman, 2015; USDA Forest Service, n.d.).
The rapid spread of these infestations has swept across the Southern States, the Northeast, Midwest, and the Western United States and Canada, all the way to Alaska and Mexico.
In the Southeastern U.S., a group of five native species of bark beetles have caused massive loss of pine trees throughout the southern states: the southern pine beetle (Dendroctonus frontalis), the eastern six-spined engraver (Ips calligraphus), the eastern five-spined engraver (Ips grandicollis), the small southern pine engraver (Ips avulsus), and the black turpentine beetle (Dendroctonus terebrans), collectively called the “southern pine bark beetle guild.” Several of these species may simultaneously infest a single stand of trees (Coulson & Klepzig, 2011). Of these five species, the southern pine beetle has caused the most damage. In the south, the bark beetles infest mainly the yellow pines, loblolly and shortleaf, but many other pine species can also be infested. These beetles have infested at least 16 species of pines, although certain species seem to be more susceptible.
While all five of these bark beetles are found throughout the Southeast, several have ranges extending throughout the Northeast and Midwest.
Ips ponderosae is found in the Western states of Montana, Wyoming, Colorado, New Mexico, Utah, South Dakota, Arizona, Nebraska. Ips calligraphus is found in a small area of California (Connor & Wilkinson, 1983).
The mountain pine beetle (Dendroctonus ponderosae), a beetle that ranges throughout the Western States and Western Canada, infects a number of western pines, including ponderosa, lodgepole, sugar, and western white pine. “High-risk lodgepole pine stands have an average age of more than 80, an average diameter at breast height of more than 8 inches.” High-risk secondary growth stands of ponderosa pine have an “average width at breast height more than 10 inches.” Mountain pine beetle has caused the loss of millions of lodgepole and ponderosa pines in recent decades (Amman, McGregor, & Dolph, 1989). It has been common in recent years to drive by entire hillsides covered with the red or yellow color of dead needles on trees. Several Ips species also cause damage to western pines, such as Ips confusus on piñon pines.
In addition to the pine bark beetles, the spruce bark beetle (Dendroctonus rufipennis) has caused severe damage in Alaska, Montana, Utah, Idaho, Arizona, and British Columbia (Holsten, Thier, Munson, & Gibson, 1999). The damage has been most extensive in Alaska and British Columbia, the locations where there are the largest stands of spruce trees. By 2010, spruce bark beetle activity was visible by aerial mapping over Alaska, showing the extent of infestations to have reached over six million acres. Over about 40 years, the spruce bark beetle caused the loss of over three billion board-feet of timber in Alaska (Alaska Department of Natural Resources, n.d.). While this beetle can infest any spruce species it encounters, the most economically valued species have been white, Sitka, and Engelmann spruce. Large diameter, slower growing spruce tend to be more susceptible (which may explain why the slender black spruce, abundant in Alaska, has not been severely affected). Unlike the situation with the southern pine forests, spruce forest infestations do occur in old-growth forests, but mainly in locations where spruce is the main type of conifer present.
The northern spruce engraver (Ips perturbatus) also affects spruce trees. Douglas firs can be affected by Dendroctonus pseudotsugae. Larch trees are affected by Dendroctonus simplex. And several species of firs are affected by the fir engraver (Scolytus ventralis).
But this is a book about invasive plant species. So why have I included a chapter on bark beetles? The answer is really quite simple. Rich Prodgers, a Montana botanist, once told me, “It’s not that we have an invasion of bark beetles that is the problem. The problem is that we have an invasion of yellow pines.”
The bark beetles causing the most destruction are not introduced insect species. The bark beetles co-evolved with many of the tree species they infest. In other words, these beetles have been a natural part of the forest ecosystem for many thousands, if not millions, of years. Yet they are viewed by many today as pests.
But let’s set the record straight. Bark beetles are a natural part of the forest thinning and decomposition processes. They speed up the decomposition of trees, particularly trees that have been weakened by lightning strikes, fire, disease, wind storms, and drought.
Researchers have recently used tree-rings and lake sediment cores to estimate the frequency of large-scale infestations over previous centuries. These studies have revealed, for example, that in the past, spruce beetle outbreaks occurred 50-80 years apart for white spruce, 70 years apart for Norway spruce in Europe, and 120 years apart for Engelmann spruce (Morris, et al, 2016). While it is apparent that the length of time between outbreaks was longer in previous centuries, little is yet known about the intensity of these previous outbreaks and little is known about forest recovery time after these outbreaks. But what is clear is that today the time period between extensive outbreaks has been shortened. And the intensity of recent outbreaks is thought to be ten times greater than in previous centuries (Oatman, 2015).
What factors have contributed to a shorter period of time between outbreaks, and the high-intensity outbreaks we have witnessed in recent decades? While various factors are known, large beetle infestations were relatively rare in the past, and the likelihood of infestation within a specific softwood population remains unpredictable. But the following human-induced conditions appear to be the leading causes of recent widespread bark beetle infestations:
Monocultures
In a natural forest ecosystem, a forest is composed of a variety of species of trees, including both softwood (conifers) and hardwood (broadleaf) trees. In addition, a wide variety of herbaceous plants, shrubs, vines, wildflowers, fungi, etc. are an important part of the system.
The southern forests are very different today than when bark beetles and pine species co-evolved. The southern forests in the past were not small monocultures, as they tend to be today. They were vast forests with mixed hardwoods and softwoods.
