For animals roughly the size of a grain of rice, bark beetles can exact a disproportionate effect on the landscapes they inhabit. In the face of a changing climate, these tiny insects are calling an entire philosophy of forest management into question, redirecting the conversation from artificial control by harvesting timber to natural control by maintaining genetic diversity.
A short trip east of Fresno, California to the western Sierra’s Shaver Lake basin helps illustrate the impact bark beetles can have on a forest. As you leave the oak woodland of the foothills and climb into the conifer forest surrounding Shaver Lake, large stands of ponderosa pines (Pinus ponderosa) come into view. The needles on many of these trees are amber colored, creating a landscape reminiscent of fall foliage. These pines, however, are evergreen. They are supposed to maintain green needles year round. Their abnormal amber color is the product of what has become an unprecedented outbreak of bark beetles.
Bark beetles are phytophagous, which means they eat plants. Specifically, they eat the inner bark or cambium layer of trees. The cambium layer is where trees transport their water and nutrients. If the beetles severely damage the cambium layer, the trees die. Some 600 species of bark beetle occur naturally in the United States and Canada, with 200 species in California alone. This degree of diversity means that many beetles use specific tree species as hosts for their life cycle. Fir engraver beetles (Scolytus ventralis), for example, complete their life cycle in true firs, such as the red fir (Abies magnifica) and white fir (A. concolor) in California’s mid- to high-elevation forests. Western pine beetles (Dendroctonus brevicomis), on the other hand, typically target ponderosa pines, such as those around Shaver Lake.
The Bark Beetle Life Cycle
The life cycle of the western pine beetle is complex. Adult beetles possess specialized chemical receptors that allow them to recognize a host tree and determine its ability to effectively defend itself. Once a female beetle identifies a suitable host tree, she uses her strong mandibles (the insect equivalent of a jaw) to penetrate its outer bark. This boring causes the tree to exude resin, forming what is known as a pitch tube. If the beetle is able to successfully enter the tree, the resin becomes infused with boring dust, called frass, and the pitch tube takes on a reddish tint.
Once successfully inside, the female releases a chemical signal, called a pheromone, which serves to attract other beetles to the tree she just entered. If conditions are right, other beetles will detect her signal, target the same tree, and release additional pheromones, resulting in an infestation of the affected tree by many more beetles. This mass arrival and colonization of an individual tree helps overcome the tree’s natural defenses. As a tree’s defenses are depleted, beetles release another pheromone that transmits a “no vacancy” signal to latecomers. This pheromone therefore helps reduce competition among the beetle’s young, which will develop within the tree.
The ultimate goal of bark beetles is to reproduce. Once they’ve successfully colonized a tree, they lay eggs in the cambium layer. And even as they bore into the bark they leave a trail of food for their progeny in the form of fungal spores. Western pine beetles have evolved specialized pouch-like structures called mycangia, which help transport these fungal spores. As the beetles chew through the bark, fungal spores are deposited in their wake, resulting in a blue “stain” of the tree’s inner tissues. These fungi help convert the tree’s tissues into nutrients, providing a source of nutrition for the next generation. These fungi also play an important role in overcoming a tree’s natural defenses by blocking the conductive vessels that connect the inner wood and outer bark, resulting in death of the host tree.
Once successfully inside a tree, adult beetles deposit their eggs in chambers, called galleries, which are bored by the entering adult. In the case of the western pine beetle, each female deposits up to 60 eggs per egg-laying period. The eggs hatch after a period of 1-2 weeks. The emerging larvae then bore their own galleries, which branch from the egg-laying gallery in distinctive patterns. These patterns are so distinctive they can be used to identify beetle species. In the case of the
western pine beetle, larval galleries form maze-like burrows winding in all directions throughout the tree’s bark.
Pupation, the last stage of development before adulthood, occurs in enlarged chambers at the ends of larval tunnels. Once pupation is complete, the beetles enter standby mode, ready to emerge as soon as conditions are favorable, usually on warm days in late spring to early fall. To leave their host tree, this new generation of beetles bore exit holes. Because the beetles emerge en masse, the process creates a buckshot pattern that dots the outer bark of the host tree. The newly emerged beetles immediately seek their next host tree and begin the cycle over again. The entire cycle, from egg to adult, takes 2-10 months, depending on ambient conditions.
