By Katy Rende
It’s about the size of a pinhead or a grain of rice.
But forest officials say that billions of the little bugs, unleashed by a warming climate, are a monster already transforming the majestic subalpine forests of the San Juans.
And the infestation has the potential to destroy up to 90 percent of San Juan County's spruce and pine stands in coming years.
The spruce beetle, one of less than one percent of the 6,000 species of bark beetles that is able to kill living trees, has already descended upon the Eastern San Juans in massive numbers, where they left behind entire mountain sides of brown pines ready to ignite.
The epidemic started in the mid- to late-1990s throughout the West. That was about the time when the climate started shifting into warmer temperatures and dryer seasons, according to Forest Service officials. While bark beetles have always existed in the Rockies, their populations are kept in check by freezing temperatures, tree defenses, natural enemies, habitat supply, and wildfire.
These problematic beetles, termed aggressive beetles, have responded to this global climate change all over the West. Here in the San Juans, the dominant aggressive bark beetle is the spruce beetle. It bores into our primarily Engelmann spruce forests, leaving only the smaller Douglas firs standing underneath the towering dead giants.
Depending on the ecosystem, many other aggressive bark beetle populations have grown to epidemic sizes in forests all over North America. For example, Northern Colorado was hit hard in recent years by the mountain pine beetle.
Each type of beetle has a different life cycle that affects how quickly it spreads. The spruce beetle only reproduces every two years. In contrast, some species reproduce up to three generations in a single year. So, while many other ecosystems have already experienced the devastation of these insects, the San Juans are in the middle of a slower migration of beetles, primarily from the east.
While beetles are a normal presence in the spruce-fir forests, Steve Hartvigsen, supervisory forester for the Pagosa and Columbine ranger districts of San Juan National Forest, believes the population exploded after the winter of 2005.
That unusually wet, heavy winter left snow clinging to tree branches well into spring. Unaccustomed to carrying such weight, many live trees fell to the forest floor. In addition, numerous heavy avalanches created sprawling fields of debris.
Spruce beetles usually only burrow into recently fallen trees, but the overwhelming abundance of grounded habitat that summer allowed for greater reproduction. It is believed that the enlarged population moved on to standing trees the following seasons. Higher winter temperatures allowed them to survive under the bark of existing conifers without the added insulation from the ground they normally depend on.
Winter temperatures need to reach -40 F for several weeks to sufficiently kill the beetle larvae under the thick bark. San Juan County has not experienced those temperatures in years.
Sub-adults will actively increase survival rates by exiting the tree's bark and re-burrowing at the base of the tree where the snowpack provides more insulation.
“Hence, the best scenario for freezing these little buggers is a winter with very cold temperatures combined with minimal snowpack,” Hartvigsen explained.
“Like many insects that have multi-year life cycles, spruce beetles metabolize their own glycol (aka antifreeze) to survive cold temps,” Hartvigsen said. “If a very cold period hits early or late relative to winter, this can also catch these insects before glycol metabolization, or after they have shifted back to typical spring/summer bodily fluids. But, we so rarely hit the temps needed to knock them back.”
Larger, older pines have both lower defenses against the beetles as well as thicker bark, providing better shelter against the cold and the beetle's predators, which include woodpeckers and other insects. So beetles tend to attack trees between 80 and 100 years old and with a diameter of at least 8-10 inches. The San Juan Mountains are dominated by trees of this age due to fire suppression efforts that began in the late 1800s.
The trees have two lines of defense against the beetles. The first is resin, called the “pitch out” phase, during which the tree uses sap to physically push the beetle out of the tree. The second defense is the tree toxins, which is the pine smell coming from the wood. It is a natural pesticide.
Tree age, overcrowding, water stress, and air composition can all decrease these tree defenses. In fact, in severely water-stressed seasons, some trees can’t produce any sap to push the beetles out, resulting in one whole line of defense shut down. Hartvigsen said he noticed this in 2002 and 2003, when many trees had beetle holes without any pitch out.
Once the beetles locate a vulnerable tree, they release aggregation pheromones that tell other adult beetles to colonize the tree. They swarm, burrow, and lay eggs. A concentrated attack has a greater chance of overcoming the tree’s defenses. After the new colony is sufficiently implanted under the bark, an anti-aggregation pheromone is released to prevent overcrowding. Similar chemical communication is common among many insects.
The beetles also have a symbiotic relationship with blue-stain fungus. Beetles carry the fungus in specialized parts of their mouths, where it helps the beetles bore into the tree. As it spreads itself deeper into the trunk, the fungus disrupts the tree's ability to carry water and nutrients to the crown, causing the tree to starve to death.
Here they come!
Yearly aerial assessments of the forest health have concluded that 10,000 acres of the 242,000 in San Juan County are already showing clear signs of infestation, although Hartvigsen warns that “it takes two years until a tree shows the effects of an attack,” which is the yellowing, then tanning and graying of pine needles. The number of acres affected has more than doubled from 2012-2013, and is expected to continue to grow for 6-8 more generations, which is 10-12 years.
On the ground level, signs of an infested tree include the “pitch outs”, where sap has accumulated at the site of a beetle attack, and a really fine boring dust, like sawdust, in the crevices and at the base of the tree.
The consequences of large-scale tree death are already being felt in other parts of the country. In Northern Colorado and Southern Wyoming, the U.S. Forest Service estimates that up to 100,000 dead trees killed by beetles fall to the ground every day. All these dead trees pose a risk to outdoor recreation, roads, and power lines.
They also increase the fire potential, especially during the time when the dry pine needles are still clinging to the tree.
“After the needles fall off, it is less flammable than it was as a green tree,” explains USFS forester Gretchen Fitzgerald. “But beetle killed spruce can stand for a long time.” The fire risk increases again after the branches and trees fall, leaving behind a heavy fuel bed.
Congress recently approved $45 million towards damage mitigation in a portion of the farm bill in February. The money, however, will be spread across 35 states seeking help.
Hartvigsen says the Forest Service will “continue to focus on lower-elevation forests closer to private property, roads, and power lines.”
They are also talking with Durango Mountain Resort officials in an effort to prevent widespread losses at such a high-impact location. Since the beetles have not traveled that far west yet, mitigation may have a greater chance of success.
The labor-intensive removal of the largest, oldest trees in a stand may reduce the chances that a beetle population can grab hold in a particular area. Fitzgerald explains they also apply MCH caps to protect large legacy trees. MCH is the isolated anti-aggregation pheromone that the beetles release after colonizing a tree. As the beetles search for another suitable habitat, they face risk of death by exposure and predators.
As far as the wilderness goes, Hartvigsen states that they are “supposed to let the wilderness respond and reflect nature’s course.”
He reminds us that during the last outbreak in the 1930s and 40s, widespread pesticide use as a way to stop the attack ended up killing a lot more beneficial insects than aggressive ones.