The southern forests of 200 years ago were mixtures of tree species – including oaks, hickories, and longleaf pine. The mixture of tree species modified the proximity between similar species of trees, making it more challenging for beetles to find another pine to invade. For example, research on the southern pine beetle (SPB) demonstrates that, while the SPB can fly as far as 2.6 kilometers, the SPB is most likely to infest adjacent trees that are no more than 6-7.5 meters away (Coulson & Klepzig, 2011, p. 29).
Most old-growth southern forests were destroyed in favor of agricultural crops. As early European populations grew and crop sizes grew, particularly tobacco, rice, and cotton, farmers burned down many existing forests to enable farming and to provide forage for livestock. By the end of the 19th century and in the early 20th century, “the remaining southern forests were extensively logged to support economic expansion” (Stanturf, Wade, Waldrop, Kennard, & Achtemeier, 2002). In the 1940s-1970s, herbicides were also used to destroy forests to make way for agriculture and to make way for monocultures of fast-growing conifers (refer to the section in these webpages on the use of herbicides for deforestation).
Today’s southern forests primarily consist of early successional pines, such as loblolly and shortleaf pine (species of yellow pines more susceptible to the southern pine beetle than longleaf pines). In addition to a more rapid growth and higher susceptibility to beetle infestations, these species also burn more readily than the longleaf pines and the hardwood trees that were prevalent in the old-growth forests (Stanturf, Wade, Waldrop, Kennard, & Achtemeier, 2002). Trees damaged by wildfire are particularly susceptible to beetle infestations.
The US Forest Service, combined with pressure from agricultural interests and the paper and lumber industries, has allowed our previously diverse forest ecosystems to gradually be transformed into monocultures of conifer trees. Fast-growing conifers have been favored by these industries, to the detriment of natural ecosystems, which originally were comprised of a rich tapestry of a variety of species of both conifers and hardwoods. In October, 2020, President Trump opened the Tongass National Forest in Southeast Alaska to road building and lumbering. The Tongass is an old-growth temperate rain forest and the largest national forest in North America. If not stopped, this could have a devastating impact, as the forest serves as a crucial carbon sink needed to alleviate the impacts of climate change. It also provides important habitat for trout, wild salmon, brown bears, and many other species.
Secondary growth pine and spruce forests (forests that have been lumbered out and replanted) have far less diversity both in species and age class, and are thus more susceptible to future beetle infestations.
In the past, within a naturally diverse forest, beetle populations remained under control because the host species were separated by some distance, with other species of trees between them. The distance and the barrier provided by other tree species helps keep the beetles from spreading. When diverse old-growth forests are replaced with few species of pines or spruce, and the trees grow closer to each other than would occur naturally, bark beetles proliferate more easily. The close proximity of the trees facilitates the beetles in spreading rapidly from tree to tree.
Poor logging practices
Bark beetles are able to spread more readily when loggers leave behind large amounts of fallen lumber or debris, when loggers leave felled trees close to live trees, or when loggers damage nearby trees or cut into roots of unfelled trees (Connor & Wilkinson, 1983).
Logging practices that foresters claim can decrease the likelihood of future large-scale beetle infestations:
Diana Six, entomologist at the University of Montana-Missoula, and Eric Biber, a law professor at UC-Berkeley, reviewed the research (Six, et al., 2014) on the effectiveness of treatments used to manage bark beetles and concluded that the recommended logging practices have had minimal beneficial effects. Focusing on research on the mountain pine beetle, they found that, although bark beetles have been part of the forest ecology for thousands of years, and, “While considerable research has been conducted on controlling bark beetles, massive gaps in knowledge remain. In particular, there is a disturbing dearth of rigorous replicated empirical studies assessing the effects of various management strategies, particularly timber harvest treatments, for bark beetle outbreak suppression. Even fewer studies have focused on how such treatments meet explicit goals or affect forest structure, function and future outbreak dynamics.” They point out that climate change has contributed to bark beetle expanded range and greater impacts. Higher temperatures make it easier for bark beetles to infest trees, as the higher temperatures result in less resin, resin that would normally create a chemical challenge to beetles.
Direct controls, which include removal of infested trees, clear-cutting, prescribed burns, fell and burn, trap trees (trees baited with pheromones), debarking, or the use of insecticides and toxins, are “extremely expensive in time, effort and resources,” and have to be repeated year after year. Six, et al., 2014, found that few rigorous studies have been conducted on the effectiveness of these strategies. Direct treatments may only serve to delay infestations. The few studies that have been conducted, mainly in Canada, have shown quite variable results, ranging from no effect to only marginal effects.
Indirect controls include thinning forests, daylighting (removing enough trees and shrubs to expose remaining trees to sunlight), removing mature and old growth trees, or replacing trees with different species of trees. Most of these result in large timber harvests. Six, et al., 2014, found similar problems with studies on the effectiveness of indirect treatments, too few studies, poor monitoring of the impact of treatments, and variable results. I recommend reading their review of the research. They found that, “long-term replicated studies monitoring beetle responses to thinned forests from non-outbreak to outbreak to post-outbreak phase are virtually non-existent.” In regards to daylighting, they found no published studies of its effectiveness. Six, et al., 2014, found that there has been little monitoring of the impact of tree removal practices, and failures have often not been documented. They concluded that removing trees “often removes more trees than the beetles would.” Unfortunately, much of the funding of studies on these treatments are short-term, often only one to two years.