Western pine beetles typically produce one to two generations annually. However, their life cycle is highly temperature dependent. At lower elevations and latitudes, they can cycle through up to four generations in one year, with beetle activity beginning as early as March and lasting as late as November.
Trees are not entirely defenseless to bark beetle attacks. In fact, healthy trees can produce enough resin to eject or drown prospecting beetles. Resin production is a tree’s primary method of defense against bark beetle colonization. It serves to flush and seal wounds created by the bark beetle’s boring, preventing colonization of the tree and subsequent egg laying. Tree resin can also contain compounds that serve as a secondary defense against beetle colonization. Some compounds, for example, are toxic to beetles and others discourage the growth of fungi. Bark beetles also have natural predators, including other insects and birds. Woodpeckers remove the outer bark of colonized trees and consume the larvae sheltered inside. Any remaining larvae are then susceptible to desiccation because their primary protection has been removed.
If a tree produces its own defenses and natural predators exist, how have so many trees in the Shaver Lake basin fallen victim to these tiny insects? The answer to that question is complicated but related to current climatic conditions.
In recent years, drought and above-average temperatures have affected large portions of California and western North America. Because the bark beetle life cycle is highly dependent on ambient conditions, these climatic changes have altered beetle response to seasonal cues. Under normal conditions, cooler temperatures resulted in either beetle mortality or delayed maturity, a process known as diapause. The changing climate has altered these important and natural population controls, reducing cold-induced mortality or delaying diapause. Longer periods of warm temperatures and drier weather mean the season of adult flight activity is longer, adults begin laying eggs earlier in the year, and beetles develop faster, more often, and more successfully.
Drier and warmer climatic conditions have also impacted host trees and the susceptibility of entire forests to bark beetle irruptions. Drought can reduce resin pressure and production in trees, weakening their primary defense mechanism and thereby reducing the number of beetles needed to successfully colonize the trees. Warming temperatures have also led to bark beetle outbreaks at higher elevations and latitudes than previously recorded, including in areas where trees have not evolved adequate beetle defenses. Other factors that have weakened trees and made them more susceptible to beetle colonization include crowding, competition with other vegetation, root damage, root disease, and soil compaction. Along with climatic changes, these effects can have synergistic impacts, which in recent years have resulted in bark beetle outbreaks that are more extensive and acute than any previously recorded in western North America. As the climate continues to change, bark beetle outbreaks are expected to become more frequent and severe. This has called into question an entire philosophy of forest and bark beetle management.
Bark beetles have generated lively discussion on the topic of forest management. Overstocked stands of even-age trees have been cited as a primary cause of group killing of trees by bark beetles. Forest management practices such as fire suppression and timber harvest that create such conditions can intensify the extent and severity of bark beetle outbreaks. On the other hand, bark beetle outbreaks are often driven by climate and other factors not readily influenced by forest management practices. For this reason, little can be done to curtail bark beetle infestations once they begin. Insecticides and chemical control are largely ineffective because bark beetles spend most of their lives under the protection of tree bark. Instead, quickly removing, burning, or chipping infested trees is thought to be the most effective method of control. In addition, some forest management strategies and plans call for preventative and prescribed harvest to reduce the density of trees in otherwise healthy forests as a necessary method of bark beetle control. Despite these forest management recommendations, ecologists have begun looking at bark beetle related forest management more broadly.
Bark beetle irruptions have been a selective force in forests for millennia. While some view outbreaks as emblematic of a forest’s declining health, ecologists view these irruptions as a form of natural forest management. Recent research has suggested that genetic variation in tree species has resulted in differing levels of resistance and susceptibility to bark beetle colonization. Some trees, for example, appear to be less susceptible to drought stress than others. And some produce more toxic resin and allocate more energy to resin-duct production than others, allowing them to effectively ward off prospecting beetles. In this way, beetles and trees are engaged in a type of arms race, with forests containing more genetic, phenotypic, and age variation better able to withstand the effects of bark beetles and, by extension, climatic change. For this reason, ecologists advocate for forest management practices that promote a mosaic of forest habitat types. They also encourage that genetic and phenotypic variation be considered when making forest management decisions. There is still much to be learned with regard to forest management and bark beetles. Fortunately, this is an active area of research and one that is increasingly important in the face of a changing climate.
— Renée Robison, Staff Scientist
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