It was an ecological war zone. Anyone visiting the Kenai Peninsula in the 1990s could not ignore the fact that something big was afoot. Valleys and mountainsides bristled with the skeletons of dead and dying trees. The forest was in the throes of the worst spruce beetle outbreak ever recorded. Millions of acres hosted billions of beetles.

Today, the region still grapples with the repercussions of that infestation. Alaska’s uneasy relationship with spruce beetles is changing as the state warms, wildfires engulf forests and beetle populations soar.

Spruce beetles in Alaska

Spruce beetles (Dendroctonus rufipennis Kirby) are one of 600 species of bark beetle native to the United States. They occur from Arizona to Alaska and across Canada. Severe beetle infestations change forest composition and structure, alter wildlife habitat and water dynamics, and open the door to catastrophic fires. Today’s increasing temperatures are leading to more beetle outbreaks than ever before.

Spruce beetles live throughout all forested parts of Alaska and dine upon spruce. Preferred trees are large, old, diseased, or stressed. Drought, flooding, and high tree density make individual trees and entire forests more susceptible to invasion. In the 1990s, the Kenai’s high number of large spruce and a prolonged warming trend created the perfect storm for a spruce beetle attack. Between 1980 and 2003, this epidemic impacted over two million acres of Southcentral.

broad pass forest with trees killed by spruce beetles
Spruce beetles killed these trees in Broad Pass along the Parks Highway in Alaska. Courtesy Susan Sommer

No larger than a grain of rice with wings, spruce beetles boast a set of mandibles to bore into and devour trees. Small populations are always present in Alaska’s spruce forests, and under normal conditions pose little threat to the landscape, kept in check by woodpeckers, predatory beetles, and cold winters. Unhealthy conditions, however, can lead to infestations of epic proportions. 

Emerging in early summer, a female beetle selects a tree and drills through the thick outer bark into the phloem, the succulent inner bark that transports sugars from needles to roots. A healthy tree repulses the attack with a gush of sap, flushing the beetle out or suffocating her in sticky pitch, but an unhealthy specimen allows entrance. Once inside, the beetle releases a pheromone inviting others to join the banquet. She gnaws a tunnel through the phloem, pausing to lay up to 150 eggs along the way. When the eggs hatch, the grub-like larvae eat their own trails through the phloem, disrupting and eventually cutting off the tree’s flow of nutrients. A second pheromone is released once the tree is colonized, warning other beetles it has reached capacity. Eventually the tree starves to death. 

Warmer conditions

Such deaths may be attributed in part to climate change. “Changing climate conditions impact the health of the trees,” says U.S. Forest Service entomologist Jessie Moan. “Heat or water stressed trees will be more susceptible to beetles.” Stressed trees don’t have enough sap to flush out intruders and once invaded, can host over 100 beetles per square foot. “Spruce beetles use climatic cues during their lifecycle to indicate when they become active and progress through life stages,” Moan says. “Even short-term weather patterns can change their biology and that certainly impacts how, when, and where they are in the environment.” 

Today’s warmer conditions allow beetles to compress their two-year lifecycle into one, leading to higher numbers of mature beetles searching for fresh tree hosts. Once beetles hit the easy targets, they turn to healthy spruce and larch, Scotch pine, and ornamentals. Entire forests can succumb, leaving behind a serious risk of fire.

“The Kenai Peninsula is a fire-adapted environment where fire will always play a role,” says Mark Cahur, U.S. Forest Service Alaska Region hazardous fuels coordinator. “Natural ecosystems require disturbance to renew and manage themselves, which on the Kenai currently includes both fire and beetle activity.” Beetle-killed forests are open and grassy with dead fallen trees, a perfect combination to fuel hot, fast-moving fires that are difficult to control. Over the past decade, the Kenai has experienced large fires, which Cahur believes could become more common as beetles ravage Alaska’s forests. He points out, however, that “the changes brought forth through the presence of fire can be considered natural as the forest goes through its lifecycle. Fire is providing for new growth and enhanced habitat.”  

Mitigation and management

fireman cuts down a tree killed by spruce beetles
The Gannett Glacier Fire Crew recently cut down 665 beetle-killed spruce and other hazard trees along a trail at the Russian River Campground in Cooper Landing as a part of a spruce beetle mitigation project for the Chugach National Forest. Courtesy Alaska Division of Forestry

Moan and Alaska’s Forest Health team conduct annual aerial detection surveys, flying over damaged forests and mapping conditions from the air. Their findings show that Southcentral’s current infestation, which began in 2016, has hit the Matanuska-Susitna valley the hardest, especially near the Susitna River. Today, over one million acres of Southcentral are experiencing beetle disturbance. As forests recover, Moan will study vegetative composition, curious to see if birch will replace spruce in some areas. While most outbreaks last five to 10 years, the current infestation could go on much longer. 

Major outbreaks are not fresh news, however. Scientists look back in time by studying dendrochronology. Reading tree rings from the past two centuries, large infestations occurred in Southcentral in the 1810s, 1870s, 1910s, and 1970s. Tree rings show these outbreaks followed a year or more of warm dry weather. Based on current evidence, Moan believes “we may have shorter intervals between outbreaks and might see more frequent outbreaks depending on host availability.”

Alaska, like much of the West, is struggling to find effective management for bark beetles. Strategies include harvesting infested trees and experimenting with semiochemicals (behavior-modifying chemicals which reduce beetle attacks). Pesticides are used to protect individual high-value trees in populated areas.

By doing what they do best — eating — spruce beetles are modifying Alaska’s forests. A drive down the Kenai Peninsula shows a blackened landscape burned by recent fires but punctuated by lush new vegetation and stands of infested trees. The coming years will see more tree mortality across Southcentral. “The landscape and viewshed are ever changing through natural disturbances, whether one week or 30 years after fire or beetles,” says Cahur. “Change will always be occurring.”


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