Six, et al., 2014, point out that, “Funding cuts to research personnel, particularly in agencies like the US Forest Service, have exacerbated this problem exactly at the time when the need for rigorous research is increasing at a rapid pace. The US Forest Service has recognized that long-term planning must include explicit goals to increase forest resilience and adaptation to disturbance, including outbreaks of the mountain pine beetle. However, with extreme cuts to budgets and personnel, they are highly constrained to meet these needs at this time. Likewise, cuts in federal funding to agencies such as United States Department of Agriculture and the National Science Foundation concurrently reduce the ability of academic researchers to address these problems.”
Six, et al., 2014, point out that, “These studies highlight a seldom considered impact of mountain pine beetle-that it can act as a natural thinning agent and seldom removes all mature trees during outbreaks. These effects are an important part of the ecological role that the beetle plays in western pine forests…. Although beetle outbreaks, like fire, can have negative impacts on timber values and aesthetics, their natural role in many forest ecosystems is seldom considered and beetle suppression is often perceived as something that must be conducted at all costs. However, as with fire, suppression of beetles over the long term may alter forests in ways that are not desirable or sustainable.”
Six, et al., 2014, conclude that, “Our survey of the relevant literature finds that there is significant uncertainty about whether the most commonly used beetle timber harvest treatments are, indeed, effective. Yet there has been little discussion of this uncertainty in the relevant policy debates. Politicians have instead latched on to beetle timber treatments as a cure-all for beetle infestations and have pushed to weaken or eliminate environmental laws that are perceived to be obstructing these treatments. Agencies such as the US Forest Service, to their credit, have been more nuanced in their support for bills that package beetle timber harvest treatments with weakened environmental laws; they have opposed several proposals to alter environmental laws to allow more treatments, but on the other hand, the agencies have at times also aggressively pushed for the implementation of treatments.
“It seems clear that the policy debates–both in the agencies and in Congress–need to be better informed by science. Researchers should be more proactive in communicating their understandings of the current science to policymakers. This does not mean that researchers need to take a position pro or con vis-à-vis beetle treatments, or even vis-à-vis specific legal proposals. In the face of uncertainty, aggressive beetle timber harvest treatments may be warranted in some instances. However, policymakers should be aware of uncertainty when they are making the relevant decisions and should also be more willing to include the voices of scientists in the development of policy.
“Given the uncertainty about the effectiveness of many beetle timber harvest treatments, the high financial costs of those treatments, the impacts on other environmental resources and values, and the possibility that in the long-run those treatments may interfere with the ability of North American forests to adapt to climate change, our position is that weakening or eliminating environmental laws to allow more beetle timber harvest treatments is the wrong choice for advancing forest health in the United States.”
Fettig, et al., 2014, dispute some of the claims of Six, et al., 2014, by pointing out that Six, et al., 2014, had missed some important research. Fettig, et al., 2014, argue for the value of some treatments, particularly forest thinning for its value, not just for beetle suppression but also for reducing fuels to reduce fire hazard. Their counter-arguments are worth reading.
Forest fire suppression
Native American tribes used controlled burns for thousands of years, to manage the landscape. Beginning about a thousand years ago, as eastern and southern tribes engaged in cultivation of native plants, fire was used to clear small tracts of land for agriculture and forage and to provide an open area for defense purposes. Early Europeans followed similar practices for the first few centuries they inhabited the New World. But in the southern and eastern forests, as populations and demand for crops grew, Europeans burned down many existing forests to enable farming and to provide forage for livestock (Stanturf, et al., 2002).
The US Forest Service was created in 1905. At first, it was severely underfunded by Congress (PBS, Ordeal by Fire). A massive northwestern fire occurred in 1910, covering about 3 million acres in the National Forests in northeastern Washington, northern Idaho, and western Montana. To combat the fire, the USFS hired many untrained men to help out, many of whom died. This large fire caused enough concern that the US Congress significantly increased funding for the USFS, and allowed for the creation of many new National Forests, expanding that model into the Eastern US, which had few National Forest designations previously. While some people argued that fire suppression was the wrong strategy for management of fires, the USFS implemented a fire suppression policy aimed at stopping every forest fire immediately. “The Clarke-McNary Act of 1924 … provided Federal funding for State fire-control efforts… By 1930, 70 million acres were protected from fire; by 1980, over 233 million acres were protected” (Stanturf, et al., 2002).
Fire suppression policy ignored scientific knowledge of the important role fire plays in forest ecosystems. As a result of decades of fire suppression, a massive amount of deadwood developed in the forests. Deadwood provides an avenue for rapid spread of bark beetles. The presence of so much deadwood, coupled with drought, resulted in the massive forest fires that have plagued the West since the 1980s. The fires in Yellowstone National Park in 1988 are a prime example of the devastation caused by these conditions.
It was not until the 1970s that the USFS started allowing some forest fires to burn. And while the USFS has implemented some prescribed burns, the rapid expanse of population centers across the country has resulted in much more restriction on the implementation of prescribed burns.
The decades of fire suppression allowed softwood species that normally were kept at bay by natural fires, to increase in number in comparison to more fire resilient species. The massive fire in Bastrop, Texas, in 2011 is a testament to what happens when fire suppression is followed by increased temperatures, leading to severe drought.
