Numerous insect epidemics appear to be damaging North American forests at unprecedented levels. Many fear that the dying trees will further increase wildfire threats, alter ecosystems, and disrupt wood supplies for current and future housing and other uses. In the past, Congress has created special research or control programs for insect epidemics. The continuing concern over wildfires and forest health may lead to congressional efforts to combat the epidemics. Some also assert that insect epidemics are exacerbated by global climate change, and that they may even contribute to that change. This report examines the mountain pine beetle ( Dendroctonus ponderosae ) epidemic because it is one of the most damaging insects affecting forests and because more is known about its life cycle than the cycles of many other forest pests. The mountain pine beetle is one of a number of bark beetles native to the western forests of North America, several of which are currently infested at epidemic levels. Mountain pine beetles inhabit most species of western pine, including ponderosa pine, white pine, sugar pine, limber pine, and whitebark pine, but epidemics are particularly associated with lodgepole pine. Lodgepole pine is a common tree from the Colorado Rockies and Sierra Nevada north to the Yukon Territory. It typically grows in dense, even-aged stands that have regenerated following intense wildfires that killed the previous stand. The mountain pine beetle is a seasonally adapted insect, successfully reproducing and occasionally reaching epidemic levels where it is univoltine —completing an entire life cycle in one year. The majority of its life is spent as larva in the phloem (innermost bark layer) of pine trees. The eggs hatch 10-14 days after being laid, and the larvae feed on the phloem. In the fall, the larvae produce glycerol , a natural antifreeze that allows them to survive winter temperatures. Larvae are vulnerable to frosts in the fall, before producing glycerol, and when they pupate. During winter, temperatures below -30°F for at least five days can kill most larvae. Larvae pupate in very late spring or early summer (usually June). Adult mountain pine beetles usually emerge in midsummer, late enough to avoid harm from late spring frosts but early enough to allow eggs to hatch into larvae; ambient air temperatures determine the timing of emergence. The adults disperse, with preference for trees of larger diameter (thicker trees have thicker phloem, and thus more food) and trees under stress (e.g., injured, diseased, or suffering from drought, and thus offering less resistance to attack). Trees less than five inches in diameter are rarely attacked, even in an epidemic. Females first attacking a tree release chemicals called aggregating pheromones , which attract males and other females in a mass attack on the tree. The beetles cannot readily attack and kill any tree. Native predators, notably woodpeckers and clerid beetles, help to control populations. In addition, the trees "have evolved significant defensive chemistry that serves to protect them from beetle attack. The mountain pine beetle, in turn, has evolved a mass-attack strategy that overwhelms tree defenses through sheer numbers of attacking beetles." Also, the beetles introduce a blue-stain fungus into the trees they attack, which clogs water transport systems and contributes to killing the trees. Thus, trees under stress—from drought, disease, injury, or other cause—are more susceptible to attack by the mountain pine beetle and blue-stain fungus. When a widespread stress (e.g., a regional drought) affects a forest with many relatively large-diameter lodgepole pine trees, the stage is set for a mountain pine beetle epidemic. Mountain pine beetle epidemics have occurred in lodgepole pine forests for thousands of years. Epidemics lasting 5 to 20 years occur at irregular intervals, affecting large areas and often killing more than 80% of the trees of more than 10 centimeters (about 4 inches) in diameter. There is wide variability in the mortality of pure lodgepole pine forests, and even forests that appear to be completely killed will have lodgepole pine seedlings and saplings released from the competition in dense stands. There is no single, simple cause for a population to reach epidemic levels. Research has provided information about the population ecology of the mountain pine beetle in lodgepole pine, but not about the transition to epidemic populations. Widespread stress in a mature forest clearly provides a setting for an outbreak, but no specific trigger has been identified. One source suggests that prolonged warm periods, especially with warm winters, may trigger epidemics, but the evidence is not conclusive. During epidemics, natural controls (e.g., woodpeckers and clerid beetles) have little effect on mountain pine beetle population levels. The current mountain pine beetle epidemic is actually three geographically or ecologically distinct epidemics, with quite different situations