The morning after I found clear evidence that spruce bark beetles had chewed their way into a pair of mature spruce trees in my front yard (described in detail in last week’s City Wilds column), I was still reeling from my discovery.
Head spinning, emotions still raw, I wanted to know if my trees were doomed, as I feared they were. I also wanted to learn more about what exactly had happened—and why it seemed to occur so quickly—and why, despite my summer-long effort to keep the spruces sufficiently hydrated and healthy by regular watering, the beetles had broken through their defenses.
And so I called the University of Alaska’s Cooperative Extension Service and spoke with its Anchorage-based integrated pest management (IPM) specialist, Jessie Moan. Jessie proved to be both knowledgeable and sympathetic. She explained things in an easy-to-follow way, while also listening patiently to my observations, worries, and questions.
After describing what I’d observed, I began my series of questions with this one (though I suspected I already knew the answer): “How many beetles does it take to kill a tree?”
Jessie’s reply, though unsatisfying, made sense: “I don’t have a great answer to that, because it depends on so many factors.” For instance: the size and age of the tree, its health, where the beetles attack the tree, how many make it to the phloem layer beneath the bark, on and on.
Still, her explanation of the beetles’ strategy made it clear that more than enough had breached my trees’ defenses to do great harm and possibly—probably---mortally wound at least one and perhaps both.
Here, informed by my interview with Jessie, I’ll present a simplified version of what happens when spruce bark beetles seek out a host to raise their young.
To begin, adult spruce bark beetles are small cylindrical insects, about a quarter-inch long and an eighth-inch wide, and reddish-brown to black in color. From birth to death they spend most of their lives below the bark of spruce trees, in the soft phloem tissue that’s critical to transporting water and nutrients within a tree.
Native to Alaska, the beetles are always present in northern forests that contain spruce trees and when their numbers are low, scientists say, they actually help maintain a forest’s health and primarily inhabit downed trees. It’s only during large outbreaks that they become a problem.
The adult beetles normally target older, mature trees during a “flight period” that lasts from May through July. If a spruce is healthy and the number of beetles is small, the tree will prevent their entry by “pitching out” the insects with sap, creating small pale, sticky masses on the bark’s exterior called “pitch tubes.” But when beetles find a vulnerable tree—which they do by sensing, and following, chemical scents put out by trees, one of many remarkable abilities the insects have evolved —they will send out a message, using chemical secretions of their own, called pheromones, that draw in other beetles.
In a successful “mass attack,” multiple beetles—sometimes a few, other times dozens, maybe even hundreds—will penetrate the tree’s bark and reach the soft phloem tissue that transports water, sugars, and other nutrients through the tree. If the swarm of insects becomes excessive, the beetles will send out another, “anti-aggregation” pheromone that in Jessie’s words, signals, “We’re full here, go somewhere else.”
Once inside the phloem, female beetles will chew a vertical path, called a gallery, and again use pheromones, this time to attract a male. After mating, the female will then lay a string of eggs along each side of the gallery. The adults will then spend the rest of their lives inside the tree.
When the eggs hatch, the larvae feed on the phloem, eating outward from the gallery in directions more or less perpendicular to it. If enough larvae are born inside the tree, they will eventually eat their way around the tree’s circumference, essentially severing the phloem and the spruce’s mechanism for feeding itself. Then the tree begins to starve.
So, Jessie pointed out, it’s the larval stage that does the great majority of the internal damage to the tree.
Alaska’s spruce bark beetles exhibit both 1- and 2-year life cycles, depending on weather conditions. Warmer years, like those we’ve experienced recently in Anchorage (and Alaska generally)—and which are more likely to occur as our climate heats up—tend to favor 1-year-cycles. What this means is that adult beetles are being produced more quickly. Larvae born this year will leave their host trees next spring or early summer, on the hunt for new spruces to occupy.
That shorter cycle, plus the increased vulnerability of spruce trees because of warmer and drier conditions, has contributed to Southcentral Alaska’s latest beetle outbreak. And many more dying spruce trees, whether in Anchorage or the Mat-Su Valleys.
A spruce tree whose defenses have been successfully breached in a mass attack is almost certainly going to die, though not necessarily right away. The tree essentially starves to death, with the length of that starvation dependent in part on the severity of the attack.
Even a tree that is “lightly attacked” has been compromised, made more vulnerable, and, Jessie noted, “Often it will be attacked again the following spring,” with fatal results.
“The bad news,” she told me, “is that it’s really hard for a spruce tree to come back from an attack.” That’s especially true during a massive outbreak like we’re experiencing in Anchorage right now, when beetles are exceedingly abundant and next summer there will be a new, large generation looking for trees to enter.
During our conversation, I recounted my shock upon noticing the brown, drooping branches and dried, brittle needles on a large section of one spruce tree. Only days earlier, both front-yard spruces had appeared to be in good health, vibrantly green and carrying hundreds of new spruce cones, and I hadn’t noticed any signs of a beetle attack.
The change seemed so sudden.
The same seemed true of the entire neighborhood: as recently as mid-July, the trees appeared healthy, at least from a distance. But now many are browned and dying. Could it really have happened so quickly? Do trees that have been “mass attacked” show signs of starvation only weeks—or days—later?
“Typically we don’t see needle drop (and browning) the same year the trees are attacked,” Jessie said. “It usually happens the following year. But if there’s been a huge mass attack, we sometimes see trees drop their needles the same year.”
I can’t be sure because I didn’t witness it happening—how could I have missed it, I wonder—but it does seem that a great swarm of spruce bark beetles passed through my Turnagain neighborhood this summer. This past spring and for much of the summer, our spruce trees appeared green and healthy, though perhaps increasingly stressed from the hot, dry summer. Now dying trees are everywhere I look, it seems.
There’s plenty more to discuss about spruce trees and spruce bark beetles, including the ways that homeowners can protect their trees; the pros and cons of using the pesticide carbaryl—something I chose not to do because of my concerns about putting toxins into the air and landscape; more observations and insights from Jessie Moan as well as Pam Miller of the Alaska Community Action on Toxics (ACAT); and some final reflections of my own.
Anchorage nature writer Bill Sherwonit is a widely published essayist and the author of more than a dozen books, including “Living with Wildness: An Alaskan Odyssey” and “Animal Stories: Encounters with Alaska’s Wildlife.” Readers wishing to send comments or questions directly to Bill may do so at email@example.com.