Since 2015, California and the northwestern states have experienced unprecedented summer temperatures and drought conditions that have enabled large fires to develop. Decades of fire suppression in Federal and State forests have provided massive amounts of fuel that has resulted in much larger fires than would occur in a natural ecosystem. And as more communities have grown up close to the forests, the result has been billions of dollars of structural damages in communities close to the forests, as well as the loss of lives, as fire frequency and intensity has increased.
Climate change
Increased temperatures and increased drought have already been shown to increase the spread of bark beetles (Bentz, et al., 2010). For example, the southern pine beetle can reproduce between one and nine times in a year, depending on the climate (Coulson & Klepzig, 2011, p. 13). Increased temperatures increase the number of new beetle generations per year. The southern pine beetle is native to the United States and Central America. In the US, southern pine beetles (SPB) range from New Jersey to Florida, and throughout the south as far west as Arizona.
In the southernmost parts of its range, seven to nine generations can emerge in a year. In years when increased temperatures allow multiple generations of beetles even in the northernmost parts of its range, the beetles can overcome the natural chemical defenses of even healthy trees. This single species can cause “periodic large-scale eruptions that encompass entire regions of the South” (Coulson & Klepzig, 2011, p. 13).
The northern limits to the range of the SPB appear to be climate specific. In winter, southern pine beetles begin to die off with temperatures below about 10 degrees Fahrenheit. At temperatures below 3 degrees, near total mortality occurs within a population. Insect mortality also tends to increase in temperatures above 86 degrees.
The western pine beetle (Dendroctonus brevicomis) and the piñon Ips (Ips confuses) can produce more than one generation per year. The spruce beetle and the mountain pine beetle can take one-three years to go through a single generation. But these cycles are regulated by temperature and seasonality of those temperatures. In other words, as regional temperatures increase, seasons also change, with temperatures warming up earlier in the spring, and cooling down later in the fall (Bentz, et al., 2010).
Thus, we can expect various bark beetle species to move further north and to higher elevations as global warming continues, and we can expect beetles to produce more generations per year. In addition, higher temperatures can increase the drought stress on trees, another factor that makes the trees weaker and thus more susceptible to infestation. In 2011, scientists observed that mountain pine beetles have expanded their range and infested jack pines, with which they did not co-evolve. This means that jack pines do not have natural deterrents to this species of beetle (Oatman, 2015).
Climate change is already causing disruption to ecosystems worldwide. In the Western US, higher temperatures, combined with fire suppression policies, and the large increase in monocultures, in grasslands, sagebrush habitats, and forests, has allowed fires to become a greater problem every summer. The higher temperatures have also allowed bark beetle populations to flourish.
Bark beetles: What does the future hold?
The bark beetle infestations of recent decades are a problem of our own making. We are, to put it in the words of William Shakespeare, hoist with our own petard.
So what should we do to avoid such frequent high-intensity outbreaks in the future?
Insecticides have limited value in the control of large-scale infestations. They are most effective in preventing an infestation. Once a widespread infestation has occurred, insecticides may be ineffective and the cost of such treatment may be higher than the value of the timber. The use of insecticides is also not effective, after an infestation takes hold, without collaboration between all landowners in the area. Without appropriate silviculture, insecticides are not effective (Amman, McGregor, & Dolph, 1989). Synthetic beetle attractants have limited value, except in managing small outbreaks.
Infested forests may be more fire prone, but we have to keep in mind that fire is nature’s way of cleaning house. Recent devastating fires in the West have increased in frequency and intensity for several of the same reasons beetle infestations have increased: historical fire suppression, the rise of forest monocultures, and climate change. Were it not for human encroachment into forest habitats, forest fires perhaps would not be perceived as the evil they seem to be viewed as today.
Morris, et al., 2016, point to research that suggests beetle infested forests can re-generate without any human intervention. Unless we are willing to tackle the big picture factors described in this section, it may be that doing nothing is better than doing something. Morris et al., 2016, concluded their analysis of the research on beetle outbreaks by stating, “Effectively communicating [to the public] the role of bark beetle disturbances in promoting forest rejuvenation” is an important aspect of forest management.
References:
Next Section on wildfires:
Bark beetles and forest ecosystems in North America
Bark beetles have caused billions of dollars of economic damage to US and Canadian forests in recent decades. Bark beetle populations expanded dramatically in the 1980s. Billions of coniferous trees have been killed by bark beetles since that time (Bentz, et al., 2010). These beetles have decimated over 40 million acres of forests over the past few decades (Oatman, 2015; USDA Forest Service, n.d.).
The rapid spread of these infestations has swept across the Southern States, the Northeast, Midwest, and the Western United States and Canada, all the way to Alaska and Mexico.
In the Southeastern U.S., a group of five native species of bark beetles have caused massive loss of pine trees throughout the southern states: the southern pine beetle (Dendroctonus frontalis), the eastern six-spined engraver (Ips calligraphus), the eastern five-spined engraver (Ips grandicollis), the small southern pine engraver (Ips avulsus), and the black turpentine beetle (Dendroctonus terebrans), collectively called the “southern pine bark beetle guild.” Several of these species may simultaneously infest a single stand of trees (Coulson & Klepzig, 2011). Of these five species, the southern pine beetle has caused the most damage. In the south, the bark beetles infest mainly the yellow pines, loblolly and shortleaf, but many other pine species can also be infested. These beetles have infested at least 16 species of pines, although certain species seem to be more susceptible.