and consequences. One is in the lodgepole pine stands of the central Rocky Mountains—primarily in Colorado and Wyoming. Another is in the lodgepole pine stands of central British Columbia. The third is in high-elevation pines, primarily in Wyoming, Montana, and Idaho. The current mountain pine beetle epidemic in Colorado and Wyoming is extensive, but it is unclear whether the current level is unprecedented. Most researchers note that mountain pine beetle epidemics are known to have occurred in lodgepole pine forests, but that the current epidemic is more extensive than has been seen in the past century. However, one source noted the loss of 15 billion board feet of lodgepole pine timber from mountain pine beetles in Idaho and Montana from 1911 to 1935. Although the current epidemic is extensive, it may be normal and natural. Some researchers have stated: Even though insect outbreaks greatly affect forest ecosystems, they may not be detrimental from a long-term ecological perspective. Such disturbances may in fact be crucial to maintaining ecosystem integrity, a situation ... described as "normative outbreaks." The current epidemic is extensive, largely because vast areas of lodgepole pine provide suitable habitat for the mountain pine beetle. This expanse of mature lodgepole pine forests in the central Rocky Mountains is the result of natural but widespread severe wildfires in the 19 th century. Thus, while the current epidemic is unsightly and may have significant consequences (as discussed below), it is an extreme version of an undesirable but normal, natural event. The epidemic may have been exacerbated by climate change. Climate change may have altered precipitation patterns, contributing to the current extended drought in the Rocky Mountain region. This drought has put stress on the trees, leaving them vulnerable to attack by insects and diseases, including the mountain pine beetle. Thus, climate change may be a contributing factor in the mountain pine beetle outbreak in the lodgepole pine ecosystems of the central Rockies. The interior forests of British Columbia (BC), stretching west from the Rocky Mountain crest to the east side of the Coast Range, are similar to U.S. forests of the Rocky Mountains, and include extensive stretches of mature lodgepole pine. As with the lodgepole pine forests of the central U.S. Rockies, the mountain pine beetle is an endemic (native) species, with periodic epidemics. However, the current epidemic is the most severe ever recorded, and is expected to kill up to 80% of the mature lodgepole pine in BC within the next decade. The current mountain pine beetle infestation has reached farther north and east than previous epidemics. This is likely the result of climate change leading to insufficient freezing temperatures farther north than previously recorded. Thus, the mountain pine beetle can become univoltine (completing its life cycle in one year) farther north than previously recorded. While the mountain pine beetle outbreak in central and southern BC may be within historic norms, as in the central U.S. Rockies, the epidemic is likely to reach farther north than its historically normal distribution because of climate change. Mountain pine beetles are also endemic to whitebark, bristlecone, and other high-elevation pines. These trees inhabit upper mountain slopes (together with spruce and subalpine fir), and at the extreme grow in the krummholz form (German for twisted wood )—the classic stunted, wind-swept pines of timberline. Whitebark pine forests have generally been "climatically unsuited for outbreak populations of the mountain pine beetle." The high elevations have typically led to short summers (preventing univoltine populations) and to cold winters (which kill mountain pine beetles). However, a beetle epidemic in whitebark pine in central Idaho in the 1930s coincided with a series of unusually warm years. The dead trees are still standing, a stark reminder that not all forests are adapted to recover from devastating natural events. As with the mountain pine beetle outbreak in extreme northern BC, climate change is likely a significant factor in the current epidemic in high-elevation pine ecosystems, because warmer temperatures allow the mountain pine beetle to become univoltine in these areas. While insignificant for timber, the whitebark pine is a critical element in the northern U.S. Rocky Mountain ecosystems, because it produces an abundance of the large, high-protein seeds that are important mainstays for several animal species, particularly grizzly bears. The loss of whitebark pine, a key food in the fall prior to hibernation, could devastate bear populations by lowering reproductive success and forcing grizzlies to forage at lower elevations where there would be more human conflicts. These forests and animals are American symbols of wilderness and endangered species, and their decline might harm these values. Little, if anything, can be done to halt or disrupt a mountain pine beetle epidemic.