While all five of these bark beetles are found throughout the Southeast, several have ranges extending throughout the Northeast and Midwest.
Ips ponderosae is found in the Western states of Montana, Wyoming, Colorado, New Mexico, Utah, South Dakota, Arizona, Nebraska. Ips calligraphus is found in a small area of California (Connor & Wilkinson, 1983).
The mountain pine beetle (Dendroctonus ponderosae), a beetle that ranges throughout the Western States and Western Canada, infects a number of western pines, including ponderosa, lodgepole, sugar, and western white pine. “High-risk lodgepole pine stands have an average age of more than 80, an average diameter at breast height of more than 8 inches.” High-risk secondary growth stands of ponderosa pine have an “average width at breast height more than 10 inches.” Mountain pine beetle has caused the loss of millions of lodgepole and ponderosa pines in recent decades (Amman, McGregor, & Dolph, 1989). It has been common in recent years to drive by entire hillsides covered with the red or yellow color of dead needles on trees. Several Ips species also cause damage to western pines, such as Ips confusus on piñon pines.
In addition to the pine bark beetles, the spruce bark beetle (Dendroctonus rufipennis) has caused severe damage in Alaska, Montana, Utah, Idaho, Arizona, and British Columbia (Holsten, Thier, Munson, & Gibson, 1999). The damage has been most extensive in Alaska and British Columbia, the locations where there are the largest stands of spruce trees. By 2010, spruce bark beetle activity was visible by aerial mapping over Alaska, showing the extent of infestations to have reached over six million acres. Over about 40 years, the spruce bark beetle caused the loss of over three billion board-feet of timber in Alaska (Alaska Department of Natural Resources, n.d.). While this beetle can infest any spruce species it encounters, the most economically valued species have been white, Sitka, and Engelmann spruce. Large diameter, slower growing spruce tend to be more susceptible (which may explain why the slender black spruce, abundant in Alaska, has not been severely affected). Unlike the situation with the southern pine forests, spruce forest infestations do occur in old-growth forests, but mainly in locations where spruce is the main type of conifer present.
The northern spruce engraver (Ips perturbatus) also affects spruce trees. Douglas firs can be affected by Dendroctonus pseudotsugae. Larch trees are affected by Dendroctonus simplex. And several species of firs are affected by the fir engraver (Scolytus ventralis).
But this is a book about invasive plant species. So why have I included a chapter on bark beetles? The answer is really quite simple. Rich Prodgers, a Montana botanist, once told me, “It’s not that we have an invasion of bark beetles that is the problem. The problem is that we have an invasion of yellow pines.”
The bark beetles causing the most destruction are not introduced insect species. The bark beetles co-evolved with many of the tree species they infest. In other words, these beetles have been a natural part of the forest ecosystem for many thousands, if not millions, of years. Yet they are viewed by many today as pests.
But let’s set the record straight. Bark beetles are a natural part of the forest thinning and decomposition processes. They speed up the decomposition of trees, particularly trees that have been weakened by lightning strikes, fire, disease, wind storms, and drought.
Researchers have recently used tree-rings and lake sediment cores to estimate the frequency of large-scale infestations over previous centuries. These studies have revealed, for example, that in the past, spruce beetle outbreaks occurred 50-80 years apart for white spruce, 70 years apart for Norway spruce in Europe, and 120 years apart for Engelmann spruce (Morris, et al, 2016). While it is apparent that the length of time between outbreaks was longer in previous centuries, little is yet known about the intensity of these previous outbreaks and little is known about forest recovery time after these outbreaks. But what is clear is that today the time period between extensive outbreaks has been shortened. And the intensity of recent outbreaks is thought to be ten times greater than in previous centuries (Oatman, 2015).
What factors have contributed to a shorter period of time between outbreaks, and the high-intensity outbreaks we have witnessed in recent decades? While various factors are known, large beetle infestations were relatively rare in the past, and the likelihood of infestation within a specific softwood population remains unpredictable. But the following human-induced conditions appear to be the leading causes of recent widespread bark beetle infestations:
- Monocultures
- Poor logging practices
- Fire suppression
- Climate change
Monocultures
In a natural forest ecosystem, a forest is composed of a variety of species of trees, including both softwood (conifers) and hardwood (broadleaf) trees. In addition, a wide variety of herbaceous plants, shrubs, vines, wildflowers, fungi, etc. are an important part of the system.
The southern forests are very different today than when bark beetles and pine species co-evolved. The southern forests in the past were not small monocultures, as they tend to be today. They were vast forests with mixed hardwoods and softwoods.
The southern forests of 200 years ago were mixtures of tree species – including oaks, hickories, and longleaf pine. The mixture of tree species modified the proximity between similar species of trees, making it more challenging for beetles to find another pine to invade. For example, research on the southern pine beetle (SPB) demonstrates that, while the SPB can fly as far as 2.6 kilometers, the SPB is most likely to infest adjacent trees that are no more than 6-7.5 meters away (Coulson & Klepzig, 2011, p. 29).
Most old-growth southern forests were destroyed in favor of agricultural crops. As early European populations grew and crop sizes grew, particularly tobacco, rice, and cotton, farmers burned down many existing forests to enable farming and to provide forage for livestock. By the end of the 19th century and in the early 20th century, “the remaining southern forests were extensively logged to support economic expansion” (Stanturf, Wade, Waldrop, Kennard, & Achtemeier, 2002). In the 1940s-1970s, herbicides were also used to destroy forests to make way for agriculture and to make way for monocultures of fast-growing conifers (refer to the section in these webpages on the use of herbicides for deforestation).
Today’s southern forests primarily consist of early successional pines, such as loblolly and shortleaf pine (species of yellow pines more susceptible to the southern pine beetle than longleaf pines). In addition to a more rapid growth and higher susceptibility to beetle infestations, these species also burn more readily than the longleaf pines and the hardwood trees that were prevalent in the old-growth forests (Stanturf, Wade, Waldrop, Kennard, & Achtemeier, 2002). Trees damaged by wildfire are particularly susceptible to beetle infestations.
The US Forest Service, combined with pressure from agricultural interests and the paper and lumber industries, has allowed our previously diverse forest ecosystems to gradually be transformed into monocultures of conifer trees. Fast-growing conifers have been favored by these industries, to the detriment of natural ecosystems, which originally were comprised of a rich tapestry of a variety of species of both conifers and hardwoods. In October, 2020, President Trump opened the Tongass National Forest in Southeast Alaska to road building and lumbering. The Tongass is an old-growth temperate rain forest and the largest national forest in North America. If not stopped, this could have a devastating impact, as the forest serves as a crucial carbon sink needed to alleviate the impacts of climate change. It also provides important habitat for trout, wild salmon, brown bears, and many other species.
Secondary growth pine and spruce forests (forests that have been lumbered out and replanted) have far less diversity both in species and age class, and are thus more susceptible to future beetle infestations.
In the past, within a naturally diverse forest, beetle populations remained under control because the host species were separated by some distance, with other species of trees between them. The distance and the barrier provided by other tree species helps keep the beetles from spreading. When diverse old-growth forests are replaced with few species of pines or spruce, and the trees grow closer to each other than would occur naturally, bark beetles proliferate more easily. The close proximity of the trees facilitates the beetles in spreading rapidly from tree to tree.
Poor logging practices
Bark beetles are able to spread more readily when loggers leave behind large amounts of fallen lumber or debris, when loggers leave felled trees close to live trees, or when loggers damage nearby trees or cut into roots of unfelled trees (Connor & Wilkinson, 1983).
Logging practices that foresters claim can decrease the likelihood of future large-scale beetle infestations:
- Rapid removal of debris and felled timber
- Making sure piles of felled timber are stored at a distance from live trees
- Thinning the secondary growth stands to increase the distances between susceptible species
- Patch-thinning, to allow the development of diversity of age and size classes, in secondary growth conditions
- Care in using equipment to avoid scarring or cutting into roots of live trees
- Thinning out, salvaging, or destroying damaged trees
Diana Six, entomologist at the University of Montana-Missoula, and Eric Biber, a law professor at UC-Berkeley, reviewed the research (Six, et al., 2014) on the effectiveness of treatments used to manage bark beetles and concluded that the recommended logging practices have had minimal beneficial effects. Focusing on research on the mountain pine beetle, they found that, although bark beetles have been part of the forest ecology for thousands of years, and, “While considerable research has been conducted on controlling bark beetles, massive gaps in knowledge remain. In particular, there is a disturbing dearth of rigorous replicated empirical studies assessing the effects of various management strategies, particularly timber harvest treatments, for bark beetle outbreak suppression. Even fewer studies have focused on how such treatments meet explicit goals or affect forest structure, function and future outbreak dynamics.” They point out that climate change has contributed to bark beetle expanded range and greater impacts. Higher temperatures make it easier for bark beetles to infest trees, as the higher temperatures result in less resin, resin that would normally create a chemical challenge to beetles.
Direct controls, which include removal of infested trees, clear-cutting, prescribed burns, fell and burn, trap trees (trees baited with pheromones), debarking, or the use of insecticides and toxins, are “extremely expensive in time, effort and resources,” and have to be repeated year after year. Six, et al., 2014, found that few rigorous studies have been conducted on the effectiveness of these strategies. Direct treatments may only serve to delay infestations. The few studies that have been conducted, mainly in Canada, have shown quite variable results, ranging from no effect to only marginal effects.
Indirect controls include thinning forests, daylighting (removing enough trees and shrubs to expose remaining trees to sunlight), removing mature and old growth trees, or replacing trees with different species of trees. Most of these result in large timber harvests. Six, et al., 2014, found similar problems with studies on the effectiveness of indirect treatments, too few studies, poor monitoring of the impact of treatments, and variable results. I recommend reading their review of the research. They found that, “long-term replicated studies monitoring beetle responses to thinned forests from non-outbreak to outbreak to post-outbreak phase are virtually non-existent.” In regards to daylighting, they found no published studies of its effectiveness. Six, et al., 2014, found that there has been little monitoring of the impact of tree removal practices, and failures have often not been documented. They concluded that removing trees “often removes more trees than the beetles would.” Unfortunately, much of the funding of studies on these treatments are short-term, often only one to two years.
Six, et al., 2014, point out that, “Funding cuts to research personnel, particularly in agencies like the US Forest Service, have exacerbated this problem exactly at the time when the need for rigorous research is increasing at a rapid pace. The US Forest Service has recognized that long-term planning must include explicit goals to increase forest resilience and adaptation to disturbance, including outbreaks of the mountain pine beetle. However, with extreme cuts to budgets and personnel, they are highly constrained to meet these needs at this time. Likewise, cuts in federal funding to agencies such as United States Department of Agriculture and the National Science Foundation concurrently reduce the ability of academic researchers to address these problems.”
Six, et al., 2014, point out that, “These studies highlight a seldom considered impact of mountain pine beetle-that it can act as a natural thinning agent and seldom removes all mature trees during outbreaks. These effects are an important part of the ecological role that the beetle plays in western pine forests…. Although beetle outbreaks, like fire, can have negative impacts on timber values and aesthetics, their natural role in many forest ecosystems is seldom considered and beetle suppression is often perceived as something that must be conducted at all costs. However, as with fire, suppression of beetles over the long term may alter forests in ways that are not desirable or sustainable.”
Six, et al., 2014, conclude that, “Our survey of the relevant literature finds that there is significant uncertainty about whether the most commonly used beetle timber harvest treatments are, indeed, effective. Yet there has been little discussion of this uncertainty in the relevant policy debates. Politicians have instead latched on to beetle timber treatments as a cure-all for beetle infestations and have pushed to weaken or eliminate environmental laws that are perceived to be obstructing these treatments. Agencies such as the US Forest Service, to their credit, have been more nuanced in their support for bills that package beetle timber harvest treatments with weakened environmental laws; they have opposed several proposals to alter environmental laws to allow more treatments, but on the other hand, the agencies have at times also aggressively pushed for the implementation of treatments.
“It seems clear that the policy debates–both in the agencies and in Congress–need to be better informed by science. Researchers should be more proactive in communicating their understandings of the current science to policymakers. This does not mean that researchers need to take a position pro or con vis-à-vis beetle treatments, or even vis-à-vis specific legal proposals. In the face of uncertainty, aggressive beetle timber harvest treatments may be warranted in some instances. However, policymakers should be aware of uncertainty when they are making the relevant decisions and should also be more willing to include the voices of scientists in the development of policy.
“Given the uncertainty about the effectiveness of many beetle timber harvest treatments, the high financial costs of those treatments, the impacts on other environmental resources and values, and the possibility that in the long-run those treatments may interfere with the ability of North American forests to adapt to climate change, our position is that weakening or eliminating environmental laws to allow more beetle timber harvest treatments is the wrong choice for advancing forest health in the United States.”
Fettig, et al., 2014, dispute some of the claims of Six, et al., 2014, by pointing out that Six, et al., 2014, had missed some important research. Fettig, et al., 2014, argue for the value of some treatments, particularly forest thinning for its value, not just for beetle suppression but also for reducing fuels to reduce fire hazard. Their counter-arguments are worth reading.
Forest fire suppression
Native American tribes used controlled burns for thousands of years, to manage the landscape. Beginning about a thousand years ago, as eastern and southern tribes engaged in cultivation of native plants, fire was used to clear small tracts of land for agriculture and forage and to provide an open area for defense purposes. Early Europeans followed similar practices for the first few centuries they inhabited the New World. But in the southern and eastern forests, as populations and demand for crops grew, Europeans burned down many existing forests to enable farming and to provide forage for livestock (Stanturf, et al., 2002).
The US Forest Service was created in 1905. At first, it was severely underfunded by Congress (PBS, Ordeal by Fire). A massive northwestern fire occurred in 1910, covering about 3 million acres in the National Forests in northeastern Washington, northern Idaho, and western Montana. To combat the fire, the USFS hired many untrained men to help out, many of whom died. This large fire caused enough concern that the US Congress significantly increased funding for the USFS, and allowed for the creation of many new National Forests, expanding that model into the Eastern US, which had few National Forest designations previously. While some people argued that fire suppression was the wrong strategy for management of fires, the USFS implemented a fire suppression policy aimed at stopping every forest fire immediately. “The Clarke-McNary Act of 1924 … provided Federal funding for State fire-control efforts… By 1930, 70 million acres were protected from fire; by 1980, over 233 million acres were protected” (Stanturf, et al., 2002).
Fire suppression policy ignored scientific knowledge of the important role fire plays in forest ecosystems. As a result of decades of fire suppression, a massive amount of deadwood developed in the forests. Deadwood provides an avenue for rapid spread of bark beetles. The presence of so much deadwood, coupled with drought, resulted in the massive forest fires that have plagued the West since the 1980s. The fires in Yellowstone National Park in 1988 are a prime example of the devastation caused by these conditions.
It was not until the 1970s that the USFS started allowing some forest fires to burn. And while the USFS has implemented some prescribed burns, the rapid expanse of population centers across the country has resulted in much more restriction on the implementation of prescribed burns.
The decades of fire suppression allowed softwood species that normally were kept at bay by natural fires, to increase in number in comparison to more fire resilient species. The massive fire in Bastrop, Texas, in 2011 is a testament to what happens when fire suppression is followed by increased temperatures, leading to severe drought.
Since 2015, California and the northwestern states have experienced unprecedented summer temperatures and drought conditions that have enabled large fires to develop. Decades of fire suppression in Federal and State forests have provided massive amounts of fuel that has resulted in much larger fires than would occur in a natural ecosystem. And as more communities have grown up close to the forests, the result has been billions of dollars of structural damages in communities close to the forests, as well as the loss of lives, as fire frequency and intensity has increased.
Climate change
Increased temperatures and increased drought have already been shown to increase the spread of bark beetles (Bentz, et al., 2010). For example, the southern pine beetle can reproduce between one and nine times in a year, depending on the climate (Coulson & Klepzig, 2011, p. 13). Increased temperatures increase the number of new beetle generations per year. The southern pine beetle is native to the United States and Central America. In the US, southern pine beetles (SPB) range from New Jersey to Florida, and throughout the south as far west as Arizona.
In the southernmost parts of its range, seven to nine generations can emerge in a year. In years when increased temperatures allow multiple generations of beetles even in the northernmost parts of its range, the beetles can overcome the natural chemical defenses of even healthy trees. This single species can cause “periodic large-scale eruptions that encompass entire regions of the South” (Coulson & Klepzig, 2011, p. 13).
The northern limits to the range of the SPB appear to be climate specific. In winter, southern pine beetles begin to die off with temperatures below about 10 degrees Fahrenheit. At temperatures below 3 degrees, near total mortality occurs within a population. Insect mortality also tends to increase in temperatures above 86 degrees.
The western pine beetle (Dendroctonus brevicomis) and the piñon Ips (Ips confuses) can produce more than one generation per year. The spruce beetle and the mountain pine beetle can take one-three years to go through a single generation. But these cycles are regulated by temperature and seasonality of those temperatures. In other words, as regional temperatures increase, seasons also change, with temperatures warming up earlier in the spring, and cooling down later in the fall (Bentz, et al., 2010).
Thus, we can expect various bark beetle species to move further north and to higher elevations as global warming continues, and we can expect beetles to produce more generations per year. In addition, higher temperatures can increase the drought stress on trees, another factor that makes the trees weaker and thus more susceptible to infestation. In 2011, scientists observed that mountain pine beetles have expanded their range and infested jack pines, with which they did not co-evolve. This means that jack pines do not have natural deterrents to this species of beetle (Oatman, 2015).
Climate change is already causing disruption to ecosystems worldwide. In the Western US, higher temperatures, combined with fire suppression policies, and the large increase in monocultures, in grasslands, sagebrush habitats, and forests, has allowed fires to become a greater problem every summer. The higher temperatures have also allowed bark beetle populations to flourish.
Bark beetles: What does the future hold?
The bark beetle infestations of recent decades are a problem of our own making. We are, to put it in the words of William Shakespeare, hoist with our own petard.
So what should we do to avoid such frequent high-intensity outbreaks in the future?
Insecticides have limited value in the control of large-scale infestations. They are most effective in preventing an infestation. Once a widespread infestation has occurred, insecticides may be ineffective and the cost of such treatment may be higher than the value of the timber. The use of insecticides is also not effective, after an infestation takes hold, without collaboration between all landowners in the area. Without appropriate silviculture, insecticides are not effective (Amman, McGregor, & Dolph, 1989). Synthetic beetle attractants have limited value, except in managing small outbreaks.
Infested forests may be more fire prone, but we have to keep in mind that fire is nature’s way of cleaning house. Recent devastating fires in the West have increased in frequency and intensity for several of the same reasons beetle infestations have increased: historical fire suppression, the rise of forest monocultures, and climate change. Were it not for human encroachment into forest habitats, forest fires perhaps would not be perceived as the evil they seem to be viewed as today.
Morris, et al., 2016, point to research that suggests beetle infested forests can re-generate without any human intervention. Unless we are willing to tackle the big picture factors described in this section, it may be that doing nothing is better than doing something. Morris et al., 2016, concluded their analysis of the research on beetle outbreaks by stating, “Effectively communicating [to the public] the role of bark beetle disturbances in promoting forest rejuvenation” is an important aspect of forest management.
References:
- Amman, G.D., McGregor, M.D., & Dolph, Jr., R.E. (1989). Forest Insect and Disease Leaflet 2. Mountain Pine Beetle. USDA Forest Service. Washington, D.C. Retrieved August 3, 2018 from https://www.barkbeetles.org/mountain/fidl2.htm
- Bentz, B. J., Regniere, J., Fettig, C.J., Hansen, M., Hayes, J.L., Hicks, J.A., Kelsey, R.G., Negron, J.F., & Seybold, S.J. (2010). Climate change and bark beetles of the western United States and Canada: Direct and indirect effects. BioScience, 60(8): 602-613.
- Connor, M.D., & Wilkinson, R.C. (1983). Forest Insect and Disease Leaflet 129. Ips Bark Beetles in the South. USDA Forest Service. Washington, D.C. Retrieved July 31, 2018 from
https://www.barkbeetles.org/ips/ipsfidl.htm - Coulson, R.N., & Klepzig, K.D., Eds. (2011). Southern Pine Beetle II. General Technical Report SRS-140. USDA Forest Service, Southern Research Station. Asheville, NC.
- Holsten, E.H., Thier, R.W., Munson, A.S., & Gibson, K.E. (1999). The Spruce Beetle. Forest Insect and Disease Leaflet 127. USDA Forest Service, Washington, D.C.. Retrieved July 31,2018 from https://www.barkbeetles.org/spruce/sbfidl127.htm
- Morris, J.L., Cottrell, S., Fettig, C.J., et al. (2016). Managing bark beetle impacts on ecosystems and society: priority questions to motivate future research. Journal of Applied Ecology. British Ecological Society. DOI: 10.1111/1365-2664.12782
- Oatman, M. (May/June, 2015). Bark beetles are decimating our forests: That might actually be a good thing. Mother Jones.
Next Section on wildfires: